Debugging on Linux for 390 by Denis Joseph Barrow (djbarrow@de.ibm.com,barrow_dj@yahoo.com) Copyright (C) 2000 IBM Deutschland Entwicklung GmbH, IBM Corporation Best viewed with fixed width fonts Overview of Document: ===================== This document is intended to give an good overview of how to debug Linux for S390 it isn't intended as a complete reference & not a tutorial on the fundamentals of C & assembly, it dosen't go into 390 IO in any detail. It is intended to compliment the following books. Enterprise Systems Architecture/390 Reference Summary SA22-7209-01 & the Enterprise Systems Architecture/390 Principles of Operation SA22-7201-05 & any other worthwhile references you get. It is intended like the Enterprise Systems Architecture/390 Reference Summary to be printed out & used as a quick cheat sheet self help style reference when problems occur. Contents ======== S390 Register Set Address Spaces on Linux Address Spaces on Linux for S390 The Linux for S390 Kernel Task Structure Register Usage & Stackframes on Linux for S390 with glossary Compiling programs for debugging on Linux for S390 Figuring out gcc compile errors Debugging Tools objdump strace Debugging under VM Stack Tracing under VM S390 IO Overview Debugging IO on S390 under VM GDB on S390 Stack chaining in gdb by hand Examining core dumps LDD Debugging modules The proc file system Starting points for debugging scripting languages etc. S390 Register Set ================ The current ESA 390 architecture has the following registers. 16 32 bit General propose registers ( r0-r15 or gpr0-gpr15) used for arithmetic & addressing 16 Control registers ( cr0-cr15 kernel usage only ) used for memory managment, interrupt control,debugging control etc. 16 Access registers ( ar0-ar15 ) not used by normal programs but potentially could be used as temporary storage. Their main purpose is their 1 to 1 association with general purpose registers and are used in the kernel for copying data between address spaces. 16 64 bit floating point registers (fp0-fp15 ) IEEE & HFP floating point format compliant on G5 upwards & a Floating point control reg (FPC) 4 64 bit registers (fp0,fp2,fp4 & fp6) HFP only on older machines. Note: Linux (currently) always uses IEEE & emulates G5 IEEE format on older machines, ( provided the kernel is configured for this ). The PSW is the most important register on the machine it is 64 bit & serves the roles of a program counter (pc), condition code register,memory space designator. In IBM standard notation I am counting bit 0 as the MSB. It has several advantages over a normal program counter in that you can change address translation & program counter in a single instruction. To change address translation, e.g. switching address translation off requires that you have a logical=physical mapping for the address you are currently running at. Bit Value 0 Reserved ( must be 0 ) otherwise specification exception occurs. 1 Program Event Recording 1 PER enabled, PER is used to facilititate debugging e.g. single stepping. 2-4 Reserved ( must be 0 ). 5 Dynamic address translation 1=DAT on. 6 Input/Output interrupt Mask 7 External interrupt Mask used primarily for interprocessor signalling & clock interupts. 8-12 PSW Key used for complex memory protection mechanism not used under linux 13 Machine Check Mask 1=enable machine check interrupts 14 Wait State set this to 1 to stop the processor except for interrupts & give time to other LPARS used in CPU idle in the kernel to increase overall usage of processor resources. 15 Problem state ( if set to 1 certain instructions are disabled ) all linux user programs run with this bit 1 ( useful info for debugging under VM ). 16-17 Address Space Control 00 Primary Space Mode when DAT on The linux kernel currently runs in this mode, CR1 is affiliated with this mode & points to the primary segment table origin etc. 01 Access register mode this mode is used in functions to copy data between kernel & user space. 10 Secondary space mode not used in linux however CR7 the register affiliated with this mode is & this & normally CR13=CR7 to allow us to copy data between kernel & user space. We do this as follows: We set ar2 to 0 to designate its affiliated gpr ( gpr2 )to point to primary=kernel space. We set ar4 to 1 to designate its affiliated gpr ( gpr4 ) to point to secondary=home=user space & then essentially do a memcopy(gpr2,gpr4,size) to copy data between the address spaces, the reason we use home space for the kernel & don't keep secondary space free is that code will not run in secondary space. 11 Home Space Mode all user programs run in this mode. it is affiliated with CR13. 18-19 Condition codes (CC) 20 Fixed point overflow mask if 1=FPU exceptions for this event occur ( normally 0 ) 21 Decimal overflow mask if 1=FPU exceptions for this event occur ( normally 0 ) 22 Exponent underflow mask if 1=FPU exceptions for this event occur ( normally 0 ) 23 Significance Mask if 1=FPU exceptions for this event occur ( normally 0 ) 24-31 Reserved Must be 0. 32 1=31 bit addressing mode 0=24 bit addressing mode (for backward compatibility ), linux always runs with this bit set to 1 33-64 Instruction address. Prefix Page ----------- This per cpu memory area is too intimately tied to the processor not to mention. It exists between the real addresses 0-4096 on the processor & is exchanged with a page in absolute storage by the set prefix instruction in linux'es startup. This page different on each processor. Bytes 0-512 ( 200 hex ) are used by the processor itself for holding such information as exception indications & entry points for exceptions. Bytes after 0xc00 hex are used by linux for per processor globals. The closest thing to this on traditional architectures is the interrupt vector table. This is a good thing & does simplify some of the kernel coding however it means that we now cannot catch stray NULL pointers in the kernel without hard coded checks. Address Spaces on Linux ======================= The traditional Intel Linux is approximately mapped as follows forgive the ascii art. 0xFFFFFFFF 4GB Himem ***************** * * * Kernel Space * * * ***************** **************** User Space Himem (typically 0xC0000000 3GB )* User Stack * * * ***************** * * * Shared Libs * * Next Process * ***************** * to * * * <== * Run * <== * User Program * * * * Data BSS * * * * Text * * * * Sections * * * 0x00000000 ***************** **************** Now it is easy to see that on Intel it is quite easy to recognise a kernel address as being one greater than user space himem ( in this case 0xC0000000). & addresses of less than this are the ones in the current running program on this processor ( if an smp box ). If using the virtual machine ( VM ) as a debugger it is quite difficult to know which user process is running as the address space you are looking at could be from any process in the run queue. Thankfully you normally get lucky as address spaces don't overlap that & you can recognise the code at by cross referencing with a dump made by objdump ( more about that later ). The limitation of Intels addressing technique is that the linux kernel uses a very simple real address to virtual addressing technique of Real Address=Virtual Address-User Space Himem. This means that on Intel the kernel linux can typically only address Himem=0xFFFFFFFF-0xC0000000=1GB & this is all the RAM these machines can typically use. They can lower User Himem to 2GB or lower & thus be able to use 2GB of RAM however this shrinks the maximum size of User Space from 3GB to 2GB they have a no win limit of 4GB unless they go to 64 Bit. On 390 our limitations & strengths make us slightly different. For backward compatibility we are only allowed use 31 bits (2GB) of our 32 bit addresses,however, we use entirely separate address spaces for the user & kernel. This means we can support 2GB of non Extended RAM, & more with the Extended memory managment swap device & 64 Bit when it comes along. Address Spaces on Linux for S390 ================================ Our addressing scheme is as follows Himem 0x7fffffff 2GB ***************** **************** * User Stack * * * ***************** * * * Shared Libs * * * ***************** * * * * * Kernel * * User Program * * * * Data BSS * * * * Text * * * * Sections * * * 0x00000000 ***************** **************** This also means that we need to look at the PSW problem state bit or the addressing mode to decide whether we are looking at user or kernel space. The Linux for S390 Kernel Task Structure ======================================== Each process/thread under Linux for S390 has its own kernel task_struct defined in linux/include/linux/sched.h The S390 on initialisation & resuming of a process on a cpu sets the __LC_KERNEL_STACK variable in the spare prefix area for this cpu ( which we use for per processor globals). The kernel stack pointer is intimately tied with the task stucture for each processor as follows. ************************ * 1 page kernel stack * * ( 4K ) * ************************ * 1 page task_struct * * ( 4K ) * 8K aligned ************************ What this means is that we don't need to dedicate any register or global variable to point to the current running process & can retrieve it with the following very simple construct static inline struct task_struct * get_current(void) { struct task_struct *current; __asm__("lhi %0,-8192\n\t" "nr %0,15" : "=r" (current) ); return current; } i.e. just anding the current kernel stack pointer with the mask -8192. Thankfully because Linux dosen't have support for nested IO interrupts & our devices have large buffers can survive interrupts being shut for short amounts of time we don't need a separate stack for interrupts. Register Usage & Stackframes on Linux for S390 ============================================== Overview: --------- This is the code that gcc produces at the top & the bottom of each function, it usually is fairly consistent & similar from function to function & if you know its layout you can probalby make some headway in finding the ultimate cause of a problem after a crash without a source level debugger. Note: To follow stackframes requires a knowledge of C or Pascal & limited knowledge of one assembly language. Glossary: --------- alloca: This is a built in compiler function for runtime allocation of extra space on the callers stack which is obviously freed up on function exit ( e.g. the caller may choose to allocate nothing of a buffer of 4k if required for temporary purposes ), it generates very efficent code ( a few cycles ) when compared to alternatives like malloc. automatics: These are local variables on the stack, i.e they aren't in registers & they aren't static. back-chain: This is a pointer to the stack pointer before entering a framed functions ( see frameless function ) prologue got by deferencing the address of the current stack pointer, i.e. got by accessing the 32 bit value at the stack pointers current location. base-pointer: This is a pointer to the back of the literal pool which is an area just behind each procedure used to store constants in each function. call-clobbered: The caller probably needs to save these registers if there is something of value in them, on the stack or elsewhere before making a call to another procedure so that it can restore it later. epilogue: The code generated by the compiler to return to the caller. frameless-function A frameless function in Linux for 390 is one which doesn't need more than the 96 bytes given to it by the caller. A frameless function never: 1) Sets up a back chain. 2) Calls alloca. 3) Calls other normal functions 4) Has automatics. GOT-pointer: This is a pointer to the global-offset-table in ELF ( Executable Linkable Format, Linux'es most common executable format ), all globals & shared library objects are found using this pointer. lazy-binding ELF shared libraries are typically only loaded when routines in the shared library are actually first called at runtime. This is lazy binding. procedure-linkage-table This is a table found from the GOT which contains pointers to routines in other shared libraries which can't be called to by easier means. prologue: The code generated by the compiler to set up the stack frame. outgoing-args: This is extra area allocated on the stack of the calling function if the parameters for the callee's cannot all be put in registers, the same area can be reused by each function the caller calls. routine-descriptor: A COFF executable format based concept of a procedure reference actually being 8 bytes or more as opposed to a simple pointer to the routine. This is typically defined as follows Routine Descriptor offset 0=Pointer to Function Routine Descriptor offset 4=Pointer to Table of Contents The table of contents/TOC is roughly equivalent to a GOT pointer. & it means that shared libraries etc. can be shared between several environments each with their own TOC. static-chain: This is used in nested functions a concept adopted from pascal by gcc not used in ansi C or C++ ( although quite useful ), basically it is a pointer used to reference local variables of enclosing functions. You might come across this stuff once or twice in your lifetime. e.g. The function below should return 11 though gcc may get upset & toss warnings about unused variables. int FunctionA(int a) { int b; FunctionC(int c) { b=c+1; } FunctionC(10); return(b); } S390 Register usage =================== r0 used by syscalls/assembly call-clobbered r1 used by syscalls/assembly call-clobbered r2 argument 0 / return value 0 call-clobbered r3 argument 1 / return value 1 (if long long) call-clobbered r4 argument 2 call-clobbered r5 argument 3 call-clobbered r6 argument 5 saved r7 pointer-to arguments 5 to ... saved r8 this & that saved r9 this & that saved r10 static-chain ( if nested function ) saved r11 frame-pointer ( if function used alloca ) saved r12 got-pointer saved r13 base-pointer saved r14 return-address saved r15 stack-pointer saved f0 argument 0 / return value ( float/double ) call-clobbered f2 argument 1 call-clobbered f4 saved f6 saved The remaining floating points f1,f3,f5 f7-f15 are call-clobbered. Notes: ------ 1) The only requirement is that registers which are used by the callee are saved, e.g. the compiler is perfectly capible of using r11 for purposes other than a frame a frame pointer if a frame pointer is not needed. 2) In functions with variable arguments e.g. printf the calling procedure is identical to one without variable arguments & the same number of parameters. However, the prologue of this function is somewhat more hairy owing to it having to move these parameters to the stack to get va_start, va_arg & va_end to work. 3) Access registers are currently unused by gcc but are used in the kernel. Possibilities exist to use them at the moment for temporary storage but it isn't recommended. 4) Only 4 of the floating point registers are used for parameter passing as older machines such as G3 only have only 4 & it keeps the stack frame compatible with other compilers. However with IEEE floating point emulation under linux on the older machines you are free to use the other 12. 5) A long long or double parameter cannot be have the first 4 bytes in a register & the second four bytes in the outgoing args area. It must be purely in the outgoing args area if crossing this boundary. 6) Floating point parameters are mixed with outgoing args on the outgoing args area in the order the are passed in as parameters. Stack Frame Layout ================== 0 back chain ( a 0 here signifies end of back chain ) 4 eos ( end of stack, not used on Linux for S390 used in other linkage formats ) 8 glue used in other linkage formats for saved routine descriptors etc. 12 glue used in other linkage formats for saved routine descriptors etc. 16 scratch area 20 scratch area 24 saved r6 of caller function 28 saved r7 of caller function 32 saved r8 of caller function 36 saved r9 of caller function 40 saved r10 of caller function 44 saved r11 of caller function 48 saved r12 of caller function 52 saved r13 of caller function 56 saved r14 of caller function 60 saved r15 of caller function 64 saved f4 of caller function 72 saved f6 of caller function 80 undefined 96 outgoing args passed from caller to callee 96+x possible stack alignment ( 8 bytes desirable ) 96+x+y alloca space of caller ( if used ) 96+x+y+z automatics of caller ( if used ) 0 back-chain A sample program with comments. =============================== Comments on the function test ----------------------------- 1) It didn't need to set up a pointer to the constant pool gpr13 as it isn't used ( :-( ). 2) This is a frameless function & no stack is bought. 3) The compiler was clever enough to recognise that it could return the value in r2 as well as use it for the passed in parameter ( :-) ). 4) The basr ( branch relative & save ) trick works as follows the instruction has a special case with r0,r0 with some instruction operands is understood as the literal value 0, some risc architectures also do this ). So now we are branching to the next address & the address new program counter is in r13,so now we subtract the size of the function prologue we have executed + the size of the literal pool to get to the top of the literal pool 0040037c int test(int b) { # Function prologue below 40037c: 90 de f0 34 stm %r13,%r14,52(%r15) # Save registers r13 & r14 400380: 0d d0 basr %r13,%r0 # Set up pointer to constant pool using 400382: a7 da ff fa ahi %r13,-6 # basr trick return(5+b); # Huge main program 400386: a7 2a 00 05 ahi %r2,5 # add 5 to r2 # Function epilogue below 40038a: 98 de f0 34 lm %r13,%r14,52(%r15) # restore registers r13 & 14 40038e: 07 fe br %r14 # return } Comments on the function main ----------------------------- 1) The compiler did this function optimally ( 8-) ) Literal pool for main. 400390: ff ff ff ec .long 0xffffffec main(int argc,char *argv[]) { # Function prologue below 400394: 90 bf f0 2c stm %r11,%r15,44(%r15) # Save necessary registers 400398: 18 0f lr %r0,%r15 # copy stack pointer to r0 40039a: a7 fa ff a0 ahi %r15,-96 # Make area for callee saving 40039e: 0d d0 basr %r13,%r0 # Set up r13 to point to 4003a0: a7 da ff f0 ahi %r13,-16 # literal pool 4003a4: 50 00 f0 00 st %r0,0(%r15) # Save backchain return(test(5)); # Main Program Below 4003a8: 58 e0 d0 00 l %r14,0(%r13) # load relative address of test from # literal pool 4003ac: a7 28 00 05 lhi %r2,5 # Set first parameter to 5 4003b0: 4d ee d0 00 bas %r14,0(%r14,%r13) # jump to test setting r14 as return # address using branch & save instruction. # Function Epilogue below 4003b4: 98 bf f0 8c lm %r11,%r15,140(%r15)# Restore necessary registers. 4003b8: 07 fe br %r14 # return to do program exit } Compiling programs for debugging on Linux for S390 ================================================== Make sure that the gcc is compiling & linking with the -g flag on. This is typically done adding/appending the flags -g to the CFLAGS & LDFLAGS variables Makefile of the program concerned. If using gdb & you would like accurate displays of registers & stack traces compile without optimisation i.e make sure that there is no -O2 or similar on the CFLAGS line of the Makefile & the emitted gcc commands, obviously this will produce worse code ( not advisable for shipment ) but it is an aid to the debugging process. This aids debugging because the compiler will copy parameters passed in in registers onto the stack so backtracing & looking at passed in parameters will work, however some larger programs which use inline functions will not compile without optimisation. Figuring out gcc compile errors =============================== If you are getting a lot of syntax errors compiling a program & the problem isn't blatantly obvious from the source. It often helps to just preprocess the file, this is done with the -E option in gcc. What this does is that it runs through the very first phase of compilation ( compilation in gcc is done in several stages & gcc calls many programs to achieve its end result ) with the -E option gcc just calls the gcc preprocessor (cpp). The c preprocessor does the following, it joins all the files #included together recursively ( #include files can #include other files ) & also the c file you wish to compile. It puts a fully qualified path of the #included files in a comment & it does macro expansion. This is useful for debugging because 1) You can double check whether the files you expect to be included are the ones that are being included ( e.g. double check that you aren't going to the i386 asm directory ). 2) Check that macro definitions aren't clashing with typedefs, 3) Check that definitons aren't being used before they are being included. 4) Helps put the line emitting the error under the microscope if it contains macros. For convenience the Linux kernel's makefile will do preprocessing automatically for you by suffixing the file you want built with .i ( instead of .o ) e.g. from the linux directory type make arch/s390/kernel/signal.i this will build s390-gcc -D__KERNEL__ -I/home1/barrow/linux/include -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer -fno-strict-aliasing -D__SMP__ -pipe -fno-strength-reduce -E arch/s390/kernel/signal.c > arch/s390/kernel/signal.i Now look at signal.i you should see something like. # 1 "/home1/barrow/linux/include/asm/types.h" 1 typedef unsigned short umode_t; typedef __signed__ char __s8; typedef unsigned char __u8; typedef __signed__ short __s16; typedef unsigned short __u16; If instead you are getting errors further down e.g. unknown instruction:2515 "move.l" or better still unknown instruction:2515 "Fixme not implemented yet, call Martin" you are probably are attempting to compile some code meant for another architecture or code that is simply not implemented, with a fixme statement stuck into the inline assembly code so that the author of the file now knows he has work to do. To look at the assembly emitted by gcc just before it is about to call gas ( the gnu assembler ) use the -S option. Again for your convenience the Linux kernel's Makefile will hold your hand & do all this donkey work for you also by building the file with the .s suffix. e.g. from the Linux directory type make arch/s390/kernel/signal.s s390-gcc -D__KERNEL__ -I/home1/barrow/linux/include -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer -fno-strict-aliasing -D__SMP__ -pipe -fno-strength-reduce -S arch/s390/kernel/signal.c -o arch/s390/kernel/signal.s This will output something like, ( please note the constant pool & the useful comments in the prologue to give you a hand at interpreting it ). .LC54: .string "misaligned (__u16 *) in __xchg\n" .LC57: .string "misaligned (__u32 *) in __xchg\n" .L$PG1: # Pool sys_sigsuspend .LC192: .long -262401 .LC193: .long -1 .LC194: .long schedule-.L$PG1 .LC195: .long do_signal-.L$PG1 .align 4 .globl sys_sigsuspend .type sys_sigsuspend,@function sys_sigsuspend: # leaf function 0 # automatics 16 # outgoing args 0 # need frame pointer 0 # call alloca 0 # has varargs 0 # incoming args (stack) 0 # function length 168 STM 8,15,32(15) LR 0,15 AHI 15,-112 BASR 13,0 .L$CO1: AHI 13,.L$PG1-.L$CO1 ST 0,0(15) LR 8,2 N 5,.LC192-.L$PG1(13) Debugging Tools: ================ objdump ======= This is a tool with many options the most useful being ( if compiled with -g). objdump --source > The whole kernel can be compiled like this ( Doing this will make a 17MB kernel & a 200 MB listing ) however you have to strip it before building the image using the strip command to make it a more reasonable size to boot it. A source/assembly mixed dump of the kernel can be done with the line objdump --source vmlinux > vmlinux.lst Also if the file isn't compiled -g this will output as much debugging information as it can ( e.g. function names ), however, this is very slow as it spends lots of time searching for debugging info, the following self explanitory line should be used instead if the code isn't compiled -g. objdump --disassemble-all --syms vmlinux > vmlinux.lst as it is much faster As hard drive space is valuble most of us use the following approach. 1) Look at the emitted psw on the console to find the crash address in the kernel. 2) Look at the file System.map ( in the linux directory ) produced when building the kernel to find the closest address less than the current PSW to find the offending function. 3) use grep or similar to search the source tree looking for the source file with this function if you don't know where it is. 4) rebuild this object file with -g on, as an example suppose the file was ( /arch/s390/kernel/signal.o ) 5) Assuming the file with the erroneous function is signal.c Move to the base of the Linux source tree 6) rm /arch/s390/kernel/signal.o 7) make /arch/s390/kernel/signal.o 8) watch the gcc command line emitted 9) type it in again or alernatively cut & paste it on the console adding the -g option. 10) objdump --source arch/s390/kernel/signal.o > signal.lst This will output the source & the assembly intermixed, as the snippet below shows This will unfortunately output addresses which aren't the same as the kernel ones you should be able to get around the mental arithmetic by playing with the --adjust-vma parameter to objdump. extern inline void spin_lock(spinlock_t *lp) { a0: 18 34 lr %r3,%r4 a2: a7 3a 03 bc ahi %r3,956 __asm__ __volatile(" lhi 1,-1\n" a6: a7 18 ff ff lhi %r1,-1 aa: 1f 00 slr %r0,%r0 ac: ba 01 30 00 cs %r0,%r1,0(%r3) b0: a7 44 ff fd jm aa saveset = current->blocked; b4: d2 07 f0 68 mvc 104(8,%r15),972(%r4) b8: 43 cc return (set->sig[0] & mask) != 0; } 6) If debugging under VM go down to that section in the document for more info. strace: ------- Q. What is it ? A. It is a tool for intercepting calls to the kernel & logging them to a file & on the screen. Q. What use is it ? A. You can used it to find out what files a particular program opens. Example 1 --------- If you wanted to know does ping work but didn't have the source strace ping -c 1 127.0.0.1 & then look at the man pages for each of the syscalls below, ( In fact this is sometimes easier than looking at some spagetti source which conditionally compiles for several architectures ) Not everything that it throws out needs to make sense immeadiately Just looking quickly you can see that it is making up a RAW socket for the ICMP protocol. Doing an alarm(10) for a 10 second timeout & doing a gettimeofday call before & after each read to see how long the replies took, & writing some text to stdout so the user has an idea what is going on. socket(PF_INET, SOCK_RAW, IPPROTO_ICMP) = 3 getuid() = 0 setuid(0) = 0 stat("/usr/share/locale/C/libc.cat", 0xbffff134) = -1 ENOENT (No such file or directory) stat("/usr/share/locale/libc/C", 0xbffff134) = -1 ENOENT (No such file or directory) stat("/usr/local/share/locale/C/libc.cat", 0xbffff134) = -1 ENOENT (No such file or directory) getpid() = 353 setsockopt(3, SOL_SOCKET, SO_BROADCAST, [1], 4) = 0 setsockopt(3, SOL_SOCKET, SO_RCVBUF, [49152], 4) = 0 fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(3, 1), ...}) = 0 mmap(0, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40008000 ioctl(1, TCGETS, {B9600 opost isig icanon echo ...}) = 0 write(1, "PING 127.0.0.1 (127.0.0.1): 56 d"..., 42PING 127.0.0.1 (127.0.0.1): 56 data bytes ) = 42 sigaction(SIGINT, {0x8049ba0, [], SA_RESTART}, {SIG_DFL}) = 0 sigaction(SIGALRM, {0x8049600, [], SA_RESTART}, {SIG_DFL}) = 0 gettimeofday({948904719, 138951}, NULL) = 0 sendto(3, "\10\0D\201a\1\0\0\17#\2178\307\36"..., 64, 0, {sin_family=AF_INET, sin_port=htons(0), sin_addr=inet_addr("127.0.0.1")}, 16) = 64 sigaction(SIGALRM, {0x8049600, [], SA_RESTART}, {0x8049600, [], SA_RESTART}) = 0 sigaction(SIGALRM, {0x8049ba0, [], SA_RESTART}, {0x8049600, [], SA_RESTART}) = 0 alarm(10) = 0 recvfrom(3, "E\0\0T\0005\0\0@\1|r\177\0\0\1\177"..., 192, 0, {sin_family=AF_INET, sin_port=htons(50882), sin_addr=inet_addr("127.0.0.1")}, [16]) = 84 gettimeofday({948904719, 160224}, NULL) = 0 recvfrom(3, "E\0\0T\0006\0\0\377\1\275p\177\0"..., 192, 0, {sin_family=AF_INET, sin_port=htons(50882), sin_addr=inet_addr("127.0.0.1")}, [16]) = 84 gettimeofday({948904719, 166952}, NULL) = 0 write(1, "64 bytes from 127.0.0.1: icmp_se"..., 5764 bytes from 127.0.0.1: icmp_seq=0 ttl=255 time=28.0 ms Example 2 --------- strace passwd 2>&1 | grep open produces the following output open("/etc/ld.so.cache", O_RDONLY) = 3 open("/opt/kde/lib/libc.so.5", O_RDONLY) = -1 ENOENT (No such file or directory) open("/lib/libc.so.5", O_RDONLY) = 3 open("/dev", O_RDONLY) = 3 open("/var/run/utmp", O_RDONLY) = 3 open("/etc/passwd", O_RDONLY) = 3 open("/etc/shadow", O_RDONLY) = 3 open("/etc/login.defs", O_RDONLY) = 4 open("/dev/tty", O_RDONLY) = 4 The 2>&1 is done to redirect stderr to stdout & grep is then filtering this input through the pipe for each line containing the string open. Example 3 --------- Getting sophistocated telnetd crashes on & I don't know why Steps ----- 1) Replace the following line in /etc/inetd.conf telnet stream tcp nowait root /usr/sbin/in.telnetd -h with telnet stream tcp nowait root /blah 2) Create the file /blah with the following contents to start tracing telnetd #!/bin/bash /usr/bin/strace -o/t1 -f /usr/sbin/in.telnetd -h 3) chmod 700 /blah to make it executable only to root 4) killall -HUP inetd or ps aux | grep inetd get inetd's process id & kill -HUP inetd to restart it. Important options ----------------- -o is used to tell strace to output to a file in our case t1 in the root directory -f is to follow children i.e. e.g in our case above telnetd will start the login process & subsequently a shell like bash. You will be able to tell which is which from the process ID's listed on the left hand side of the strace output. -p will tell strace to attach to a running process, yup this can be done provided it isn't being traced or debugged already & you have enough privileges, the reason 2 processes cannot trace or debug the same program is that strace becomes the parent process of the one being debugged & processes ( unlike people ) can have only one parent. However the file /t1 will get big quite quickly to test it telnet 127.0.0.1 now look at what files in.telnetd execve'd 413 execve("/usr/sbin/in.telnetd", ["/usr/sbin/in.telnetd", "-h"], [/* 17 vars */]) = 0 414 execve("/bin/login", ["/bin/login", "-h", "localhost", "-p"], [/* 2 vars */]) = 0 Whey it worked!. Other hints: ------------ If the program is not very interactive ( i.e. not much keyboard input ) & is crashing in one architecture but not in another you can do an strace of both programs under as identical a scenario as you can on both architectures outputting to a file then. do a diff of the two traces using the diff program i.e. diff output1 output2 & maybe you'll be able to see where the call paths differed, this is possibly near the cause of the crash. More info --------- Look at man pages for strace & the various syscalls e.g. man strace, man alarm, man socket. Debugging under VM ================== Notes ----- Addresses & values in the VM debugger are always hex never decimal Address ranges are of the format - or . e.g. The address range 0x2000 to 0x3000 can be described described as 2000-3000 or 2000.1000 The VM Debugger is case insensitive. VM's strengths are usually other debuggers weaknesses you can get at any resource no matter how sensitive e.g. memory managment resources,change address translation in the PSW. For kernel hacking you will reap dividends if you get good at it. The VM Debugger displays operators but not operands, probably because some of it was written when memory was expensive & the programmer was probably proud that it fitted into 2k of memory & the programmers & didn't want to shock hardcore VM'ers by changing the interface :-), also the debugger displays useful information on the same line & the author of the code probably felt that it was a good idea not to go over the 80 columns on the screen. As some of you are probably in a panic now this isn't as unintuitive as it may seem as the 390 instructions are easy to decode mentally & you can make a good guess at a lot of them as all the operands are nibble ( half byte aligned ) & if you have an objdump listing also it is quite easy to follow, if you don't have an objdump listing keep a copy of the ESA Reference Summary & look at between pages 2 & 7 or alternatively the ESA principles of operation. e.g. even I can guess that 0001AFF8' LR 180F CC 0 is a ( load register ) lr r0,r15 Also it is very easy to tell the length of a 390 instruction from the 2 most significant bits in the instruction ( not that this info is really useful except if you are trying to make sense of a hexdump of code ). Here is a table Bits Instruction Length ------------------------------------------ 00 2 Bytes 01 4 Bytes 10 4 Bytes 11 6 Bytes The debugger also displays other useful info on the same line such as the addresses being operated on destination addresses of branches & condition codes. e.g. 00019736' AHI A7DAFF0E CC 1 000198BA' BRC A7840004 -> 000198C2' CC 0 000198CE' STM 900EF068 >> 0FA95E78 CC 2 Useful VM debugger commands =========================== I suppose I'd better mention this before I start to list the current active traces do Q TR there can be a maximum of 255 of these per set ( more about trace sets later ). To stop traces issue a TR END. To delete a particular breakpoint issue TR DEL The PA1 key drops to CP mode so you can issue debugger commands, Doing alt c (on my 3270 console at least ) clears the screen. hitting b comes back to the running operating system from cp mode ( in our case linux ). It is typically useful to add shortcuts to your profile.exec file if you have one ( this is roughly equivalent to autoexec.bat in DOS ). file here are a few from mine. /* this gives me command history on issuing f12 */ set pf12 retrieve /* this continues */ set pf8 imm b /* goes to trace set a */ set pf1 imm tr goto a /* goes to trace set b */ set pf2 imm tr goto b /* goes to trace set c */ set pf3 imm tr goto c Instruction Tracing ------------------- Setting a simple breakpoint TR I PSWA
To debug a particular function try TR I R TR I on its own will single step. TR I DATA will trace for particular mnemonics e.g. TR I DATA 4D R 0197BC.4000 will trace for BAS'es ( opcode 4D ) in the range 0197BC.4000 if you were inclined you could add traces for all branch instructions & suffix them with the run prefix so you would have a backtrace on screen when a program crashes. TR BR will trace branches into or out of an address. e.g. TR BR INTO 0 is often quite useful if a program is getting awkward & deciding to branch to 0 & crashing as this will stop at the address before in jumps to 0. Displaying & modifying Registers -------------------------------- D G will display all the gprs D X will display all the control registers D AR will display all the access registers D AR4-7 will display access registers 4 to 7 CPU ALL D G will display the GRPS of all CPUS in the configuration D PSW will display the current PSW st PSW 2000 will put the value 2000 into the PSW & cause crash your machine. D PREFIX Displaying Memory ----------------- To display memory mapped using the current PSW's mapping try D To make VM display a message each time it hits a particular address & continue try D I will disassemble/display a range of instructions. ST addr 32 bit word will store a 32 bit aligned address D T will display the EBCDIC in an address ( if you are that way inclined ) D R will display real addresses ( without DAT ) but with prefixing. There are other complex options to display if you need to get at say home space but are in primary space the easiest thing to do is to temporarily modify the PSW to the other addressing mode, display the stuff & then restore it. Hints ----- If you want to issue a debugger command without halting your virtual machine with the PA1 key try prefixing the command with #CP e.g. #cp tr i pswa 2000 also suffixing most debugger commands with RUN will cause them not to stop just display the mnemonic at the current instruction on the console. If you have several breakpoints you want to put into your program & you get fed up of cross referencing with System.map you can do the following trick for several symbols. grep do_signal System.map which emits the following among other things 0001f4e0 T do_signal now you can do TR I PSWA 0001f4e0 cmd msg * do_signal This sends a message to your own console each time do_signal is entered. ( As an aside I wrote a perl script once which automatically generated a REXX script with breakpoints on every kernel procedure, this isn't a good idea because there are thousands of these routines & VM can only set 255 breakpoints at a time so you nearly had to spend as long pruning the file down as you would entering the msg's by hand ),however, the trick might be useful for a single object file. Tracing Program Exceptions -------------------------- If you get a crash which says something like illegal operation or specification exception followed by a register dump You can restart linux & trace these using the tr prog trace option. The most common ones you will normally be tracing for is 1=operation exception 2=privileged operation exception 4=protection exception 5=addressing exception 6=specification exception 10=segment translation exception 11=page translation exception The full list of these is on page 22 of the current ESA Reference Summary. e.g. tr prog 10 will trace segment translation exceptions. tr prog on its own will trace all program interruption codes. Trace Sets ---------- On starting VM you are initially in the INITIAL trace set. You can do a Q TR to verify this. If you have a complex tracing situation where you wish to wait for instance till a driver is open before you start tracing IO, but know in your heart that you are going to have to make several runs through the code till you have a clue whats going on. What you can do is TR I PSWA hit b to continue till breakpoint reach the breakpoint now do your TR GOTO B TR IO 7c08-7c09 or whatever & trace tour IO to got back to the initial trace set do TR GOTO INITIAL & the TR I PSWA will be the only active breakpoint again. Tracing linux syscalls under VM ------------------------------- Syscalls are implemented on Linux for S390 by the Supervisor call instruction (SVC) there 256 possibilities of these as the instruction is made up of a 0xA opcode & the second byte being the syscall number. They are traced using the simple command. TR SVC the syscalls are defined in linux/include/asm-s390/unistd.h e.g. to trace all file opens just do TR SVC 5 ( as this is the syscall number of open ) SMP Specific commands --------------------- To find out how many cpus you have Q CPUS displays all the CPU's available to your virtual machine To find the cpu that the current cpu VM debugger commands are being directed at do Q CPU to change the current cpu cpu VM debugger commands are being directed at do CPU On a SMP guest issue a command to all CPUs try prefixing the command with cpu all. To issue a command to a particular cpu try cpu e.g. CPU 01 TR I R 2000.3000 If you are running on a guest with several cpus & you have a IO related problem & cannot follow the flow of code but you know it isnt smp related. from the bash prompt issue shutdown -h now or halt. do a Q CPUS to find out how many cpus you have detach each one of them from cp except cpu 0 by issueing a DETACH CPU 01-(number of cpus in configuration) & reboot linux. TR SIGP will trace inter processor signal processor instructions. Help for displaying ascii textstrings ------------------------------------- As textstrings are cannot be displayed in ASCII under the VM debugger ( I love EBDIC too ) I have written this little program which will convert a command line of hex digits to ascii text which can be compiled under linux & you can copy the hex digits from your x3270 terminal to your xterm if you are debugging from a linuxbox. This is quite useful when looking at a parameter passed in as a text string under VM ( unless you are good at decoding ASCII in your head ). e.g. consider tracing an open syscall TR SVC 5 We have stopped at a breakpoint 000151B0' SVC 0A05 -> 0001909A' CC 0 D 20.8 to check the SVC old psw in the prefix area & see was it from userspace ( for the layout of the prefix area consult P18 of the ESA 390 Reference Summary if you have it available ). V00000020 070C2000 800151B2 The problem state bit wasn't set & it's also too early in the boot sequence for it to be a userspace SVC if it was we would have to temporarily switch the psw to user space addressing so we could get at the first parameter of the open in gpr2. Next do a D G2 GPR 2 = 00014CB4 Now display what gpr2 is pointing to D 00014CB4.20 V00014CB4 2F646576 2F636F6E 736F6C65 00001BF5 V00014CC4 FC00014C B4001001 E0001000 B8070707 Now copy the text till the first 00 hex ( which is the end of the string to an xterm & do hex2ascii on it. hex2ascii 2F646576 2F636F6E 736F6C65 00 outputs Decoded Hex:=/ d e v / c o n s o l e 0x00 We were opening the console device, You can compile the code below yourself for practice :-), /* * hex2ascii.c * a useful little tool for converting a hexadecimal command line to ascii * * Author(s): Denis Joseph Barrow (djbarrow@de.ibm.com,barrow_dj@yahoo.com) * (C) 2000 IBM Deutschland Entwicklung GmbH, IBM Corporation. */ #include int main(int argc,char *argv[]) { int cnt1,cnt2,len,toggle=0; int startcnt=1; unsigned char c,hex; if(argc>1&&(strcmp(argv[1],"-a")==0)) startcnt=2; printf("Decoded Hex:="); for(cnt1=startcnt;cnt1='0'&&c<='9') c=c-'0'; if(c>='A'&&c<='F') c=c-'A'+10; if(c>='a'&&c<='F') c=c-'a'+10; switch(toggle) { case 0: hex=c<<4; toggle=1; break; case 1: hex+=c; if(hex<32||hex>127) { if(startcnt==1) printf("0x%02X ",(int)hex); else printf("."); } else { printf("%c",hex); if(startcnt==1) printf(" "); } toggle=0; break; } } } printf("\n"); } Stack tracing under VM ---------------------- A basic backtrace ----------------- Here are the tricks I use 9 out of 10 times it works pretty well, When your backchain reaches a dead end -------------------------------------- This can happen when an exception happens in the kernel & the kernel is entered twice if you reach the NULL pointer at the end of the back chain you should be able to sniff further back if you follow the following tricks. 1) A kernel address should be easy to recognise since it is in primary space & the problem state bit isn't set & also The Hi bit of the address is set. 2) Another backchain should also be easy to recognise since it is an address pointing to another address approximately 100 bytes or 0x70 hex behind the current stackpointer. Here is some practice. boot the kernel & hit PA1 at some random time d g to display the gprs, this should display something like GPR 0 = 00000001 00156018 0014359C 00000000 GPR 4 = 00000001 001B8888 000003E0 00000000 GPR 8 = 00100080 00100084 00000000 000FE000 GPR 12 = 00010400 8001B2DC 8001B36A 000FFED8 Note that GPR14 is a return address but as we are real men we are going to trace the stack. display 0x40 bytes after the stack pointer. V000FFED8 000FFF38 8001B838 80014C8E 000FFF38 V000FFEE8 00000000 00000000 000003E0 00000000 V000FFEF8 00100080 00100084 00000000 000FE000 V000FFF08 00010400 8001B2DC 8001B36A 000FFED8 Ah now look at whats in sp+56 (sp+0x38) this is 8001B36A our saved r14 if you look above at our stackframe & also agrees with GPR14. now backchain d 000FFF38.40 we now are taking the contents of SP to get our first backchain. V000FFF38 000FFFA0 00000000 00014995 00147094 V000FFF48 00147090 001470A0 000003E0 00000000 V000FFF58 00100080 00100084 00000000 001BF1D0 V000FFF68 00010400 800149BA 80014CA6 000FFF38 This displays a 2nd return address of 80014CA6 now do d 000FFFA0.40 for our 3rd backchain V000FFFA0 04B52002 0001107F 00000000 00000000 V000FFFB0 00000000 00000000 FF000000 0001107F V000FFFC0 00000000 00000000 00000000 00000000 V000FFFD0 00010400 80010802 8001085A 000FFFA0 our 3rd return address is 8001085A as the 04B52002 looks suspiciously like rubbish it is fair to assume that the kernel entry routines for the sake of optimisation dont set up a backchain. now look at System.map to see if the addresses make any sense. grep -i 0001b3 System.map outputs among other things 0001b304 T cpu_idle so 8001B36A is cpu_idle+0x66 ( quiet the cpu is asleep, don't wake it ) grep -i 00014 System.map produces among other things 00014a78 T start_kernel so 0014CA6 is start_kernel+some hex number I can't add in my head. grep -i 00108 System.map this produces 00010800 T _stext so 8001085A is _stext+0x5a Congrats you've done your first backchain. S390 IO Overview ================ I am not going to give a course in 390 IO architecture as this would take me quite a while & I'm no expert. Instead I'll give a 390 IO architecture summary for Dummies if you have the ESA principles of operation available read this instead. If nothing else you may find a few useful keywords in here & be able to use them on a web search engine like altavista to find more useful information. Unlike other bus architectures modern 390 systems do their IO using mostly fibre optics & devices such as tapes & disks can be shared between several mainframes, also S390 can support upto 65536 devices while a high end PC based system might be choking with around 64. Here is some of the common IO terminology Subchannel: This is the logical number most IO commands use to talk to an IO device there can be upto 0x10000 (65536) of these in a configuration typically there is a few hundred. Under VM for simplicity they are allocated contiguously, however on the native hardware they are not they typically stay consistent between boots provided no new hardware is inserted or removed. Under Linux for 390 we use these as IRQ's & also when issuing an IO command (CLEAR SUBCHANNEL, HALT SUBCHANNEL,MODIFY SUBCHANNEL,RESUME SUBCHANNEL,START SUBCHANNEL,STORE SUBCHANNEL & TEST SUBCHANNEL ) we use this as the ID of the device we wish to talk to, the most important of these instructions are START SUBCHANNEL ( to start IO ), TEST SUBCHANNEL ( to check whether the IO completed successfully ), & HALT SUBCHANNEL ( to kill IO ), a subchannel can have up to 8 channel paths to a device this offers redunancy if one is not available. Device Number: This number remains static & Is closely tied to the hardware, there are 65536 of these also they are made up of a CHPID ( Channel Path ID, the most significant 8 bits ) & another lsb 8 bits. These remain static even if more devices are inserted or removed from the hardware, there is a 1 to 1 mapping between Subchannels & Device Numbers provided devices arent inserted or removed. Channel Control Words: CCWS are linked lists of instructions initially pointed to by an operation request block (ORB), which is initially given to Start Subchannel (SSCH) command along with the subchannel number for the IO subsystem to process while the CPU continues executing normal code. These come in two flavours, Format 0 ( 24 bit for backward ) compatibility & Format 1 ( 31 bit ). These are typically used to issue read & write ( & many other instructions ) they consist of a length field & an absolute address field. For each IO typically get 1 or 2 interrupts one for channel end ( primary status ) when the channel is idle & the second for device end ( secondary status ) sometimes you get both concurrently, you check how the IO went on by issueing a TEST SUBCHANNEL at each interrupt, from which you receive an Interruption response block (IRB). If you get channel & device end status in the IRB without channel checks etc. your IO probably went okay. If you didn't you probably need a doctorto examine the IRB & extended status word etc. If an error occurs more sophistocated control units have a facitity known as concurrent sense this means that if an error occurs Extended sense information will be presented in the Extended status word in the IRB if not you have to issue a subsequent SENSE CCW command after the test subchannel. TPI( Test pending interrupt) can also be used for polled IO but in multitasking multiprocessor systems it isn't recommended except for checking special cases ( i.e. non looping checks for pending IO etc. ). Store Subchannel & Modify Subchannel can be used to examine & modify operating characteristics of a subchannel ( e.g. channel paths ). Other IO related Terms: Sysplex: S390's Clustering Technology QDIO: S390's new high speed IO architecture to support devices such as gigabit ethernet, this architecture is also designed to be forward compatible with up & coming 64 bit machines. General Concepts Input Output Processors (IOP's) are responsible for communicating between the mainframe CPU's & the channel & relieve the mainframe CPU's from the burden of communicating with IO devices directly, this allows the CPU's to concentrate on data processing. IOP's can use one or more links ( known as channel paths ) to talk to each IO device. It first checks for path availability & chooses an available one, then starts ( & sometimes terminates IO ). There are two types of channel path ESCON & the Paralell IO interface. IO devices are attached to control units, control units provide the logic to interface the channel paths & channel path IO protocols to the IO devices, they can be integrated with the devices or housed separately & often talk to several similar devices ( typical examples would be raid controllers or a control unit which connects to 1000 3270 terminals ). +---------------------------------------------------------------+ | +-----+ +-----+ +-----+ +-----+ +----------+ +----------+ | | | CPU | | CPU | | CPU | | CPU | | Main | | Expanded | | | | | | | | | | | | Memory | | Storage | | | +-----+ +-----+ +-----+ +-----+ +----------+ +----------+ | |---------------------------------------------------------------+ | IOP | IOP | IOP | |--------------------------------------------------------------- | C | C | C | C | C | C | C | C | C | C | C | C | C | C | C | C | ---------------------------------------------------------------- || || || Bus & Tag Channel Path || ESCON || ====================== || Channel || || || || Path +----------+ +----------+ +----------+ | | | | | | | CU | | CU | | CU | | | | | | | +----------+ +----------+ +----------+ | | | | | +----------+ +----------+ +----------+ +----------+ +----------+ |I/O Device| |I/O Device| |I/O Device| |I/O Device| |I/O Device| +----------+ +----------+ +----------+ +----------+ +----------+ CPU = Central Processing Unit C = Channel IOP = IP Processor CU = Control Unit The 390 IO systems come in 2 flavours the current 390 machines support both The Older 360 & 370 Interface,sometimes called the paralell I/O interface, sometimes called Bus-and Tag & sometimes Original Equipment Manufacturers Interface (OEMI). This byte wide paralell channel path/bus has parity & data on the "Bus" cable & control lines on the "Tag" cable. These can operate in byte multiplex mode for sharing between several slow devices or burst mode & monopolize the channel for the whole burst. Upto 256 devices can be addressed on one of these cables. These cables are about one inch in diameter. The maximum unextended length supported by these cables is 125 Meters but this can be extended up to 2km with a fibre optic channel extended such as a 3044. The maximum burst speed supported is 4.5 megabytes per second however some really old processors support only transfer rates of 3.0, 2.0 & 1.0 MB/sec. One of these paths can be daisy chained to up to 8 control units. ESCON if fibre optic it is also called FICON Was introduced by IBM in 1990. Has 2 fibre optic cables & uses either leds or lasers for communication at a signaling rate of upto 200 megabits/sec. As 10bits are transferred for every 8 bits info this drops to 160 megabits/sec & to 18.6 Megabytes/sec once control info & CRC are added. ESCON only operates in burst mode. ESCONs typical max cable length is 3km for the led version & 20km for the laser version known as XDF ( extended distance facility ). This can be further extended by using an ESCON director which triples the above mentioned ranges. Unlike Bus & Tag as ESCON is serial it uses a packet switching architecture the standard Bus & Tag control protocol is however present within the packets. Upto 256 devices can be attached to each control unit that uses one of these interfaces. Common 390 Devices include: Network adapters typically OSA2,3172's,2116's & OSA-E gigabit ethernet adapters, Consoles 3270 & 3215 ( a teletype emulated under linux for a line mode console ). DASD's direct access storage devices ( otherwise known as hard disks ). Tape Drives. CTC ( Channel to Channel Adapters ), ESCON or Paralell Cables used as a very high speed serial link between 2 machines. We use 2 cables under linux to do a bi-directional serial link. Debugging IO on S390 under VM ============================= Now we are ready to go on with IO tracing commands under VM A few self explanatory queries: Q OSA Q CTC Q DISK Q DASD Q osa on my machine returns OSA 7C08 ON OSA 7C08 SUBCHANNEL = 0000 OSA 7C09 ON OSA 7C09 SUBCHANNEL = 0001 OSA 7C14 ON OSA 7C14 SUBCHANNEL = 0002 OSA 7C15 ON OSA 7C15 SUBCHANNEL = 0003 Now using the device numbers returned by this command we will Trace the io starting up on the first device 7c08 & 7c09 In our simplest case we can trace the start subchannels like TR SSCH 7C08-7C09 or the halt subchannels or TR HSCH 7C08-7C09 MSCH's ,STSCH's I think you can guess the rest Ingo's favourite trick is tracing all the IO's & CCWS & spooling them into the reader of another VM guest so he can ftp the logfile back to his own machine.I'll do a small bit of this & give you a look at the output. 1) Spool stdout to VM guest linux4's reader SP PRT TO * 2) Fill linux4's reader with the trace TR IO 7c08-7c09 INST INT CCW PRT RUN 3) Start up linux i 00c 4) Finish the trace TR END 5) close the reader C PRT 6) list reader contents RDRLIST 7) copy it to linux4's minidisk RECEIVE / LOG TXT A1 ( replace 8) filel & press F11 to look at it You should see someting like. 00020942' SSCH B2334000 0048813C CC 0 SCH 0000 DEV 7C08 CPA 000FFDF0 PARM 00E2C9C4 KEY 0 FPI C0 LPM 80 CCW 000FFDF0 E4200100 00487FE8 0000 E4240100 ........ IDAL 43D8AFE8 IDAL 0FB76000 00020B0A' I/O DEV 7C08 -> 000197BC' SCH 0000 PARM 00E2C9C4 00021628' TSCH B2354000 >> 00488164 CC 0 SCH 0000 DEV 7C08 CCWA 000FFDF8 DEV STS 0C SCH STS 00 CNT 00EC KEY 0 FPI C0 CC 0 CTLS 4007 00022238' STSCH B2344000 >> 00488108 CC 0 SCH 0000 DEV 7C08 If you don't like messing up your readed ( because you possibly booted from it ) you can alternatively spool it to another readers guest. GDB on S390 =========== N.B. if compiling for debugging gdb works better without optimisation ( see Compiling programs for debugging ) invocation ---------- gdb Online help ----------- help: gives help on commands e.g. help help display Note gdb's online help is very good use it. Assembly -------- info registers: displays registers other than floating point. info all-registers: displays floating points as well. disassemble: dissassembles e.g. disassemble without parameters will disassemble the current function disassemble $pc $pc+10 Viewing & modifying variables ----------------------------- print or p: displays variable or register e.g. p/x $sp will display the stack pointer display: prints variable or register each time program stops e.g. display/x $pc will display the program counter display argc undisplay : undo's display's info breakpoints: shows all current breakpoints info stack: shows stack back trace ( if this dosent work too well, I'll show you the stacktrace by hand below ). info locals: displays local variables. info args: display current procedure arguments. set args: will set argc & argv each time the victim program is invoked. set =value set argc=100 set $pc=0 Modifying execution ------------------- step: steps n lines of sourcecode step steps 1 line. step 100 steps 100 lines of code. next: like step except this will not step into subroutines stepi: steps a single machine code instruction. e.g. stepi 100 nexti: steps a single machine code instruction but will not step into subroutines. finish: will run until exit of the current routine run: (re)starts a program cont: continues a program quit: exits gdb. breakpoints ------------ break sets a breakpoint e.g. break main break *$pc break *0x400618 heres a really useful one for large programs rbr Set a breakpoint for all functions matching REGEXP e.g. rbr 390 will set a breakpoint with all functions with 390 in their name. info breakpoints lists all breakpoints delete: delete breakpoint by number or delete them all e.g. delete 1 will delete the first breakpoint delete will delete them all watch: This will set a watchpoint ( usually hardware assisted ), This will watch a variable till it changes e.g. watch cnt, will watch the variable cnt till it changes. As an aside unfortunately gdb's, architecture independent watchpoint code is inconsistent & not very good, watchpoints usually work but not always. info watchpoints: Display currently active watchpoints condition: ( another useful one ) Specify breakpoint number N to break only if COND is true. Usage is `condition N COND', where N is an integer and COND is an expression to be evaluated whenever breakpoint N is reached. User defined functions/macros ----------------------------- define: ( Note this is very very useful,simple & powerful ) usage define end examples which you should consider putting into .gdbinit in your home directory define d stepi disassemble $pc $pc+10 end define e nexti disassemble $pc $pc+10 end Other hard to classify stuff ---------------------------- signal n: sends the victim program a signal. e.g. signal 3 will send a SIGQUIT. info signals: what gdb does when the victim receives certain signals. list: e.g. list lists current function source list 1,10 list first 10 lines of curret file. list test.c:1,10 directory: Adds directories to be searched for source if gdb cannot find the source. (note it is a bit sensititive about slashes ) e.g. To add the root of the filesystem to the searchpath do directory // call This calls a function in the victim program, this is pretty powerful e.g. (gdb) call printf("hello world") outputs: $1 = 11 You might now be thinking that the line above didn't work, something extra had to be done. (gdb) call fflush(stdout) hello world$2 = 0 As an aside the debugger also calls malloc & free under the hood to make space for the "hello world" string. hints ----- 1) command completion works just like bash ( if you are a bad typist like me this really helps ) e.g. hit br & cursor up & down :-). 2) if you have a debugging problem that takes a few steps to recreate put the steps into a file called .gdbinit in your current working directory if you have defined a few extra useful user defined commands put these in your home directory & they will be read each time gdb is launched. A typical .gdbinit file might be. break main run break runtime_exception cont stack chaining in gdb by hand ----------------------------- This is done using a the same trick described for VM p/x (*($sp+56))&0x7fffffff get the first backchain. this outputs $5 = 0x528f18 on my machine. Now you can use info symbol (*($sp+56))&0x7fffffff you might see something like. rl_getc + 36 in section .text telling you what is located at address 0x528f18 Now do. p/x (*(*$sp+56))&0x7fffffff This outputs $6 = 0x528ed0 Now do. info symbol (*(*$sp+56))&0x7fffffff rl_read_key + 180 in section .text now do p/x (*(**$sp+56))&0x7fffffff & so on. Note: Remember gdb has history just like bash you don't need to retype the whole line just use the up & down arrows. For more info ------------- From your linuxbox do man gdb or info gdb. core dumps ---------- What a core dump ?, A core dump is a file generated by the kernel ( if allowed ) which contains the registers, & all active pages of the program which has crashed. From this file gdb will allow you to look at the registers & stack trace & memory of the program as if it just crashed on your system, it is usually called core & created in the current working directory. This is very useful in that a customer can mail a core dump to a technical support department & the technical support department can reconstruct what happened. Provided the have an indentical copy of this program with debugging symbols compiled in & the source base of this build is available. In short it is far more useful than something like a crash log could ever hope to be. In theory all that is missing to restart a core dumped program is a kernel patch which will do the following. 1) Make a new kernel task structure 2) Reload all the dumped pages back into the kernels memory managment structures. 3) Do the required clock fixups 4) Get all files & network connections for the process back into an identical state ( really difficult ). 5) A few more difficult things I haven't thought of. Why have I never seen one ?. Probably because you haven't used the command ulimit -c unlimited in bash to allow core dumps, now do ulimit -a to verify that the limit was accepted. A sample core dump To create this I'm going to do ulimit -c unlimited gdb to launch gdb (my victim app. ) now be bad & do the following from another telnet/xterm session to the same machine ps -aux | grep gdb kill -SIGSEGV or alternatively use killall -SIGSEGV gdb if you have the killall command. Now look at the core dump. ./gdb ./gdb core Displays the following GNU gdb 4.18 Copyright 1998 Free Software Foundation, Inc. GDB is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. Type "show copying" to see the conditions. There is absolutely no warranty for GDB. Type "show warranty" for details. This GDB was configured as "s390-ibm-linux"... Core was generated by `./gdb'. Program terminated with signal 11, Segmentation fault. Reading symbols from /usr/lib/libncurses.so.4...done. Reading symbols from /lib/libm.so.6...done. Reading symbols from /lib/libc.so.6...done. Reading symbols from /lib/ld-linux.so.2...done. #0 0x40126d1a in read () from /lib/libc.so.6 Setting up the environment for debugging gdb. Breakpoint 1 at 0x4dc6f8: file utils.c, line 471. Breakpoint 2 at 0x4d87a4: file top.c, line 2609. (top-gdb) info stack #0 0x40126d1a in read () from /lib/libc.so.6 #1 0x528f26 in rl_getc (stream=0x7ffffde8) at input.c:402 #2 0x528ed0 in rl_read_key () at input.c:381 #3 0x5167e6 in readline_internal_char () at readline.c:454 #4 0x5168ee in readline_internal_charloop () at readline.c:507 #5 0x51692c in readline_internal () at readline.c:521 #6 0x5164fe in readline (prompt=0x7ffff810 "\177x\177\177x") at readline.c:349 #7 0x4d7a8a in command_line_input (prrompt=0x564420 "(gdb) ", repeat=1, annotation_suffix=0x4d6b44 "prompt") at top.c:2091 #8 0x4d6cf0 in command_loop () at top.c:1345 #9 0x4e25bc in main (argc=1, argv=0x7ffffdf4) at main.c:635 LDD === This is a program which lists the shared libraries which a library needs. e.g. ldd ./gdb outputs libncurses.so.4 => /usr/lib/libncurses.so.4 (0x40018000) libm.so.6 => /lib/libm.so.6 (0x4005e000) libc.so.6 => /lib/libc.so.6 (0x40084000) /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000) Debugging modules ================= As modules are dynamically loaded into the kernel their address can be anywhere to get around this use the -m option with insmod to emit a load map which can be piped into a file if required. The proc file system ==================== What is it ?. It is a filesystem created by the kernel with files which are created on demand by the kernel if read, or can be used to modify kernel parameters, it is a powerful concept. e.g. cat /proc/sys/net/ipv4/ip_forward On my machine outputs 0 telling me ip_forwarding is not on to switch it on I can do echo 1 > /proc/sys/net/ipv4/ip_forward cat it again cat /proc/sys/net/ipv4/ip_forward On my machine now outputs 1 IP forwarding is on. There is a lot of useful info in here best found by going in & having a look around, so I'll take you through some entries I consider important. All the processes running on the machine have there own entry defined by /proc/ So lets have a look at the init process cd /proc/1 cat cmdline emits init [2] cd /proc/1/fd This contains numerical entries of all the open files, some of these you can cat e.g. stdout (2) cat /proc/29/maps on my machine emits 00400000-00478000 r-xp 00000000 5f:00 4103 /bin/bash 00478000-0047e000 rw-p 00077000 5f:00 4103 /bin/bash 0047e000-00492000 rwxp 00000000 00:00 0 40000000-40015000 r-xp 00000000 5f:00 14382 /lib/ld-2.1.2.so 40015000-40016000 rw-p 00014000 5f:00 14382 /lib/ld-2.1.2.so 40016000-40017000 rwxp 00000000 00:00 0 40017000-40018000 rw-p 00000000 00:00 0 40018000-4001b000 r-xp 00000000 5f:00 14435 /lib/libtermcap.so.2.0.8 4001b000-4001c000 rw-p 00002000 5f:00 14435 /lib/libtermcap.so.2.0.8 4001c000-4010d000 r-xp 00000000 5f:00 14387 /lib/libc-2.1.2.so 4010d000-40111000 rw-p 000f0000 5f:00 14387 /lib/libc-2.1.2.so 40111000-40114000 rw-p 00000000 00:00 0 40114000-4011e000 r-xp 00000000 5f:00 14408 /lib/libnss_files-2.1.2.so 4011e000-4011f000 rw-p 00009000 5f:00 14408 /lib/libnss_files-2.1.2.so 7fffd000-80000000 rwxp ffffe000 00:00 0 Showing us the shared libraries init uses where they are in memory & memory access permissions for each virtual memory area. /proc/1/cwd is a softlink to the current working directory. /proc/1/root is the root of the filesystem for this process. /proc/1/mem is the current running processes memory which you can read & write to like a file. strace uses this sometimes as it is a bit faster than the rather inefficent ptrace interface for peeking at DATA. cat status Name: init State: S (sleeping) Pid: 1 PPid: 0 Uid: 0 0 0 0 Gid: 0 0 0 0 Groups: VmSize: 408 kB VmLck: 0 kB VmRSS: 208 kB VmData: 24 kB VmStk: 8 kB VmExe: 368 kB VmLib: 0 kB SigPnd: 0000000000000000 SigBlk: 0000000000000000 SigIgn: 7fffffffd7f0d8fc SigCgt: 00000000280b2603 CapInh: 00000000fffffeff CapPrm: 00000000ffffffff CapEff: 00000000fffffeff User PSW: 070de000 80414146 task: 004b6000 tss: 004b62d8 ksp: 004b7ca8 pt_regs: 004b7f68 User GPRS: 00000400 00000000 0000000b 7ffffa90 00000000 00000000 00000000 0045d9f4 0045cafc 7ffffa90 7fffff18 0045cb08 00010400 804039e8 80403af8 7ffff8b0 User ACRS: 00000000 00000000 00000000 00000000 00000001 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 Kernel BackChain CallChain BackChain CallChain 004b7ca8 8002bd0c 004b7d18 8002b92c 004b7db8 8005cd50 004b7e38 8005d12a 004b7f08 80019114 Showing among other things memory usage & status of some signals & the processes'es registers from the kernel task_structure as well as a backchain which may be useful if a process crashes in the kernel for some unknown reason. Starting points for debugging scripting languages etc. ====================================================== bash/sh bash -x e.g. bash -x /usr/bin/bashbug displays the following lines as it executes them. + MACHINE=i586 + OS=linux-gnu + CC=gcc + CFLAGS= -DPROGRAM='bash' -DHOSTTYPE='i586' -DOSTYPE='linux-gnu' -DMACHTYPE='i586-pc-linux-gnu' -DSHELL -DHAVE_CONFIG_H -I. -I. -I./lib -O2 -pipe + RELEASE=2.01 + PATCHLEVEL=1 + RELSTATUS=release + MACHTYPE=i586-pc-linux-gnu perl -d runs the perlscript in a fully intercative debugger . Type 'h' in the debugger for help. for debugging java type jdb another fully interactive gdb style debugger. & type ? in the debugger for help. References: ----------- Enterprise Systems Architecture Reference Summary Enterprise Systems Architecture Principles of Operation Hartmut Penners 390 stack frame sheet. IBM Mainframe Channel Attachment a technology brief from a CISCO webpage Various bits of man & info pages of Linux. Linux & GDB source. Various info & man pages. CMS Help on tracing commands.