Debugging on Linux for s/390 & z/Architecture by Denis Joseph Barrow (djbarrow@de.ibm.com,barrow_dj@yahoo.com) Copyright (C) 2000-2001 IBM Deutschland Entwicklung GmbH, IBM Corporation Best viewed with fixed width fonts Overview of Document: ===================== This document is intended to give a good overview of how to debug Linux for s/390 & z/Architecture. It isn't intended as a complete reference & not a tutorial on the fundamentals of C & assembly. It doesn't go into 390 IO in any detail. It is intended to complement the documents in the reference section below & 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 ======== Register Set Address Spaces on Intel Linux Address Spaces on Linux for s/390 & z/Architecture The Linux for s/390 & z/Architecture Kernel Task Structure Register Usage & Stackframes on Linux for s/390 & z/Architecture A sample program with comments Compiling programs for debugging on Linux for s/390 & z/Architecture Figuring out gcc compile errors Debugging Tools objdump strace Performance Debugging Debugging under VM s/390 & z/Architecture IO Overview Debugging IO on s/390 & z/Architecture under VM GDB on s/390 & z/Architecture Stack chaining in gdb by hand Examining core dumps ldd Debugging modules The proc file system Starting points for debugging scripting languages etc. SysRq References Special Thanks Register Set ============ The current architectures have the following registers. 16 General propose registers, 32 bit on s/390 64 bit on z/Architecture, r0-r15 or gpr0-gpr15 used for arithmetic & addressing. 16 Control registers, 32 bit on s/390 64 bit on z/Architecture, ( cr0-cr15 kernel usage only ) used for memory management, interrupt control,debugging control etc. 16 Access registers ( ar0-ar15 ) 32 bit on s/390 & z/Architecture 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 kernel & user address spaces. Access register 0 ( & access register 1 on z/Architecture ( needs 64 bit pointer ) ) is currently used by the pthread library as a pointer to the current running threads private area. 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 on s/390 & 128 bit on z/Architecture & 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 s/390 z/Architecture 0 0 Reserved ( must be 0 ) otherwise specification exception occurs. 1 1 Program Event Recording 1 PER enabled, PER is used to facilitate debugging e.g. single stepping. 2-4 2-4 Reserved ( must be 0 ). 5 5 Dynamic address translation 1=DAT on. 6 6 Input/Output interrupt Mask 7 7 External interrupt Mask used primarily for interprocessor signalling & clock interrupts. 8-11 8-11 PSW Key used for complex memory protection mechanism not used under linux 12 12 1 on s/390 0 on z/Architecture 13 13 Machine Check Mask 1=enable machine check interrupts 14 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 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 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 18-19 Condition codes (CC) 20 20 Fixed point overflow mask if 1=FPU exceptions for this event occur ( normally 0 ) 21 21 Decimal overflow mask if 1=FPU exceptions for this event occur ( normally 0 ) 22 22 Exponent underflow mask if 1=FPU exceptions for this event occur ( normally 0 ) 23 23 Significance Mask if 1=FPU exceptions for this event occur ( normally 0 ) 24-31 24-30 Reserved Must be 0. 31 Extended Addressing Mode 32 Basic Addressing Mode Used to set addressing mode PSW 31 PSW 32 0 0 24 bit 0 1 31 bit 1 1 64 bit 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. 33-63 Reserved must be 0 64-127 Address In 24 bits mode bits 64-103=0 bits 104-127 Address In 31 bits mode bits 64-96=0 bits 97-127 Address Note: unlike 31 bit mode on s/390 bit 96 must be zero when loading the address with LPSWE otherwise a specification exception occurs, LPSW is fully backward compatible. Prefix Page(s) -------------- This per cpu memory area is too intimately tied to the processor not to mention. It exists between the real addresses 0-4096 on s/390 & 0-8192 z/Architecture & is exchanged with a 1 page on s/390 or 2 pages on z/Architecture in absolute storage by the set prefix instruction in linux'es startup. This page is mapped to a different prefix for each processor in an SMP configuration ( assuming the os designer is sane of course :-) ). Bytes 0-512 ( 200 hex ) on s/390 & 0-512,4096-4544,4604-5119 currently on z/Architecture 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 on s/390 & z/Architecture ( there is a gap on z/Architecture too currently between 0xc00 & 1000 which linux uses ). 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 Intel 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. 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 on s/390, & more with the Extended memory management swap device & currently 4TB of physical memory currently on z/Architecture. Address Spaces on Linux for s/390 & z/Architecture ================================================== Our addressing scheme is as follows Himem 0x7fffffff 2GB on s/390 ***************** **************** currently 0x3ffffffffff (2^42)-1 * User Stack * * * on z/Architecture. ***************** * * * 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. Virtual Addresses on s/390 & z/Architecture =========================================== A virtual address on s/390 is made up of 3 parts The SX ( segment index, roughly corresponding to the PGD & PMD in linux terminology ) being bits 1-11. The PX ( page index, corresponding to the page table entry (pte) in linux terminology ) being bits 12-19. The remaining bits BX (the byte index are the offset in the page ) i.e. bits 20 to 31. On z/Architecture in linux we currently make up an address from 4 parts. The region index bits (RX) 0-32 we currently use bits 22-32 The segment index (SX) being bits 33-43 The page index (PX) being bits 44-51 The byte index (BX) being bits 52-63 Notes: 1) s/390 has no PMD so the PMD is really the PGD also. A lot of this stuff is defined in pgtable.h. 2) Also seeing as s/390's page indexes are only 1k in size (bits 12-19 x 4 bytes per pte ) we use 1 ( page 4k ) to make the best use of memory by updating 4 segment indices entries each time we mess with a PMD & use offsets 0,1024,2048 & 3072 in this page as for our segment indexes. On z/Architecture our page indexes are now 2k in size ( bits 12-19 x 8 bytes per pte ) we do a similar trick but only mess with 2 segment indices each time we mess with a PMD. 3) As z/Architecture supports up to a massive 5-level page table lookup we can only use 3 currently on Linux ( as this is all the generic kernel currently supports ) however this may change in future this allows us to access ( according to my sums ) 4TB of virtual storage per process i.e. 4096*512(PTES)*1024(PMDS)*2048(PGD) = 4398046511104 bytes, enough for another 2 or 3 of years I think :-). to do this we use a region-third-table designation type in our address space control registers. The Linux for s/390 & z/Architecture 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 structure for each processor as follows. s/390 ************************ * 1 page kernel stack * * ( 4K ) * ************************ * 1 page task_struct * * ( 4K ) * 8K aligned ************************ z/Architecture ************************ * 2 page kernel stack * * ( 8K ) * ************************ * 2 page task_struct * * ( 8K ) * 16K 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 for s/390 & one very similar for z/Architecture. 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 doesn'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 s/390 & z/Architecture ================================================================= 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 probably 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. It should be noted that there are some differences between the s/390 & z/Architecture stack layouts as the z/Architecture stack layout didn't have to maintain compatibility with older linkage formats. 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 efficient 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 dereferencing 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 s390 & z/Architecture is one which doesn't need more than the register save area ( 96 bytes on s/390, 160 on z/Architecture ) 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); } s/390 & z/Architecture 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 4 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 z/Architecture argument 2 saved f6 z/Architecture argument 3 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 capable 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. 7) Floating point arguments 2 & 3 are saved in the outgoing args area for z/Architecture Stack Frame Layout ------------------ s/390 z/Architecture 0 0 back chain ( a 0 here signifies end of back chain ) 4 8 eos ( end of stack, not used on Linux for S390 used in other linkage formats ) 8 16 glue used in other s/390 linkage formats for saved routine descriptors etc. 12 24 glue used in other s/390 linkage formats for saved routine descriptors etc. 16 32 scratch area 20 40 scratch area 24 48 saved r6 of caller function 28 56 saved r7 of caller function 32 64 saved r8 of caller function 36 72 saved r9 of caller function 40 80 saved r10 of caller function 44 88 saved r11 of caller function 48 96 saved r12 of caller function 52 104 saved r13 of caller function 56 112 saved r14 of caller function 60 120 saved r15 of caller function 64 128 saved f4 of caller function 72 132 saved f6 of caller function 80 undefined 96 160 outgoing args passed from caller to callee 96+x 160+x possible stack alignment ( 8 bytes desirable ) 96+x+y 160+x+y alloca space of caller ( if used ) 96+x+y+z 160+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 } Compiler updates ---------------- main(int argc,char *argv[]) { 4004fc: 90 7f f0 1c stm %r7,%r15,28(%r15) 400500: a7 d5 00 04 bras %r13,400508 400504: 00 40 04 f4 .long 0x004004f4 # compiler now puts constant pool in code to so it saves an instruction 400508: 18 0f lr %r0,%r15 40050a: a7 fa ff a0 ahi %r15,-96 40050e: 50 00 f0 00 st %r0,0(%r15) return(test(5)); 400512: 58 10 d0 00 l %r1,0(%r13) 400516: a7 28 00 05 lhi %r2,5 40051a: 0d e1 basr %r14,%r1 # compiler adds 1 extra instruction to epilogue this is done to # avoid processor pipeline stalls owing to data dependencies on g5 & # above as register 14 in the old code was needed directly after being loaded # by the lm %r11,%r15,140(%r15) for the br %14. 40051c: 58 40 f0 98 l %r4,152(%r15) 400520: 98 7f f0 7c lm %r7,%r15,124(%r15) 400524: 07 f4 br %r4 } Hartmut ( our compiler developer ) also has been threatening to take out the stack backchain in optimised code as this also causes pipeline stalls, you have been warned. 64 bit z/Architecture code disassembly -------------------------------------- If you understand the stuff above you'll understand the stuff below too so I'll avoid repeating myself & just say that some of the instructions have g's on the end of them to indicate they are 64 bit & the stack offsets are a bigger, the only other difference you'll find between 32 & 64 bit is that we now use f4 & f6 for floating point arguments on 64 bit. 00000000800005b0 : int test(int b) { return(5+b); 800005b0: a7 2a 00 05 ahi %r2,5 800005b4: b9 14 00 22 lgfr %r2,%r2 # downcast to integer 800005b8: 07 fe br %r14 800005ba: 07 07 bcr 0,%r7 } 00000000800005bc
: main(int argc,char *argv[]) { 800005bc: eb bf f0 58 00 24 stmg %r11,%r15,88(%r15) 800005c2: b9 04 00 1f lgr %r1,%r15 800005c6: a7 fb ff 60 aghi %r15,-160 800005ca: e3 10 f0 00 00 24 stg %r1,0(%r15) return(test(5)); 800005d0: a7 29 00 05 lghi %r2,5 # brasl allows jumps > 64k & is overkill here bras would do fune 800005d4: c0 e5 ff ff ff ee brasl %r14,800005b0 800005da: e3 40 f1 10 00 04 lg %r4,272(%r15) 800005e0: eb bf f0 f8 00 04 lmg %r11,%r15,248(%r15) 800005e6: 07 f4 br %r4 } Compiling programs for debugging on Linux for s/390 & z/Architecture ==================================================================== -gdwarf-2 now works it should be considered the default debugging format for s/390 & z/Architecture as it is more reliable for debugging shared libraries, normal -g debugging works much better now Thanks to the IBM java compiler developers bug reports. This is typically done adding/appending the flags -g or -gdwarf-2 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. Debugging with optimisation has since much improved after fixing some bugs, please make sure you are using gdb-5.0 or later developed after Nov'2000. 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 definitions 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) Adding -g to the above output makes the output even more useful e.g. typing make CC:="s390-gcc -g" kernel/sched.s which compiles. s390-gcc -g -D__KERNEL__ -I/home/barrow/linux-2.3/include -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer -fno-strict-aliasing -pipe -fno-strength-reduce -S kernel/sched.c -o kernel/sched.s also outputs stabs ( debugger ) info, from this info you can find out the offsets & sizes of various elements in structures. e.g. the stab for the structure struct rlimit { unsigned long rlim_cur; unsigned long rlim_max; }; is .stabs "rlimit:T(151,2)=s8rlim_cur:(0,5),0,32;rlim_max:(0,5),32,32;;",128,0,0,0 from this stab you can see that rlimit_cur starts at bit offset 0 & is 32 bits in size rlimit_max starts at bit offset 32 & is 32 bits in size. 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). This is very slow as it spends lots of time searching for debugging info. The following self explanatory line should be used instead if the code isn't compiled -g, as it is much faster: objdump --disassemble-all --syms vmlinux > vmlinux.lst As hard drive space is valuable 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 alternatively 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. static 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. I now have a tool which takes the pain out of --adjust-vma & you are able to do something like make /arch/s390/kernel/traps.lst & it automatically generates the correctly relocated entries for the text segment in traps.lst. This tool is now standard in linux distro's in scripts/makelst 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 use 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 spaghetti source which conditionally compiles for several architectures ). Not everything that it throws out needs to make sense immediately. 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 sophisticated telnetd crashes & 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. Performance Debugging ===================== gcc is capable of compiling in profiling code just add the -p option to the CFLAGS, this obviously affects program size & performance. This can be used by the gprof gnu profiling tool or the gcov the gnu code coverage tool ( code coverage is a means of testing code quality by checking if all the code in an executable in exercised by a tester ). Using top to find out where processes are sleeping in the kernel ---------------------------------------------------------------- To do this copy the System.map from the root directory where the linux kernel was built to the /boot directory on your linux machine. Start top Now type fU You should see a new field called WCHAN which tells you where each process is sleeping here is a typical output. 6:59pm up 41 min, 1 user, load average: 0.00, 0.00, 0.00 28 processes: 27 sleeping, 1 running, 0 zombie, 0 stopped CPU states: 0.0% user, 0.1% system, 0.0% nice, 99.8% idle Mem: 254900K av, 45976K used, 208924K free, 0K shrd, 28636K buff Swap: 0K av, 0K used, 0K free 8620K cached PID USER PRI NI SIZE RSS SHARE WCHAN STAT LIB %CPU %MEM TIME COMMAND 750 root 12 0 848 848 700 do_select S 0 0.1 0.3 0:00 in.telnetd 767 root 16 0 1140 1140 964 R 0 0.1 0.4 0:00 top 1 root 8 0 212 212 180 do_select S 0 0.0 0.0 0:00 init 2 root 9 0 0 0 0 down_inte SW 0 0.0 0.0 0:00 kmcheck The time command ---------------- Another related command is the time command which gives you an indication of where a process is spending the majority of its time. e.g. time ping -c 5 nc outputs real 0m4.054s user 0m0.010s sys 0m0.010s 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 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 management 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 s/390 Reference Summary & look at between pages 2 & 7 or alternatively the s/390 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. TR I R
RUN cmd d g single steps a range of addresses but stays running & displays the gprs on each step. Displaying & modifying Registers -------------------------------- D G will display all the gprs Adding a extra G to all the commands is necessary to access the full 64 bit content in VM on z/Architecture obviously this isn't required for access registers as these are still 32 bit. e.g. DGG instead of DG 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 displays the prefix offset 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. On linux'es 3270 emulator x3270 there is a very useful option under the file ment Save Screens In File this is very good of keeping a copy of traces. From CMS help will give you online help on a particular command. e.g. HELP DISPLAY Also CP has a file called profile.exec which automatically gets called on startup of CMS ( like autoexec.bat ), keeping on a DOS analogy session CP has a feature similar to doskey, it may be useful for you to use profile.exec to define some keystrokes. e.g. SET PF9 IMM B This does a single step in VM on pressing F8. SET PF10 ^ This sets up the ^ key. which can be used for ^c (ctrl-c),^z (ctrl-z) which can't be typed directly into some 3270 consoles. SET PF11 ^- This types the starting keystrokes for a sysrq see SysRq below. SET PF12 RETRIEVE This retrieves command history on pressing F12. Sometimes in VM the display is set up to scroll automatically this can be very annoying if there are messages you wish to look at to stop this do TERM MORE 255 255 This will nearly stop automatic screen updates, however it will cause a denial of service if lots of messages go to the 3270 console, so it would be foolish to use this as the default on a production machine. Tracing particular processes ---------------------------- The kernel's text segment is intentionally at an address in memory that it will very seldom collide with text segments of user programs ( thanks Martin ), this simplifies debugging the kernel. However it is quite common for user processes to have addresses which collide this can make debugging a particular process under VM painful under normal circumstances as the process may change when doing a TR I R
. Thankfully after reading VM's online help I figured out how to debug I particular process. Your first problem is to find the STD ( segment table designation ) of the program you wish to debug. There are several ways you can do this here are a few 1) objdump --syms | grep main To get the address of main in the program. tr i pswa
Start the program, if VM drops to CP on what looks like the entry point of the main function this is most likely the process you wish to debug. Now do a D X13 or D XG13 on z/Architecture. On 31 bit the STD is bits 1-19 ( the STO segment table origin ) & 25-31 ( the STL segment table length ) of CR13. now type TR I R STD 0.7fffffff e.g. TR I R STD 8F32E1FF 0.7fffffff Another very useful variation is TR STORE INTO STD
for finding out when a particular variable changes. An alternative way of finding the STD of a currently running process is to do the following, ( this method is more complex but could be quite convenient if you aren't updating the kernel much & so your kernel structures will stay constant for a reasonable period of time ). grep task /proc//status from this you should see something like task: 0f160000 ksp: 0f161de8 pt_regs: 0f161f68 This now gives you a pointer to the task structure. Now make CC:="s390-gcc -g" kernel/sched.s To get the task_struct stabinfo. ( task_struct is defined in include/linux/sched.h ). Now we want to look at task->active_mm->pgd on my machine the active_mm in the task structure stab is active_mm:(4,12),672,32 its offset is 672/8=84=0x54 the pgd member in the mm_struct stab is pgd:(4,6)=*(29,5),96,32 so its offset is 96/8=12=0xc so we'll hexdump -s 0xf160054 /dev/mem | more i.e. task_struct+active_mm offset to look at the active_mm member f160054 0fee cc60 0019 e334 0000 0000 0000 0011 hexdump -s 0x0feecc6c /dev/mem | more i.e. active_mm+pgd offset feecc6c 0f2c 0000 0000 0001 0000 0001 0000 0010 we get something like now do TR I R STD 0.7fffffff i.e. the 0x7f is added because the pgd only gives the page table origin & we need to set the low bits to the maximum possible segment table length. TR I R STD 0f2c007f 0.7fffffff on z/Architecture you'll probably need to do TR I R STD 0.ffffffffffffffff to set the TableType to 0x1 & the Table length to 3. 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 s/390 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 inst int run or whatever the IO channels you wish to trace are & hit b 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/arch/s390/include/asm/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 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 isn't 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 issuing a DETACH CPU 01-(number of cpus in configuration) & boot linux again. TR SIGP will trace inter processor signal processor instructions. DEFINE CPU 01-(number in configuration) will get your guests cpus back. Help for displaying ascii textstrings ------------------------------------- On the very latest VM Nucleus'es VM can now display ascii ( thanks Neale for the hint ) by doing D TX. e.g. D TX0.100 Alternatively ============= Under older VM debuggers ( I love EBDIC too ) you can use this little program I wrote 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 s/390 390 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 don't 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. s/390 & z/Architecture 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 s/390 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 S39