反汇编ipanic中的 16进制数code

Kernel中出现的panic 都会带Code段, 如红色字体所示.

The following is an example of a protection fault in a loadable module
processed by klogd:
---------------------------------------------------------------------------
Aug 29 09:51:01 blizard kernel: Unable to handle kernel paging request at virtual address f15e97cc
Aug 29 09:51:01 blizard kernel: current->tss.cr3 = 0062d000, %cr3 = 0062d000
Aug 29 09:51:01 blizard kernel: *pde = 00000000
Aug 29 09:51:01 blizard kernel: Oops: 0002
Aug 29 09:51:01 blizard kernel: CPU:    0
Aug 29 09:51:01 blizard kernel: EIP:    0010:[oops:_oops+16/3868]
Aug 29 09:51:01 blizard kernel: EFLAGS: 00010212
Aug 29 09:51:01 blizard kernel: eax: 315e97cc   ebx: 003a6f80   ecx: 001be77b   edx: 00237c0c
Aug 29 09:51:01 blizard kernel: esi: 00000000   edi: bffffdb3   ebp: 00589f90   esp: 00589f8c
Aug 29 09:51:01 blizard kernel: ds: 0018   es: 0018   fs: 002b   gs: 002b   ss: 0018
Aug 29 09:51:01 blizard kernel: Process oops_test (pid: 3374, process nr: 21, stackpage=00589000)
Aug 29 09:51:01 blizard kernel: Stack: 315e97cc 00589f98 0100b0b4 bffffed4 0012e38e 00240c64 003a6f80 00000001
Aug 29 09:51:01 blizard kernel:        00000000 00237810 bfffff00 0010a7fa 00000003 00000001 00000000 bfffff00
Aug 29 09:51:01 blizard kernel:        bffffdb3 bffffed4 ffffffda 0000002b 0007002b 0000002b 0000002b 00000036
Aug 29 09:51:01 blizard kernel: Call Trace: [oops:_oops_ioctl+48/80] [_sys_ioctl+254/272] [_system_call+82/128]
Aug 29 09:51:01 blizard kernel: Code: c7 00 05 00 00 00 eb 08 90 90 90 90 90 90 90 90 89 ec 5d c3
---------------------------------------------------------------------------

 

如果要把这段二进制指令反汇编为汇编语言可以通过下面方法:

1.  把 这段数据保存到文本文件, 比如下面例子中的/home/decode.txt文件中

2.  使用linux代码中的decodecode脚本(路径: linux/kernel/scripts) 来对这段指令反汇编, 如下面内容所示. 其中code段有<8b>比较特殊, <>框住的指令代表ipanic是在这条指令上出现的问题.

fred@fred:~/linux/kernel/scripts$ ./decodecode </home/decode.txt
Code: f9 0f 8d f9 00 00 00 8d 42 0c e8 dd 26 11 c7 a1 60 ea 2b f9 8b 50 08 a1 64 ea 2b f9 8d 34 82 8b 1e 85 db 74 6d 8b 15 60 ea 2b f9 <8b> 43 04 39 42 54 7e 04 40 89 42 54 8b 43 04 3b 05 00 f6 52 c0
All code
========
   0:   f9                      stc
   1:   0f 8d f9 00 00 00       jge    0x100
   7:   8d 42 0c                lea    0xc(%rdx),%eax
   a:   e8 dd 26 11 c7          callq  0xffffffffc71126ec
   f:   a1 60 ea 2b f9 8b 50    movabs 0xa108508bf92bea60,%eax
  16:   08 a1
  18:   64                      fs
  19:   ea                      (bad)
  1a:   2b f9                   sub    %ecx,%edi
  1c:   8d 34 82                lea    (%rdx,%rax,4),%esi
  1f:   8b 1e                   mov    (%rsi),%ebx
  21:   85 db                   test   %ebx,%ebx
  23:   74 6d                   je     0x92
  25:   8b 15 60 ea 2b f9       mov    -0x6d415a0(%rip),%edx        # 0xfffffffff92bea8b
  2b:*  8b 43 04                mov    0x4(%rbx),%eax           <-- trapping instruction
  2e:   39 42 54                cmp    %eax,0x54(%rdx)
  31:   7e 04                   jle    0x37
  33:   40 89 42 54             rex mov %eax,0x54(%rdx)
  37:   8b 43 04                mov    0x4(%rbx),%eax
  3a:   3b 05 00 f6 52 c0       cmp    -0x3fad0a00(%rip),%eax   # 0xffffffffc052f640

Code starting with the faulting instruction
===========================================
   0:   8b 43 04                mov    0x4(%rbx),%eax
   3:   39 42 54                cmp    %eax,0x54(%rdx)
   6:   7e 04                   jle    0xc
   8:   40 89 42 54             rex mov %eax,0x54(%rdx)
   c:   8b 43 04                mov    0x4(%rbx),%eax
   f:   3b 05 00 f6 52 c0       cmp    -0x3fad0a00(%rip),%eax   # 0xffffffffc052f615

 

翻译来于: linux/kernel/Documentation/oops-tracing.txt

---------------------------------

How to track down an Oops.. [originally a mail to linux-kernel]

The main trick is having 5 years of experience with those pesky oops
messages ;-)

Actually, there are things you can do that make this easier. I have two
separate approaches:

        gdb /usr/src/linux/vmlinux
        gdb> disassemble <offending_function>

That's the easy way to find the problem, at least if the bug-report is
well made (like this one was - run through ksymoops to get the
information of which function and the offset in the function that it
happened in).

Oh, it helps if the report happens on a kernel that is compiled with the
same compiler and similar setups.

The other thing to do is disassemble the "Code:" part of the bug report:
ksymoops will do this too with the correct tools, but if you don't have
the tools you can just do a silly program:

        char str[] = "\xXX\xXX\xXX...";
        main(){}

and compile it with gcc -g and then do "disassemble str" (where the "XX"
stuff are the values reported by the Oops - you can just cut-and-paste
and do a replace of spaces to "\x" - that's what I do, as I'm too lazy
to write a program to automate this all).

Alternatively, you can use the shell script in scripts/decodecode.
Its usage is:  decodecode < oops.txt

The hex bytes that follow "Code:" may (in some architectures) have a series
of bytes that precede the current instruction pointer as well as bytes at and
following the current instruction pointer.  In some cases, one instruction
byte or word is surrounded by <> or (), as in "<86>" or "(f00d)".  These
<> or () markings indicate the current instruction pointer.  Example from
i386, split into multiple lines for readability:

Code: f9 0f 8d f9 00 00 00 8d 42 0c e8 dd 26 11 c7 a1 60 ea 2b f9 8b 50 08 a1
64 ea 2b f9 8d 34 82 8b 1e 85 db 74 6d 8b 15 60 ea 2b f9 <8b> 43 04 39 42 54
7e 04 40 89 42 54 8b 43 04 3b 05 00 f6 52 c0

Finally, if you want to see where the code comes from, you can do

        cd /usr/src/linux
        make fs/buffer.s        # or whatever file the bug happened in

and then you get a better idea of what happens than with the gdb
disassembly.

Now, the trick is just then to combine all the data you have: the C
sources (and general knowledge of what it _should_ do), the assembly
listing and the code disassembly (and additionally the register dump you
also get from the "oops" message - that can be useful to see _what_ the
corrupted pointers were, and when you have the assembler listing you can
also match the other registers to whatever C expressions they were used
for).

Essentially, you just look at what doesn't match (in this case it was the
"Code" disassembly that didn't match with what the compiler generated).
Then you need to find out _why_ they don't match. Often it's simple - you
see that the code uses a NULL pointer and then you look at the code and
wonder how the NULL pointer got there, and if it's a valid thing to do
you just check against it..

Now, if somebody gets the idea that this is time-consuming and requires
some small amount of concentration, you're right. Which is why I will
mostly just ignore any panic reports that don't have the symbol table
info etc looked up: it simply gets too hard to look it up (I have some
programs to search for specific patterns in the kernel code segment, and
sometimes I have been able to look up those kinds of panics too, but
that really requires pretty good knowledge of the kernel just to be able
to pick out the right sequences etc..)

_Sometimes_ it happens that I just see the disassembled code sequence
from the panic, and I know immediately where it's coming from. That's when
I get worried that I've been doing this for too long ;-)

                Linus