Hi all,

With the attached patch, I'd like to add a new format letter "j" to
the "x" command to dump instructions in backward direction.
This feature is basically equal to "ub" command in Windows debugger.

Below is the sample output:

(gdb) disas main
Dump of assembler code for function main(int, char**):
   0x4040a7 <+0>: push   %rbp
   0x4040a8 <+1>: mov    %rsp,%rbp
   0x4040ab <+4>: sub    $0x10,%rsp
   0x4040af <+8>: mov    %edi,-0x4(%rbp)
   0x4040b2 <+11>:  mov    %rsi,-0x10(%rbp)
=> 0x4040b6 <+15>:  lea    0x116(%rip),%rdi        # 0x4041d3
   0x4040bd <+22>:  callq  0x400e30 <puts@plt>
   0x4040c2 <+27>:  mov    $0x0,%eax
   0x4040c7 <+32>:  leaveq 
   0x4040c8 <+33>:  retq   
End of assembler dump.
(gdb) x/6j $rip
   0x4040a6 <previous_function(...)+119>:  retq   
   0x4040a7 <main(int, char**)>:  push   %rbp
   0x4040a8 <main(int, char**)+1>:  mov    %rsp,%rbp
   0x4040ab <main(int, char**)+4>:  sub    $0x10,%rsp
   0x4040af <main(int, char**)+8>:  mov    %edi,-0x4(%rbp)
   0x4040b2 <main(int, char**)+11>: mov    %rsi,-0x10(%rbp)
(gdb) x/2j
   0x4040a2 <previous_function(...)+115>:  add    %rdx,%rax
   0x4040a5 <previous_function(...)+118>:  leaveq 

Unlike forward disassemble, theoretically it's impossible to
solve a correct frame only using the code. In the example below, we cannot
decide whether the instruction before "pop" is "nop" or "mov".

<correct frame>
   0x4045bb <+89>:  eb b4  jmp    0x404571
   0x4045bd <+91>:  90     nop
   0x4045be <+92>:  5d     pop    %rbp
   0x4045bf <+93>:  c3     retq   

<incorrect frame>
   0x4045bc <+90>:  b4 90  mov    $0x90,%ah
   0x4045be <+92>:  5d     pop    %rbp
   0x4045bf <+93>:  c3     retq 

To solve this, the change takes the help of line number information.

I introduced a new function "disassemble_backward" which goes backward
by referring the start address of each line.If we go out of the symbol range, we cancel backtrack and dump
the smallest address with the message.

Tested on x86_64 GNU/Linux.


Thanks,
Toshihito