* issue with conditional breakpoint
@ 2014-12-15 23:30 Javier Aguinaga
2014-12-30 17:15 ` Doug Evans
0 siblings, 1 reply; 2+ messages in thread
From: Javier Aguinaga @ 2014-12-15 23:30 UTC (permalink / raw)
To: gdb
Hi guys, I'm new in gdb. The issue is the following
I'm trying to debug qemu, specificly I want to stop in the open
syscall when the argument is "/proc/meminfo", I tried the following
without successfull:
$ gdb qemu
gdb$ catch syscall open
Catchpoint 1 (syscall 'open' [2])
gdb$ cond 1 strcmp("/proc/meminfo", (char*)($rdi)) == 0
gdb$ r
-----------------------------------------------------------------------------------------------------------------------[regs]
RAX: 0x00007FFFF7FFA4A0 RBX: 0x0000555555562AA1 RBP:
0x00007FFFFFFFD8C8 RSP: 0x00007FFFFFFFD8D0 o d I t s z a P c
RDI: 0xFFFFFFFFFFFFFFF8 RSI: 0x000000000000000E RDX:
0xFFFF800008005B60 RCX: 0x00007FFFF7FFA4AE RIP: 0x00005555555FE644
R8 : 0x00007FFFF7FFE570 R9 : 0x0000000000000000 R10:
0xFFFFFFFFFFFFFFB0 R11: 0x0000000000000246 R12: 0x00007FFFF7FFE090
R13: 0x00007FFFF7FFD040 R14: 0x0000000000000000 R15: 0x00007FFFF7FFE570
CS: 0033 DS: 0000 ES: 0000 FS: 0000 GS: 0000 SS: 002B
-----------------------------------------------------------------------------------------------------------------------[code]
=> 0x5555555fe644: xor ebp,ebp
0x5555555fe646: mov r9,rdx
0x5555555fe649: pop rsi
0x5555555fe64a: mov rdx,rsp
0x5555555fe64d: and rsp,0xfffffffffffffff0
0x5555555fe651: push rax
0x5555555fe652: push rsp
0x5555555fe653: mov r8,QWORD PTR [rip+0x6c630e] # 0x555555cc4968
-----------------------------------------------------------------------------------------------------------------------------
Program received signal SIGSEGV, Segmentation fault.
-----------------------------------------------------------------------------------------------------------------------[regs]
RAX: 0x0000000000000000 RBX: 0x0000555555562AA1 RBP:
0x00007FFFFFFFD8C8 RSP: 0x00007FFFFFFFD8C8 o d I t s Z a P c
RDI: 0x00007FFFF7FFA4A0 RSI: 0x00007FFFF7DF72D0 RDX:
0x0000000000000001 RCX: 0xFFFFFFFFFFFFFFFF RIP: 0x00000000000A07F0
R8 : 0x00007FFFF7FFE570 R9 : 0x0000000000000000 R10:
0xFFFFFFFFFFFFFFB0 R11: 0x0000000000000246 R12: 0x00007FFFF7FFE090
R13: 0x00007FFFF7FFD040 R14: 0x0000000000000000 R15: 0x00007FFFF7FFE570
CS: 0033 DS: 0000 ES: 0000 FS: 0000 GS: 0000 SS: 002B Error
while running hook_stop:
Cannot access memory at address 0xa07f0
0x00000000000a07f0 in ?? ()
Error in testing breakpoint condition:
The program being debugged was signaled while in a function called from GDB.
GDB remains in the frame where the signal was received.
To change this behavior use "set unwindonsignal on".
Evaluation of the expression containing the function
(at 0x0xa07f0) will be abandoned.
When the function is done executing, GDB will silently stop.
-----------------------------------------------------------------------------------------------------------------------[regs]
RAX: 0x0000000000000000 RBX: 0x0000555555562AA1 RBP:
0x00007FFFFFFFD8C8 RSP: 0x00007FFFFFFFD8C8 o d I t s Z a P c
RDI: 0x00007FFFF7FFA4A0 RSI: 0x00007FFFF7DF72D0 RDX:
0x0000000000000001 RCX: 0xFFFFFFFFFFFFFFFF RIP: 0x00000000000A07F0
R8 : 0x00007FFFF7FFE570 R9 : 0x0000000000000000 R10:
0xFFFFFFFFFFFFFFB0 R11: 0x0000000000000246 R12: 0x00007FFFF7FFE090
R13: 0x00007FFFF7FFD040 R14: 0x0000000000000000 R15: 0x00007FFFF7FFE570
CS: 0033 DS: 0000 ES: 0000 FS: 0000 GS: 0000 SS: 002B Error
while running hook_stop:
Cannot access memory at address 0xa07f0
Catchpoint 1 (returned from syscall munmap), 0x00000000000a07f0 in ?? ()
gdb$
GDB stop in the condition (x/1s $rdi ==> "/proc/meminfo") but after
that I can't continue
My machine is a Debian x86_64, and I probe with v7.4 and v7.8
Anyone knows what is the problem?
P.S.: with "call strcmp("foo","foo")" I have:
(gdb) call strcmp("foo","foo")
Program received signal SIGSEGV, Segmentation fault.
0x00000000000a07f6 in ?? ()
The program being debugged was signaled while in a function called from GDB.
GDB remains in the frame where the signal was received.
To change this behavior use "set unwindonsignal on".
Evaluation of the expression containing the function
(strcmp@plt) will be abandoned.
When the function is done executing, GDB will silently stop.
(gdb)
Regards
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: issue with conditional breakpoint
2014-12-15 23:30 issue with conditional breakpoint Javier Aguinaga
@ 2014-12-30 17:15 ` Doug Evans
0 siblings, 0 replies; 2+ messages in thread
From: Doug Evans @ 2014-12-30 17:15 UTC (permalink / raw)
To: Javier Aguinaga; +Cc: gdb
On Mon, Dec 15, 2014 at 3:30 PM, Javier Aguinaga
<javier.4guinaga@gmail.com> wrote:
> Hi guys, I'm new in gdb. The issue is the following
>
> I'm trying to debug qemu, specificly I want to stop in the open
> syscall when the argument is "/proc/meminfo", I tried the following
> without successfull:
>
> $ gdb qemu
> gdb$ catch syscall open
> Catchpoint 1 (syscall 'open' [2])
> gdb$ cond 1 strcmp("/proc/meminfo", (char*)($rdi)) == 0
> gdb$ r
> -----------------------------------------------------------------------------------------------------------------------[regs]
> RAX: 0x00007FFFF7FFA4A0 RBX: 0x0000555555562AA1 RBP:
> 0x00007FFFFFFFD8C8 RSP: 0x00007FFFFFFFD8D0 o d I t s z a P c
> RDI: 0xFFFFFFFFFFFFFFF8 RSI: 0x000000000000000E RDX:
> 0xFFFF800008005B60 RCX: 0x00007FFFF7FFA4AE RIP: 0x00005555555FE644
> R8 : 0x00007FFFF7FFE570 R9 : 0x0000000000000000 R10:
> 0xFFFFFFFFFFFFFFB0 R11: 0x0000000000000246 R12: 0x00007FFFF7FFE090
> R13: 0x00007FFFF7FFD040 R14: 0x0000000000000000 R15: 0x00007FFFF7FFE570
> CS: 0033 DS: 0000 ES: 0000 FS: 0000 GS: 0000 SS: 002B
> -----------------------------------------------------------------------------------------------------------------------[code]
> => 0x5555555fe644: xor ebp,ebp
> 0x5555555fe646: mov r9,rdx
> 0x5555555fe649: pop rsi
> 0x5555555fe64a: mov rdx,rsp
> 0x5555555fe64d: and rsp,0xfffffffffffffff0
> 0x5555555fe651: push rax
> 0x5555555fe652: push rsp
> 0x5555555fe653: mov r8,QWORD PTR [rip+0x6c630e] # 0x555555cc4968
> -----------------------------------------------------------------------------------------------------------------------------
>
> Program received signal SIGSEGV, Segmentation fault.
> -----------------------------------------------------------------------------------------------------------------------[regs]
> RAX: 0x0000000000000000 RBX: 0x0000555555562AA1 RBP:
> 0x00007FFFFFFFD8C8 RSP: 0x00007FFFFFFFD8C8 o d I t s Z a P c
> RDI: 0x00007FFFF7FFA4A0 RSI: 0x00007FFFF7DF72D0 RDX:
> 0x0000000000000001 RCX: 0xFFFFFFFFFFFFFFFF RIP: 0x00000000000A07F0
> R8 : 0x00007FFFF7FFE570 R9 : 0x0000000000000000 R10:
> 0xFFFFFFFFFFFFFFB0 R11: 0x0000000000000246 R12: 0x00007FFFF7FFE090
> R13: 0x00007FFFF7FFD040 R14: 0x0000000000000000 R15: 0x00007FFFF7FFE570
> CS: 0033 DS: 0000 ES: 0000 FS: 0000 GS: 0000 SS: 002B Error
> while running hook_stop:
> Cannot access memory at address 0xa07f0
> 0x00000000000a07f0 in ?? ()
> Error in testing breakpoint condition:
> The program being debugged was signaled while in a function called from GDB.
> GDB remains in the frame where the signal was received.
> To change this behavior use "set unwindonsignal on".
> Evaluation of the expression containing the function
> (at 0x0xa07f0) will be abandoned.
> When the function is done executing, GDB will silently stop.
> -----------------------------------------------------------------------------------------------------------------------[regs]
> RAX: 0x0000000000000000 RBX: 0x0000555555562AA1 RBP:
> 0x00007FFFFFFFD8C8 RSP: 0x00007FFFFFFFD8C8 o d I t s Z a P c
> RDI: 0x00007FFFF7FFA4A0 RSI: 0x00007FFFF7DF72D0 RDX:
> 0x0000000000000001 RCX: 0xFFFFFFFFFFFFFFFF RIP: 0x00000000000A07F0
> R8 : 0x00007FFFF7FFE570 R9 : 0x0000000000000000 R10:
> 0xFFFFFFFFFFFFFFB0 R11: 0x0000000000000246 R12: 0x00007FFFF7FFE090
> R13: 0x00007FFFF7FFD040 R14: 0x0000000000000000 R15: 0x00007FFFF7FFE570
> CS: 0033 DS: 0000 ES: 0000 FS: 0000 GS: 0000 SS: 002B Error
> while running hook_stop:
> Cannot access memory at address 0xa07f0
>
> Catchpoint 1 (returned from syscall munmap), 0x00000000000a07f0 in ?? ()
> gdb$
>
> GDB stop in the condition (x/1s $rdi ==> "/proc/meminfo") but after
> that I can't continue
>
> My machine is a Debian x86_64, and I probe with v7.4 and v7.8
>
> Anyone knows what is the problem?
>
> P.S.: with "call strcmp("foo","foo")" I have:
> (gdb) call strcmp("foo","foo")
>
> Program received signal SIGSEGV, Segmentation fault.
> 0x00000000000a07f6 in ?? ()
> The program being debugged was signaled while in a function called from GDB.
> GDB remains in the frame where the signal was received.
> To change this behavior use "set unwindonsignal on".
> Evaluation of the expression containing the function
> (strcmp@plt) will be abandoned.
> When the function is done executing, GDB will silently stop.
> (gdb)
>
> Regards
Hi.
I'll address the strcmp segv in another email, I'm travelling at the
moment, but two things occur to me off the top of my head.
1) in order for a function call with a string argument to work gdb has
to malloc space for the string and copy the string to the malloc'd
memory. There is no mechanism for automagically freeing this memory
(at least none that I'm aware of) so using strcmp as a breakpoint
condition will leak memory. That's not necessarily a bad thing,
especially if it helps one find the problem, but it is something to be
aware of.
2) there is the $_streq convenience function:
(gdb) cond 1 $_streq("/proc/meminfo", (char*)($rdi))
$_streq is implemented in Python and comes with recent gdbs (and thus
requires gdb to be built with python support). There are two nice
properties of $_streq:
a) no inferior function calls, which can perturb inferior state in
undesirable ways
b) no leakage of memory
^ permalink raw reply [flat|nested] 2+ messages in thread
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