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Non-stop mode disfunctional ?


Sorry to follow up my own posting...

... I'm hoping to discover if the fact that non-stop doesn't work is
(a) because I'm not using it correctly, or (b) a well known problem I
should know about, or (c) an actual bug, or (d) something else ?

Thanks,

Chris

> -----Original Message-----
> From: gdb-owner@sourceware.org [mailto:gdb-owner@sourceware.org] On
> Behalf Of Chris Hall
> Sent: 07 March 2011 11:01
> To: gdb@sourceware.org
> Subject: SIGSEGV on exit from subroutines -- problem with non-stop ?
> 
> Hi,
> 
> I am using gdb 7.2-14.fc14 to work on a large multi-threaded
> application, in C, x86-64.
> 
> I have .gdbinit, per the book:
> 
>   set target async 1
>   set pagination off
>   set non-stop on
> 
> When I step using 's' or 'n', as it leaves some subroutines I keep
> getting SIGSEGV, such as:
> 
>   Program received signal SIGSEGV, Segmentation fault.
>   signal_set (signo=Cannot access memory at address
> 0xffffffffffffff5c)
>   at ...
> 
> When I 'disass' the current instruction is a leaveq.  Examining the
> registers I observe that rbp is zero, which is clearly nonsense.
> 
> I found one instance which was repeatable, which happened to be
> before
> any threads were started: if I 'ni' through a particular function,
> it
> gets to the leaveq, and gets stuck there.  Each time I do ni, the
> rsp
> and the rbp are updated by the repeated leaveq, until it goes bang.
> 
> So... I began to think this isn't something complicated to do with
> multiple threads... so here is a test:
> 
> <<--test.c-----------------------------------------------
> #include <stdio.h>
> #include <stdlib.h>
> 
> static void
> target(const char* message) {
> 	printf("%s ...BANG!\n", message) ;
> }
> 
> int main(int argc, char* argv[]) {
> 
> 	target("Light the blue touch paper") ;
> 
> 	return 0 ;
> }
> ------------------------------------------------------->>
> 
> Compiled by gcc 4.5.1 "-g -O0".
> 
> If I do "gdb test", stepping by "n":
> 
> <<-------------------------------------------------------
> (gdb) show non-stop
> Controlling the inferior in non-stop mode is on.
> (gdb) b target
> Breakpoint 1 at 0x4004d0: file test.c, line 6.
> (gdb) run
> Starting program: ...........test
> 
> Breakpoint 1, target (message=0x400615 "Light the blue touch paper")
> at test.c:6
> 6		printf("%s ...BANG!\n", message) ;
> (gdb) n
> Light the blue touch paper ...BANG!
> 7	}
> (gdb) n
> 
> Program received signal SIGSEGV, Segmentation fault.
> target (message=Cannot access memory at address 0xfffffffffffffff8
> ) at test.c:7
> 7	}
> (gdb) info reg
> ....
> rbp   0x0         	0x0
> rsp   0x7fffffffe248	0x7fffffffe248
> ....
> rip   0x4004e9		0x4004e9 	<target+37>
> ....
> ------------------------------------------------------->>
> 
> Or, stepping by 'ni':
> 
> <<-------------------------------------------------------
> (gdb) show non-stop
> Controlling the inferior in non-stop mode is on.
> (gdb) b target
> Breakpoint 1 at 0x4004d0: file test.c, line 6.
> (gdb) disass target
> Dump of assembler code for function target:
>    0x00000000004004c4 <+0>:	push   %rbp
>    0x00000000004004c5 <+1>:	mov    %rsp,%rbp
>    0x00000000004004c8 <+4>:	sub    $0x10,%rsp
>    0x00000000004004cc <+8>:	mov    %rdi,-0x8(%rbp)
>    0x00000000004004d0 <+12>:	mov    $0x400608,%eax
>    0x00000000004004d5 <+17>:	mov    -0x8(%rbp),%rdx
>    0x00000000004004d9 <+21>:	mov    %rdx,%rsi
>    0x00000000004004dc <+24>:	mov    %rax,%rdi
>    0x00000000004004df <+27>:	mov    $0x0,%eax
>    0x00000000004004e4 <+32>:	callq  0x4003b8 <printf@plt>
>    0x00000000004004e9 <+37>:	leaveq
>    0x00000000004004ea <+38>:	retq
> End of assembler dump.
> (gdb) disp/i $pc
> (gdb) run
> Starting program: .......test
> 
> Breakpoint 1, target (message=0x400615 "Light the blue touch paper")
> at test.c:6
> 6		printf("%s ...BANG!\n", message) ;
> .....
> 1: x/i $pc
> => 0x4004e4 <target+32>:	callq  0x4003b8 <printf@plt>
> (gdb) ni
> Light the blue touch paper ...BANG!
> 7	}
> 1: x/i $pc
> => 0x4004e9 <target+37>:	leaveq
> (gdb) ni
> target (message=0x100000000 <Address 0x100000000 out of bounds>) at
> test.c:7
> 7	}
> 1: x/i $pc
> => 0x4004e9 <target+37>:	leaveq
> (gdb) ni
> Cannot access memory at address 0x8
> (gdb) ni
> The program is not being run.
> ------------------------------------------------------->>
> 
> I note that if I turn off the "non-stop" option, it works.  So this
> is
> something to do with debugging multi-threaded !
> 
> I note also that if I change the target to:
> 
>   static int
>   target(const char* message) {
>           printf("%s ...BANG!\n", message) ;
>           return 0 ;
>   }
> 
> the problem goes away... so one extra instruction between the callq
> and the leaveq makes a difference:
> 
>    0x00000000004004dc <+24>:	mov    %rax,%rdi
>    0x00000000004004df <+27>:	mov    $0x0,%eax
>    0x00000000004004e4 <+32>:	callq  0x4003b8 <printf@plt>
>    0x00000000004004e9 <+37>:	mov    $0x0,%eax
>    0x00000000004004ee <+42>:	leaveq
>    0x00000000004004ef <+43>:	retq
> 
> This goes some way to explaining why it appeared to be a sporadic
> problem.
> 
> Is this me, or is this a bug ?  It used to work :-(
> 
> Thanks,
> 
> Chris


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