Non-stop mode disfunctional ?

Chris Hall
2011-03-14T14:20:27Z

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