Re: Non-stop mode disfunctional ?

Pedro Alves
2011-03-14T14:52:36Z

On Monday 14 March 2011 14:20:11, Chris Hall wrote:
>
> Sorry to follow up my own posting...

It'd be better to not start a new thread for each post.

> ... 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 ?

I only read your original post diagonally, but I'd suspect
a problem with displaced-stepping. Try with both non-stop and
target-async off (the default), but enabling
"set breakpoint always-inserted on", and "set displaced-stepping on".

--
Pedro Alves

>
> 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
>
>