RE: Non-stop mode disfunctional ?

chris.hall.list
2011-03-14T16:17:10Z

Pedro Alves wrote (on Mon 14-Mar-2011 at 14:52):
> > ... 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".

I tried:

set non-stop off
set target-async off
set breakpoint always-inserted on
set displaced-stepping on

and gdb worked just fine.

I note that "breakpoint always-inserted" and "displaced-stepping"
default to "on" for "non-stop", so I tried:

set non-stop on
set target-async on
set breakpoint always-inserted off
set displaced-stepping off

and gdb goes BANG (SIGSEGV).

As far as I can tell, with "non-stop" "on", single stepping is borked
(at least on leaving a void function immediately after calling another
function).

In case it makes a difference, I am running AMD Phenom II X6 1090T
(Stepping 0).

Thanks,

Chris

> --
> 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
> >
> >