"finish" command leads to SIGTRAP

David Griffiths dgriffiths@undo.io
Thu Feb 21 15:55:00 GMT 2019


It's something to do with the nature of single stepping through a "popfq"
instruction. Given the following instructions:

   0x7fffe104638f:    add    $0x8,%rsp
   0x7fffe1046393:    popfq
   0x7fffe1046394:    pop    %rbp
   0x7fffe1046395:    jmpq   *%rax

If I set a breakpoint at the first of that set and single step through, I
end up with:

eflags         0x346    [ PF ZF TF IF ]

but if I set a breakpoint on the last instruction and avoid single stepping
I get:

eflags         0x246    [ PF ZF IF ]

and I think it's that TF that is causing the SIGTRAP?


On Thu, 21 Feb 2019 at 13:12, Pedro Alves <palves@redhat.com> wrote:

> Might be unrelated, but ISTR that there used to be a kernel bug
> that would lead to the cpu's trace flag getting stuck set
> when you step in a signal handler.  That would result in
> SIGTRAP happening at every step from that point on.  Could
> that be the case here?
>
> I'd look at "set debug displaced on" too.  Otherwise, it's a matter
> at staring at the logs, and trying to understand what is happening.
> Basically, "finish" sets a breakpoint at the caller and runs to it.
> But all sorts of other things can happen behind the scenes.
>
> Thanks,
> Pedro Alves
>


-- 

David Griffiths, Senior Software Engineer

Undo <https://undo.io> | Resolve even the most challenging software defects
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