From: David Griffiths <dgriffiths@undo.io>
To: Pedro Alves <palves@redhat.com>
Cc: gdb@sourceware.org
Subject: Re: "finish" command leads to SIGTRAP
Date: Thu, 21 Feb 2019 15:55:00 -0000 [thread overview]
Message-ID: <CA++j6c7Vazk-A3e8T5xzMNJzhorB9DW-y8Hye7v-rwUKVnEzgA@mail.gmail.com> (raw)
In-Reply-To: <78e1f522-f6f5-d38d-0644-d083c1e4ab5d@redhat.com>
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
with software flight recorder technology
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next prev parent reply other threads:[~2019-02-21 15:55 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-21 11:21 David Griffiths
2019-02-21 11:24 ` Pedro Alves
2019-02-21 12:13 ` David Griffiths
2019-02-21 12:17 ` David Griffiths
2019-02-21 13:12 ` Pedro Alves
2019-02-21 15:55 ` David Griffiths [this message]
2019-02-21 17:50 ` Pedro Alves
2019-02-21 18:03 ` Pedro Alves
2019-02-21 18:22 ` David Griffiths
2019-02-21 18:50 ` John Baldwin
2019-02-21 19:34 ` Pedro Alves
2019-02-21 20:50 ` John Baldwin
2019-02-22 15:09 ` Pedro Alves
2019-02-22 16:42 ` John Baldwin
2019-02-22 17:38 ` David Griffiths
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