Re: [PATCH] ptrace(2) single-stepping into signal handlers

John Blackwood
2005-06-06T12:54:20Z

> Subject: Re: [PATCH] ptrace(2) single-stepping into signal handlers
> From: Andi Kleen <ak@suse.de>
> Date: Sun, 5 Jun 2005 13:21:19 +0200
> To: John Blackwood <john.blackwood@ccur.com>
> CC: linux-kernel@vger.kernel.org, roland@redhat.com, ak@suse.de,
akpm@osdl.org, bugsy@ccur.com
>
> On Fri, Jun 03, 2005 at 01:21:20PM -0400, John Blackwood wrote:
>
> >> 1. Make the default behavior that we single-step to the next
instruction
> >> in the main (non-signal handler) stream of execution, instead of
> >> single-stepping into the user's signal handler.
>
>
> I think it is better to not change the default behaviour. You risk
> breaking programs and there are much more that use ptrace than just gdb.

O.K. Yes, it seems that this is definitely not a popular idea. :-)

> >> 2. Add a new ptrace PTRACE_SETOPTIONS command flag,
>
>
> I think it would be better to just define a new ptrace singlestep command
> with the new semantics instead of adding options.

Yes, this is a good idea... I just might re-post this approach
as a suggestion.

> >> - The ptraced process has a pending signal and
> >> it stops to notify the debugger.
> >>
> >> - The debugger then single-steps into the ptraced process's signal
handler.
> >>
> >> - On the next eventual continue (PTRACE_CONT) command, we run
through the
> >> signal handler, and we stop once again at the next instruction
before
> >> we changed our execution stream and single-stepped into the user's
> >> signal handler.
> >>
> >> - At this point, we can no longer continue the ptraced process
> >> with a PTRACE_CONT command. Instead, all ptrace(2) PTRACE_CONT
calls
> >> are treated as if we had made a ptrace(2) PTRACE_SINGLESTEP call.
>
>
> Have you actually seen this in practice?

Yes, but Andrew Morton suggested I try out the above test scenario on:

ftp://ftp.kernel.org/pub/linux/kernel/v2.6/testing/linux-2.6.12-rc5.tar.bz2

And the problem has indeed been fixed - there is some new code in the
i386 version of handle_signal() that fixes this problem.

(I saw the problem on earlier 2.6.11.10 and 2.6.11.11 kernels.)

Thanks for the feedback, Andrew.