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

[John Blackwood]

> Subject: Re: [PATCH] ptrace(2) single-stepping into signal handlers
> From: Andi Kleen <ak@xxxxxxx>
> Date: Sun, 5 Jun 2005 13:21:19 +0200
> To: John Blackwood <john.blackwood@xxxxxxxx>
> CC: linux-kernel@xxxxxxxxxxxxxxx, roland@xxxxxxxxxx, ak@xxxxxxx, 
akpm@xxxxxxxx, bugsy@xxxxxxxx
>
> 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.


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