From: Pedro Alves <pedro@codesourcery.com>
Organization: CodeSourcery
To: Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH 03/10] ptrace: implement PTRACE_SEIZE
Date: Tue, 24 May 2011 13:36:03 +0100
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On Tuesday 24 May 2011 13:00:13, Tejun Heo wrote:
> Hello,
> 
> On Tue, May 24, 2011 at 10:49:58AM +0100, Pedro Alves wrote:
> > A couple interface questions that just crossed my mind:
> > 
> >  - on a fork/vfork/clone, if PTRACE_EVENT_FORK|VFORK|CLONE have been
> >    enabled, will the tracer still see the new child stop with a
> >    SIGSTOP, or will it see a PTRACE_EVENT_INTERRUPT?
> 
> This won't change, so SIGSTOP although we probably want to improve it
> such that this can be distinguished from SIGTRAP from userland.

(I assume you meant SIGSTOP from userland.)  So that if a SIGSTOPs
from userland is sent before the tracer waits for the child, the
tracer sees a siginfo corresponding to the userland SIGSTOP?  Sounds
like it might work.

> >  - is PTRACE_INTERRUPT on PTRACE_TRACEME-traced-child planed to
> >    be allowed (for convenience)?
> >    A PTRACE_O_TRACEINTERRUPT, or some such PTRACE_SETOPTIONS
> >    option might be necessary to get PTRACE_EVENT_INTERRUPT instead
> >    of SIGSTOP in the point above.
> 
> I'm currently leaning toward deprecating PTRACE_TRACEME.  If a task
> can PTRACE_TRACEME, it may as well just do pause(2) and let the parent
> SEIZE it.

Debuggers will want to nurse the child through a couple of
execs (shell, then real debuggee), so that scheme requires a bit
more synchronization, because SEIZE hides the magic exec SIGTRAP,
and so the tracer needs to set the O_TRACEXEC option before the first
exec, and make sure external signals don't break the synchronization.
Reading/writing to/from blocking pipes for that initial synchronization
is what GDB uses instead for e.g., hpux/ttrace support, which looks
similar to using PTRACE_SEIZE for PTRACE_TRACEME.  A bit more
cumbersome, though doable, I suppose.

Thanks.

-- 
Pedro Alves
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