Re: [PATCH 03/10] ptrace: implement PTRACE_SEIZE

[Tejun Heo <tj@kernel.org>]

Hello,

On Tue, May 24, 2011 at 01:36:03PM +0100, Pedro Alves wrote:
> 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.

Now that thinking more about it, TRAP_STOP (INTERRUPT trap) would
probably be better.  I'll think more about it.  For fork, it doesn't
really matter but deliverying SIGSTOP on CLONE isn't too good.  From
user's POV, TRAP_STOP should work too, right?

> > 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.

Yes, it would require some sort of synchronization.  I was thinking
more along the line of ptracer modifying tracee so that it exits
pause(2) loop after ptracer issues PTRACE_CONT, but I agree using
pipes would be more straight-forward.

Thank you.

-- 
tejun