linux-kernel - Re: [PATCH 03/10] ptrace: implement PTRACE

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Date:	Wed, 25 May 2011 20:18:56 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Pedro Alves <pedro@...esourcery.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	jan.kratochvil@...hat.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	indan@....nu, bdonlan@...il.com
Subject: Re: [PATCH 03/10] ptrace: implement PTRACE_SEIZE

Sorry for delay again, I was a bit offline.

On 05/24, Tejun Heo wrote:
>
> 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 am a bit confused, sorry if I missed/misunderstood the context...

I thought, we already discussed this. Obviously, PT_SEIZED should be
inherited by the child during the auto-attach (and it is indeed copied
by ptrace_init_task). Why should we send SIGSTOP to the child then?
Even in the fork case, this leads to the same problems as the current
PTRACE_ATTACH has with the bogus SIGSTOP it sends. I think TRAP_STOP
is the only sane option, no?


Btw. Speaking of SEIZE->execvd->INTERRUPT which makes the tracee see
a SIGTRAP. Stupid question. Perhaps PTRACE_SEIZE should set
PT_TRACESYSGOOD | PT_TRACE_EXEC along with PT_SEIZED automatically?
PT_SEIZED implies the new behaviour anyway.

Oleg.

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