linux-kernel - Re: Race in ptrace.

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Message-ID: <20100211185530.GA22055@redhat.com>
Date:	Thu, 11 Feb 2010 19:55:30 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Salman Qazi <sqazi@...gle.com>
Cc:	taviso@...gle.com, Roland Dreier <rolandd@...co.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Race in ptrace.

On 02/11, Salman Qazi wrote:
>
> >> >                // the tracee is STOPPED as requested, not TRACED,
> >> >                // SIGCONT wakes it up
> >> >                kill(pid, SIGCONT);
>
> I am still missing something.  There's probably a gap in my
> understanding, so let's try to clarify it.  The last "kill" call,
> sends a SIGCONT.  But, shouldn't this SIGCONT be intercepted by the
> tracer before the tracee sees it?

No. The tracee resumes (again: because it was STOPPED, not TRACED),
dequeues SIGCONT, reports the signal and stops in TASK_TRACED,
see ptrace_signal(). Meanwhile, until it calls ptrace_stop(), it is
TASK_RUNNING and ptrace() fails.

Oleg.

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