linux-kernel - Re: [PATCH 1/1] ptrace: make sure do_wait() won't hang after PTRACE

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Message-Id: <20110215173149.239601807B7@magilla.sf.frob.com>
Date:	Tue, 15 Feb 2011 09:31:49 -0800 (PST)
From:	Roland McGrath <roland@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	jan.kratochvil@...hat.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org
Subject: Re: [PATCH 1/1] ptrace: make sure do_wait() won't hang after
 PTRACE_ATTACH

> Actually I was thinking along this line.  We can allow
> PTRACE_GETSIGINFO to proceed without forcing the tracee into TRACED
> state, the rationale being the operation is required to tell between
> group stop and ptrace trap.  Am I missing something?

The reason for the transition to TASK_TRACED is to prevent a race with
SIGCONT waking the task.  There is always a race with SIGKILL waking it,
but the circumstances where that can really matter are far fewer.
You need to make sure that the work PTRACE_GETSIGINFO does to access
last_siginfo cannot race with that pointer disappearing or the stack
space it points to becoming invalid.  I think the use of siglock ensures
that, but Oleg should verify it.

Thanks,
Roland
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