linux-kernel - Re: [PATCH v2] Make PTRACE_SEIZE set ptrace options specified in 'data' parameter

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Date:	Sun, 11 Sep 2011 11:05:23 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Denys Vlasenko <vda.linux@...glemail.com>,
	linux-kernel@...r.kernel.org, dvlasenk@...hat.com
Subject: Re: [PATCH v2] Make PTRACE_SEIZE set ptrace options specified in
 'data' parameter

Hello,

On Thu, Sep 08, 2011 at 08:17:47PM +0200, Oleg Nesterov wrote:
> > There are places which assume ->ptrace is protected by siglock.
> 
> Really? Once again, I agree. But _afaics_ currently this is not strictly
> needed. PT_PTRACED/PT_SEIZED should not go away under ->siglock, yes, but
> it seems that it is fine to set them.

Hmmm.... I haven't checked each direction.  Maybe we don't strictly
need it on setting it but I definitely was assuming that ->ptrace was
protected by siglock while coding recent ptrace changes.  Can't the
following happen?

* ptracer seizes child, sets PT_PTRACED and then OR PT_SEIZED.

* ptracee enters signal delivery path with group stop scheduled.
  PT_PTRACED is visible and group stop is transformed into
  JOBCTL_TRAP_STOP.

* ptracee enters do_jobct_trap().  PT_SEIZED is still not visible and
  it takes the path for the old behavior.

* ptracer SEIZE'd and expects PTRACE_EVENT_STOP but it gets the old
  no-siginfo trap.

> > and linking are protected by siglock
> 
> Hmm. Could you explain this? Why do want __ptrace_link under ->siglock ?

Because it's much simpler to assume that w/ siglock locked, everything
including ->parent is set up properly w.r.t. ->ptrace.

Thanks.

-- 
tejun
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