linux-kernel - Re: [PATCH 1/3] ptrace_detach: the wrong wakeup breaks the ERESTARTxxx logic

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Date:	Mon, 9 Feb 2009 03:05:15 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Roland McGrath <roland@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jerome Marchand <jmarchan@...hat.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] ptrace_detach: the wrong wakeup breaks the
	ERESTARTxxx logic

On 02/08, Roland McGrath wrote:
>
> > This is because ptrace_detach does:
> >
> > 	if (!child->exit_state)
> > 		wake_up_process(child);
>
> I'm pretty sure that all these uses of wake_up_process were just blindly
> copied from an original use in ptrace code (what's now ptrace_resume).
> That original use just dates from the beforetime, the long long ago.
> (I don't think it indicates any coherent original intent.)
>
> It's many kinds of wrong.  It's also always been wrong in case of a
> simultaneous SIGKILL that already woke the child, which has then blocked on
> some mutex or semaphore or whatnot.  I don't know what the stated general
> policy about spurious wakeups from schedule() is supposed to be.  Perhaps
> to be pedantic, the sys_pause() code has been wrong to return without
> checking signal_pending().

Yes, I thought about fixing sys_pause() too, but I'm afraid we can have
the similar code.

> Frankly, I've always been afraid of strange cruft that might unexpectedly
> turn out to rely on this "wrong" (unconditional) wake-up.  Probably the
> things like that historically were all just to do with the stopped/traced
> bookkeeping and would be covered by explicitly dealing with PTRACE_CONT vs
> group stop et al.  But FWIW my reaction to fiddling the wake_up_process
> bogons in the past has been, "Be afraid."

Yes, I am afraid, seriously ;)

This can reveal other subtle problems, of course. But there is another reason
why this wakeup is wrong. It clearly breaks the SIGNAL_GROUP_STOPPED logic
in ptrace_untrace().

Oleg.

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