linux-kernel - Re: mm_release() call in exit

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Date:	Mon, 12 Nov 2007 16:48:02 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Jesper Juhl <jesper.juhl@...il.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: mm_release() call in exit_mm() looks dangerous

Jesper Juhl wrote:
> In kernel/exit.c we have this code :
>
> static void exit_mm(struct task_struct * tsk)
> {
>         struct mm_struct *mm = tsk->mm;
>
>         mm_release(tsk, mm);
>         if (!mm)
>                 return;
> ...
>
>
> But, mm_release() may dereference it's second argument ('mm'), so
> shouldn't we be doing the "!mm" test *before* we call mm_release() and
> not after?
> I don't know the mm code well enough to be able to tell if some of the
> other stuff mm_release does needs to be done always and the mm
> dereference can't actually happen, but maybe someone else who knows
> the code better can tell...  In any case, what's currently there looks
> a little shaky..
>   

Yeah, it looks wrong.  mm_release() calls deactivate_mm() as its first
act, which could well dereference mm (though it often doesn't).

    J
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