linux-kernel - Re: [RFC] situation with fput() locking (was Re: [PULL REQUEST] : ima-appraisal patches)

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Date:	Fri, 20 Apr 2012 19:07:48 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-fsdevel@...r.kernel.org, James Morris <jmorris@...ei.org>,
	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	David Safford <safford@...ux.vnet.ibm.com>,
	Dmitry Kasatkin <dmitry.kasatkin@...el.com>,
	Mimi Zohar <zohar@...ux.vnet.ibm.com>,
	David Miller <davem@...emloft.net>
Subject: Re: [RFC] situation with fput() locking (was Re: [PULL REQUEST] :
 ima-appraisal patches)

On Fri, Apr 20, 2012 at 10:21:35AM -0700, Linus Torvalds wrote:
> On Fri, Apr 20, 2012 at 9:42 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > Actually, I like the per-CPU spinlock variant better; the thing is,
> > with that scheme we get normal fput() (i.e. non-nodefer variant)
> > non-blocking. ?How about this:
> 
> What's the advantage of a per-cpu lock?
> 
> If you make the work be per-cpu, then you're better with no locking at
> all: just disable interrupts (which you do anyway).

Point taken.

> The thing I do not like about the schedule_work approach is that it
> (a) totally hides the real cost  - which is the scheduling - and (b)
> it is so asynchronous that it will happen potentially long after the
> task dropped the reference.

[snip]

> This is why I suggested you look at Oleg's patches. If we guarantee
> that things won't be delayed past re-entering user mode, all those
> issues go away.

I've looked at them.  One obvious problem is that it tracehook_notify_resume()
is not universally called.  AFAICS, hexagon, m68k, microblaze, score, um
and xtensa never call tracehook_notify_resume().  Out of those, hexagon is
manually checking TIF_NOTIFY_RESUME and does key_replace_session_keyring(),
so the call could be easily added into the same place; the rest of those
guys don't even look at TIF_NOTIFY_RESUME anywhere near their signal*.c
and m68k/um/xtensa don't even have it defined, let alone handled.  So this
stuff depends on some amount of asm glue hacking on several architectures ;-/

Another is that final fput() can, indeed, happen in kernel threads, so
pure switch to task_work_add() won't be enough.  I agree that having this
stuff flushed before we return to userland would be a good thing; the
question is, can we easily tell if there will be such a return to userland?
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