netdev - Re: [PATCH] net: fix setsockopt() locking errors

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Date:	Tue, 27 Jan 2009 09:08:46 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Martin MOKREJŠ 
	<mmokrejs@...osome.natur.cuni.cz>,
	Vegard Nossum <vegard.nossum@...il.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] net: fix setsockopt() locking errors

On Tue, Jan 27, 2009 at 09:52:49AM +0100, Peter Zijlstra wrote:
> On Tue, 2009-01-27 at 08:45 +0000, Jarek Poplawski wrote:
> > On Mon, Jan 26, 2009 at 10:30:30PM +0100, Martin MOKREJŠ wrote:
> > > The patch really did not help:
> > > http://bugzilla.kernel.org/show_bug.cgi?id=12515#c5
> > > Martin
> > 
> > Actually, there is a little change: the warning triggerd in another
> > place (sock_setsockopt() -> sk_attach_filter()). So we could go deeper
> > with these changes, but I'm not sure this is the right way to fix.
> > 
> > It looks like the scenario is very old, but probably wasn't reported
> > (maybe there is some lockdep improvement):
> 
> Yes, they likely are very old, and yes we added a lockdep annotation to
> copy_to/from_user() to catch these.
> 
> > A) sys_mmap2() -> mm->mmap_sem -> packet_mmap() -> sk_lock
> > B) sock_setsockopt() -> sk_lock -> copy_from_user() -> mm->mmap_sem
> > 
> > packet_mmap() (net/packet/af_packet.c) seems to be the only place in
> > net to implement mmap method, and using this lock order btw. On the
> > other hand copy_from_user() could be more popular under sk_lock, and
> > I'm not sure these changes are necessary.
> > 
> > Since I don't know enough neither sock/packet nor sys_mmap, I guess
> > some advice would be precious. It looks like Peter Zijlstra solved
> > similar problems in nfs, so I CC him.
> 
> The NFS/sunrpc case was special in that it did copy_to/from_kernel, that
> is, it never actually touched user memory -- we taught the might_fault()
> annotation about that.
> 
> Can't you simply do the copy_from_user() before you take the sk_lock?
> 

Since it's really needed, and Vegard started doing it like this, I
guess he will try to add the missing pieces.

Thanks again,
Jarek P.
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