Date: Tue, 23 Nov 2004 18:55:22 +0100 (MET)
From: "Michael Kerrisk" <michael.kerrisk@gmx.net>
To: Andrew Morton <akpm@osdl.org>
Cc: mtk-lkml@gmx.net, torvalds@osdl.org, linux-kernel@vger.kernel.org,
       manfred@colorfullife.com, hugh@veritas.com
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References: <20041123090449.1672494f.akpm@osdl.org>
Subject: Re: [PATCH 2.6.10-rc2] RLIMIT_MEMLOCK accounting of shmctl() SHM_LOCK is broken
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> "Michael Kerrisk" <mtk-lkml@gmx.net> wrote:
> >
> > The accounting of shmctl() SHM_LOCK memory locks against the
> >  user structure is broken.  The problem is that the check
> >  of the size of the to-be-locked region is based on 
> >  the size of the segment as specified when it was created
> >  by shmget() (this size is *not* rounded up to a page 
> >  boundary).  This size is then rounded down (>> PAGE_SHIFT)
> >  to PAGE_SIZE during the check in 
> >  mm/mlock.c::user_shm_lock().
> 
> True.  We should make the same change to user_shm_unlock(), and we may as
> well tweak the excessive spinlock coverage in there too.

Thanks for the confirmation Andrew.

[...]

> and then ask Hugh and Manfred to double-check.
> 
> Looking at the callers, we do:
> 
> 	user_shm_lock(inode->i_size, ...);
> 
> then, later:
> 
> 	user_shm_unlock(inode->i_size, ...);
> 
> which does make one wonder "what happens if the file got larger while it
> was locked"?

Not sure if I'm missing your point, but, just considering it from 
the point of view of System V shared memory, the segment can't 
change in size after it has been created.

Cheers,

Michael

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