linux-kernel - Re: [PATCH] sys_remap_file_pages: fix ->vm

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Date:	Sun, 3 Feb 2008 21:21:35 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Matt Helsley <matthltc@...ibm.com>
Cc:	Miklos Szeredi <mszeredi@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	William Lee Irwin III <wli@...omorphy.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] sys_remap_file_pages: fix ->vm_file accounting

(remove stable@...nel.org from CC)

On 02/02, Matt Helsley wrote:
> 
> On Wed, 2008-01-30 at 20:26 +0300, Oleg Nesterov wrote:
> > 
> > Offtopic. I noticed this problem while looking at this patch:
> > 
> > 	http://marc.info/?l=linux-mm-commits&m=120141116911711
> > 
> > So this (the old vma could be removed before we create the new mapping)
> > means that the patch above has another problem: if we are remapping the
> > whole VM_EXECUTABLE vma, removed_exe_file_vma() can clear ->exe_file
> > while it shouldn't (Matt Helsley cc'ed).
> > 
> > Oleg.
> 
> 	Looking at sys_remap_file_pages() it appears that the shared flag must
> be set in order to remap. Executable mappings are always MAP_PRIVATE and
> hence lack the shared flag so that any modifications to those areas
> don't get written back to the executable. I don't think userspace can
> change this flag

Yes, userspace can't change it. But if MVFS changes ->vm_file it could also
change vm_flags... But I think you are right anyway, we shouldn't care.


So I have to try to find another bug ;) Suppose that ->load_binary() does
a series of do_mmap(MAP_EXECUTABLE). It is possible that mmap_region() can
merge 2 vmas. In that case we "leak" ->num_exe_file_vmas. Unless I missed
something, mmap_region() should do removed_exe_file_vma() when vma_merge()
succeds (near fput(file)).

Oleg.

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