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

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Date:	Sat, 02 Feb 2008 13:17:45 -0800
From:	Matt Helsley <matthltc@...ibm.com>
To:	Oleg Nesterov <oleg@...sign.ru>
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,
	stable@...nel.org, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] sys_remap_file_pages: fix ->vm_file accounting


On Wed, 2008-01-30 at 20:26 +0300, Oleg Nesterov wrote:
> On 01/30, Miklos Szeredi wrote:
> > 
> > On Wed, 2008-01-30 at 17:20 +0300, Oleg Nesterov wrote:
> > > Fix ->vm_file accounting, mmap_region() may do do_munmap().
> > 
> > There's a small problem with the patch: the vma itself is freed at
> > unmap, so the fput(vma->vm_file) may crash.  Here's an updated patch.
> 
> Ah, indeed, thanks!
> 
> 
> 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 -- even using plain mremap. So, unless there's a way to
change that flag, I don't think there's anything related to
VM_EXECUTABLE vmas that needs to be done here.

Cc'ing linux-mm.

Cheers,
	-Matt Helsley

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