linux-kernel - Re: [PATCH] mm/mmap: Check for RLIMIT

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Date:	Tue, 2 Apr 2013 11:56:25 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Cyril Hrubis <chrubis@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/mmap: Check for RLIMIT_AS before unmapping

On Tue, Apr 02, 2013 at 11:54:03AM +0200, Cyril Hrubis wrote:
> This patch fixes corner case for MAP_FIXED when requested mapping length
> is larger than rlimit for virtual memory. In such case any overlapping
> mappings are unmapped before we check for the limit and return ENOMEM.
> 
> The check is moved before the loop that unmaps overlapping parts of
> existing mappings. When we are about to hit the limit (currently mapped
> pages + len > limit) we scan for overlapping pages and check again
> accounting for them.
> 
> This fixes situation when userspace program expects that the previous
> mappings are preserved after the mmap() syscall has returned with error.
> (POSIX clearly states that successfull mapping shall replace any
> previous mappings.)
> 
> This corner case was found and can be tested with LTP testcase:
> 
> testcases/open_posix_testsuite/conformance/interfaces/mmap/24-2.c
> 
> In this case the mmap, which is clearly over current limit, unmaps
> dynamic libraries and the testcase segfaults right after returning into
> userspace.
> 
> I've also looked at the second instance of the unmapping loop in the
> do_brk(). The do_brk() is called from brk() syscall and from vm_brk().
> The brk() syscall checks for overlapping mappings and bails out when
> there are any (so it can't be triggered from the brk syscall). The
> vm_brk() is called only from binmft handlers so it shouldn't be
> triggered unless binmft handler created overlapping mappings.
> 
> Signed-off-by: Cyril Hrubis <chrubis@...e.cz>

Reviewed-by: Mel Gorman <mgorman@...e.de>

-- 
Mel Gorman
SUSE Labs
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