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Date: Thu, 24 Sep 2009 22:59:00 +0300
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Message-ID: <84144f020909241259p1912bfcn8085f6670a97970e@mail.gmail.com>
Subject: Re: [PATCH] NOMMU: Fix MAP_PRIVATE mmap() of objects where the data 
	can be mapped directly
From: Pekka Enberg <penberg@cs.helsinki.fi>
To: David Howells <dhowells@redhat.com>
Cc: torvalds@osdl.org, akpm@linux-foundation.org, graff.yang@gmail.com,
       linux-kernel@vger.kernel.org, Paul Mundt <lethal@linux-sh.org>,
       Mel Gorman <mel@csn.ul.ie>, Greg Ungerer <gerg@snapgear.com>
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Hi David,

On Thu, Sep 24, 2009 at 5:13 PM, David Howells <dhowells@redhat.com> wrote:
> Fix MAP_PRIVATE mmap() of files and devices where the data in the backing store
> might be mapped directly. ?Use the BDI_CAP_MAP_DIRECT capability flag to govern
> whether or not we should be trying to map a file directly. ?This can be used to
> determine whether or not a region has been filled in at the point where we call
> do_mmap_shared() or do_mmap_private().
>
> The BDI_CAP_MAP_DIRECT capability flag is cleared by validate_mmap_request() if
> there's any reason we can't use it. ?It's also cleared in do_mmap_pgoff() if
> f_op->get_unmapped_area() fails.
>
>
> Without this fix, attempting to run a program from a RomFS image on a
> non-mappable MTD partition results in a BUG as the kernel attempts XIP, and
> this can be caught in gdb:
>
> Program received signal SIGABRT, Aborted.
> 0xc005dce8 in add_nommu_region (region=<value optimized out>) at mm/nommu.c:547
> (gdb) bt
> #0 ?0xc005dce8 in add_nommu_region (region=<value optimized out>) at mm/nommu.c:547
> #1 ?0xc005f168 in do_mmap_pgoff (file=0xc31a6620, addr=<value optimized out>, len=3808, prot=3, flags=6146, pgoff=0) at mm/nommu.c:1373
> #2 ?0xc00a96b8 in elf_fdpic_map_file (params=0xc33fbbec, file=0xc31a6620, mm=0xc31bef60, what=0xc0213144 "executable") at mm.h:1145
> #3 ?0xc00aa8b4 in load_elf_fdpic_binary (bprm=0xc316cb00, regs=<value optimized out>) at fs/binfmt_elf_fdpic.c:343
> #4 ?0xc006b588 in search_binary_handler (bprm=0x6, regs=0xc33fbce0) at fs/exec.c:1234
> #5 ?0xc006c648 in do_execve (filename=<value optimized out>, argv=0xc3ad14cc, envp=0xc3ad1460, regs=0xc33fbce0) at fs/exec.c:1356
> #6 ?0xc0008cf0 in sys_execve (name=<value optimized out>, argv=0xc3ad14cc, envp=0xc3ad1460) at arch/frv/kernel/process.c:263
> #7 ?0xc00075dc in __syscall_call () at arch/frv/kernel/entry.S:897

I have no idea about the BDI_CAP_MAP_DIRECT bits but...

> Note that this fix does the following commit differently:
>
> ? ? ? ?commit a190887b58c32d19c2eee007c5eb8faa970a69ba
> ? ? ? ?Author: David Howells <dhowells@redhat.com>
> ? ? ? ?Date: ? Sat Sep 5 11:17:07 2009 -0700
> ? ? ? ?nommu: fix error handling in do_mmap_pgoff()

...changes to this part of the code look like nice cleanup to me.

Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>
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