linux-kernel - Re: [PATCH] NOMMU: Fix MAP_PRIVATE mmap() of objects where the data can be mapped directly

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

Hi David,

On Thu, Sep 24, 2009 at 5:13 PM, David Howells <dhowells@...hat.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@...hat.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@...helsinki.fi>
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