linux-kernel - Re: [PATCH] mmap: add sysctl for controlling ~VM

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Message-ID: <CABqD9hba3PgTQjaWCmqQCgiaPnvdcOPd1VrQio-X-NY__T7HmA@mail.gmail.com>
Date:	Tue, 16 Aug 2011 14:50:27 -0500
From:	Will Drewry <wad@...omium.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Roland McGrath <mcgrathr@...gle.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Al Viro <viro@...iv.linux.org.uk>,
	Eric Paris <eparis@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Nitin Gupta <ngupta@...are.org>,
	Hugh Dickins <hughd@...gle.com>,
	Shaohua Li <shaohua.li@...el.com>, linux-mm@...ck.org
Subject: Re: [PATCH] mmap: add sysctl for controlling ~VM_MAYEXEC taint

On Tue, Aug 16, 2011 at 2:40 PM, Mel Gorman <mel@....ul.ie> wrote:
> On Tue, Aug 16, 2011 at 10:07:46AM -0700, Roland McGrath wrote:
>> On Tue, Aug 16, 2011 at 2:33 AM, Mel Gorman <mel@....ul.ie> wrote:
>> > Is using shm_open()+mmap instead of open()+mmap() to open a file on
>> > /dev/shm really that difficult?
>> >
>> > int shm_open(const char *name, int oflag, mode_t mode);
>> > int open(const char *pathname, int flags, mode_t mode);
>>
>> I cannot figure out the rationale behind this question at all.
>> Both of these library functions result in the same system call.
>>
>
> They might result in the same system call but one of them creates
> the file under /dev/shm which should not have the same permissions
> problem. The library really appears to want to create a shared
> executable object, using shm_open does not appear that unreasonable
> to me.

If /dev/shm is mounted noexec, the resulting file will have VM_MAYEXEC
stripped.  I don't believe it is capable of doing anything special
that will cause the mmap code path to find a different containing
mountpoint.  If it could, then that would certainly be preferable, but
it would also make this VM_MAYEXEC calculation less effective in the
default case.

thanks!

>> > An ordinary user is not going to know that a segfault from an
>> > application can be fixed with this sysctl. This looks like something
>> > that should be fixed in the library so that it can work on kernels
>> > that do not have the sysctl.
>>
>> I think the expectation is that the administrator or system builder
>> who decides to set the (non-default) noexec mount option will also
>> set the sysctl at the same time.
>>
>
> Which then needs to be copied in each distro wanting to do the same
> thing and is not backwards compatible where as using shm_open is.
>
> --
> Mel Gorman
> SUSE Labs
>
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