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

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Date:	Tue, 16 Aug 2011 14:50:52 -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 2:50 PM, Will Drewry <wad@...omium.org> wrote:
> 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

Err VMA from mmaping the file from shm_open().

> 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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