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

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Message-ID: <20110816194050.GB4484@csn.ul.ie>
Date:	Tue, 16 Aug 2011 20:40:51 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Roland McGrath <mcgrathr@...gle.com>
Cc:	Will Drewry <wad@...omium.org>, 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 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.

> > 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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