linux-kernel - Re: [PATCH] Simplify /proc/<pid|self>/exe symlink code

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Message-ID: <20070713020710.GA21668@ftp.linux.org.uk>
Date:	Fri, 13 Jul 2007 03:07:10 +0100
From:	Al Viro <viro@....linux.org.uk>
To:	Matt Helsley <matthltc@...ibm.com>
Cc:	Andrew Morton <akpm@...l.org>, Chris Wright <chrisw@...s-sol.org>,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>,
	"Hallyn, Serge" <serue@...ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] Simplify /proc/<pid|self>/exe symlink code

On Thu, Jul 12, 2007 at 07:00:12PM -0700, Matt Helsley wrote:
> This patch avoids holding the mmap semaphore while walking VMAs in response to
> programs which read or follow the /proc/<pid|self>/exe symlink. This also allows
> us to merge mmu and nommu proc_exe_link() functions. The costs are holding the
> task lock, a separate reference to the executable file stored in the task
> struct, and increased code in fork, exec, and exit paths.

I don't think it's a food idea.  Consider a program that deliberately
creates an executable anon memory, copies the binary there, jumps there
and unmaps the original.   In the current tree you'll get nothing
pinning the binary; with your patch it will remained busy.

It's not a common situation, of course, but there are legitimate uses
for such technics...
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