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

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Date:	Mon, 16 Jul 2007 12:31:14 -0700
From:	Matt Helsley <matthltc@...ibm.com>
To:	Al Viro <viro@....linux.org.uk>
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 Fri, 2007-07-13 at 03:07 +0100, Al Viro wrote:
> 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.

	Yes, it will prevent the filesystem with the executable file from being
unmounted. Do you have an example where the original filesystem urgently
needs to be unmounted while this unusual executable is running? Or is
umount -l sufficient here?

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

	Yes, I'm aware of at least one example where this technique has
legitimate uses: libhugetlbfs. I'm interested in testing others you may
be able to recommend as well.

	Furthermore, in your example the VMA walk would make /proc/self/exe a
symlink to the file that backs the next executable VMA: libc, libdl,
etc. That seems rather odd to me. In contrast, with my
patch /proc/self/exe would always be a symlink to the original
executable.

Cheers,
	-Matt Helsley

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