linux-kernel - Re: [PATCH] allow execve'ing "/proc/self/exe" even if /proc is not mounted

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Message-ID: <20090624235856.GZ8633@ZenIV.linux.org.uk>
Date:	Thu, 25 Jun 2009 00:58:56 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Frysinger <vapier@...too.org>
Subject: Re: [PATCH] allow execve'ing "/proc/self/exe" even if /proc is not
	mounted

On Thu, Jun 25, 2009 at 01:00:56AM +0200, Denys Vlasenko wrote:
> More elegant way is to execute /proc/self/exe.
> This works just fine as long as /proc is mounted.

So mount it.

> But it breaks if /proc isn't mounted, and this can happen in real-world
> usage. For example, when shell invoked very early in initrd/initramfs.

So mount it.

> With this patch, it is possible to execute /proc/self/exe
> even if /proc is not mounted.

> How patch does it: when execve syscall discovers that opening of binary
> image fails, a small bit of code is added to special case "/proc/self/exe"
> string. If binary name is *exactly* that string, and if error is ENOENT
> or EACCES, then exec will still succeed, using current binary's image.
> 
> Please apply.

No.  This is just plain sick.  Magical pathnames have no business being
in the kernel.  If procfs is too much for your sensitive soul, do an
extremely trimmed-down version that would consist of *one* *file* (yes,
as root and only node on fs).  Said file being a procfs-style symlink,
doing exactly what /proc/self/exec would do.

On such system you can just mkdir /proc/self, touch /proc/self/exec,
mount -t self_exec none /proc/self/exec and be done with that.  No
magic needed, end of the story.
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