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

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Date:	Thu, 25 Jun 2009 11:02:14 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
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

Denys Vlasenko <vda.linux@...glemail.com> writes:

> In some circumstances running process needs to re-execute
> its image.
>
> Among other useful cases, it is _crucial_ for NOMMU arches.
>
> They need it to perform daemonization. Classic sequence
> of "fork, parent dies, child continues" can't be used
> due to lack of fork on NOMMU, and instead we have to do
> "vfork, child re-exec itself (with a flag to not daemonize)
> and therefore unblocks parent, parent dies".

Why?

I would expect a simple assembly wrapper around clone would work.
I.e.  Create a new process but share the MM and reuse the same
stack. When clone returns if we are the parent exit, otherwise
continue on with life.

> Another crucial use case on NOMMU is POSIX shell support.
> Imagine a shell command of the form "func1 | func2 | func3".
> This can be implemented on NOMMU by vforking thrice,
> re-executing the shell in every child in the form
> "<shell> -c 'body of funcN'", and letting parent wait and collect
> exitcodes and such. As far as I can see, it's the only way
> to implement it correctly on NOMMU.
>
> The program may re-execute itself by name if it knows the name,
> but we generally may be unsure about it. Binary may be renamed,
> or even deleted while it is being run.

It really sounds like you want to implement fork for NOMMU.
If you have a base address register which everything is relative
to I can't imagine it would be too hard.  Heck you might even be
able to do it in user space.  

Eric

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