linux-kernel - Re: [PATCH] exec: Force single empty string when argv is empty

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Date:   Wed, 2 Feb 2022 12:31:48 -0800
From:   Kees Cook <keescook@...omium.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Ariadne Conill <ariadne@...eferenced.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Christian Brauner <brauner@...nel.org>,
        Rich Felker <dalias@...c.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH] exec: Force single empty string when argv is empty

On Tue, Feb 01, 2022 at 09:17:47AM +0000, David Laight wrote:
> From: Kees Cook
> > Sent: 01 February 2022 00:10
> ...
> > While the initial code searches[6][7] turned up what appeared to be
> > mostly corner case tests, trying to that just reject argv == NULL
> > (or an immediately terminated pointer list) quickly started tripping[8]
> > existing userspace programs.
> > 
> > The next best approach is forcing a single empty string into argv and
> > adjusting argc to match. The number of programs depending on argc == 0
> > seems a smaller set than those calling execve with a NULL argv.
> 
> Has anyone considered using the pathname for argv[0]?
> So converting:
> 	execl(path, NULL);
> into:
> 	execl(path, path, NULL);
> 
> I've not spotted any such suggestion.

It came up on some IRC discussions at some point. I'm personally not a
fan of this because it creates a bit of "new" ABI that has a lot of
variability (depending on "" is one thing, but depending on a "missing"
argv matching the exec path is very different). I think there were also
concerns about dealing with fd-based exec ("what is the 'right' name"),
etc.

I'd prefer we stay as simple as possible for this change.

-- 
Kees Cook