linux-kernel - Re: [PATCHv10 man-pages 5/5] execveat.2: initial man page for execveat(2)

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Date:	Sat, 10 Jan 2015 20:15:59 -0500
From:	Rich Felker <dalias@...ifal.cx>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	David Drysdale <drysdale@...gle.com>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Andy Lutomirski <luto@...capital.net>,
	Meredydd Luff <meredydd@...atehouse.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Kees Cook <keescook@...omium.org>,
	Arnd Bergmann <arnd@...db.de>,
	Christoph Hellwig <hch@...radead.org>, X86 ML <x86@...nel.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	sparclinux@...r.kernel.org
Subject: Re: [PATCHv10 man-pages 5/5] execveat.2: initial man page for
 execveat(2)

On Sat, Jan 10, 2015 at 04:27:23PM -0600, Eric W. Biederman wrote:
> Rich Felker <dalias@...ifal.cx> writes:
> 
> > On Sat, Jan 10, 2015 at 04:14:57AM +0000, Al Viro wrote:
> 
> >> Except that if your interpreter does stat(2) (or access(2), or getxattr(2),
> >> etc.) before bothering with open(2), you'll get screwed.
> >
> > Yes, but I think that would be very bad interpreter design.
> > stat/getxattr/access/whatever followed by open is always a TOCTOU
> > race. The correct sequence of actions is always open followed by
> > fstat/fgetxattr/...
> 
> Sigh.  I think everyone who has looked at this has been blind.
> 
> If userspace is reasonable all we have to do is fix /proc/self/exe
> for shell scripts to point at the actual script,
> and then pass /proc/self/exe on the shell scripts command line.
> 
> At a practical level we have to worry about backwards compability and
> chroot jails.  But the existence of a clean implementation with
> /proc/self/exe serves a proof of concept that it would not be too
> difficult.  When someone cares enough to implement it.

Is /proc/self/exe a "magic symlink" that's bound to the inode, or just
a regular symlink? In the latter case it defeats the whole purpose of
using O_EXEC fds and fexecve rather than pathnames.

Rich
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