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

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Message-ID: <87oaq6oypl.fsf@x220.int.ebiederm.org>
Date:	Sat, 10 Jan 2015 20:09:10 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Rich Felker <dalias@...ifal.cx>
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\@vger.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)

Rich Felker <dalias@...ifal.cx> writes:

> 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.

In implementation /proc/self/exe is a named rather than a numbered file
descriptor.  Essentially when loading an elf executable the file
descriptor is duped to the name /proc/self/exe.  The implementation
otherwise is the same as /proc/self/fd/N.

The downside of course is that I expect if we were actually to change
/proc/self/exe from to point at the script instead of the shell some
piece of software somewhere would come melting down.  I am totally not
ready to consider that kind of mine field today.

Eric

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