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

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Message-ID: <20150109224252.GY4574@brightrain.aerifal.cx>
Date:	Fri, 9 Jan 2015 17:42:52 -0500
From:	Rich Felker <dalias@...ifal.cx>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	David Drysdale <drysdale@...gle.com>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.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 Fri, Jan 09, 2015 at 10:33:00PM +0000, Al Viro wrote:
> On Fri, Jan 09, 2015 at 05:17:28PM -0500, Rich Felker wrote:
> > > Back then the procfs-free environments had been pushed as a serious argument
> > > in favour of merging the damn thing.  Now you guys turn around and say that
> > > we not only need procfs mounted, we need a yet-to-be-added kludge in there
> > > to cope with the actual intended uses.
> > 
> > Reverting does not fix the problem. There is no way to make fexecve
> > work for scripts without kernel support, and the needed kernel support
> > without fexecve would be even nastier, since handling of /proc/self/fd
> > magic-symlinks would need to be special-cased. The added fexecveat
> > syscall supports fully /proc-less operation for non-scripts.
> 
> Oh, yes it does.  It's not *our* problem if it's out of tree and not
> a part of ABI.  That way if you need it, *you* get to come up with clean
> implementation.  If it's in-tree you get leverage to push ugly kludges
> further in.  And frankly, I don't trust you to abstain from using that
> leverage in rather nasty ways.
> 
> Out of curiosity, how would you expect that "open only once" to work?
> All reliable variants I see are beyond sick...

Here's a very simple way it could work -- it could put the O_PATH fd
on a previously-unused fd number, and put a special flag on the fd,
like FD_CLOEXEC, but that causes the kernel to close it whenever it's
opened. The pathname passed could then simply be /dev/fd/%d or
/proc/self/fd/%d, and although this is presently dependent on /proc
being mounted, virtual /dev/fd/* could someday be something completely
independent of procfs. The kernel keeps all the freedom to choose how
to pass the name to the interpreter. I'm not proposing any kernel
API/ABI lock-in and I'm with you in opposing such lock-in.

Rich
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