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

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Message-ID: <20150109210941.GL22149@ZenIV.linux.org.uk>
Date:	Fri, 9 Jan 2015 21:09:41 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Rich Felker <dalias@...ifal.cx>
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 03:59:26PM -0500, Rich Felker wrote:

> > For fsck sake, folks, if you have bloody /proc, you don't need that shite
> > at all!  Just do execve on /proc/self/fd/n, and be done with that.
> > 
> > The sole excuse for merging that thing in the first place had been
> > "would anybody think of children^Wsclerotic^Whardened environments
> > where they have no /proc at all".
> 
> That doesn't work. With O_CLOEXEC, /proc/self/fd/n is already gone at
> the time the interpreter runs, whether you're using fexecveat or
> execve with "/proc/self/fd/n" to implement POSIX fexecve(). That's the
> problem. This breaks the intended idiom for fexecve.

Just what will your magical symlink do in case when the file is opened,
unlinked and marked O_CLOEXEC?  When should actual freeing of disk blocks,
etc. happen?  And no, you can't assume that interpreter will open the
damn thing even once - there's nothing to oblige it to do so.

Al, more and more tempted to ask reverting the whole thing - this hardcoded
/dev/fd/... (in fs/exec.c, no less) is disgraceful enough, but threats of
even more revolting kludges in the name of "intended idiom for fexecve"...
--
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