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

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Message-ID: <20150109204815.GR4574@brightrain.aerifal.cx>
Date:	Fri, 9 Jan 2015 15:48:15 -0500
From:	Rich Felker <dalias@...ifal.cx>
To:	David Drysdale <drysdale@...gle.com>
Cc:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andy Lutomirski <luto@...capital.net>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	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 05:46:28PM +0000, David Drysdale wrote:
> > It's AT_EXECFN,
> > /proc/self/exe, and filenames shown elsewhere in /proc that may be
> > derived in odd ways.
> >
> > I would also move the text about O_CLOEXEC to a BUGS or NOTES section
> > rather than the main description. The long-term intent should be that
> > script execution this way should work. IIRC this was discussed earlier
> > in the thread.
> 
> I may be misremembering, but I thought we hoped to be able to fix
> execveat of a script without /proc in future, but didn't expect to fix
> execveat of a script via an O_CLOEXEC fd (because in the latter
> case the fd gets closed before the script interpreter runs, so even
> if the interpreter (or a special filesystem) does clever things for names
> starting with "/dev/fd/..." the file descriptor is already gone).

I think this is a case that needs to be fixed, though it's hard. The
normal correct usage for fexecve is to always pass an O_CLOEXEC file
descriptor, and the caller can't really be expected to know whether
the file is a script or not. We discussed workarounds before and one
idea I proposed was having fexecve provide a "one open only" magic
symlink in /proc/self/ to pass to the interpreter. It would behave
like an O_PATH file descriptor magic symlink in /proc/self/fd, but
would automatically cease to exist on the first open (at which point
the interpreter would have a real O_RDONLY file descriptor for the
underlying file).

Rich
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