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

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Message-ID: <87lhlbvbzs.fsf@x220.int.ebiederm.org>
Date:	Fri, 09 Jan 2015 16:13:27 -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 Fri, Jan 09, 2015 at 09:09:41PM +0000, Al Viro wrote:

> The "magic open-once magic symlink" approach is really the cleanest
> solution I can find. In the case where the interpreter does not open
> the script, nothing terribly bad happens; the magic symlink just
> sticks around until _exit or exec. In the case where the interpreter
> opens it more than once, you get a failure, but as far as I know
> existing interpreters don't do this, and it's arguably bad design. In
> any case it's a caught error.

And it doesn't work without introducing security vulnerabilities into
the kernel, because it breaks close-on-exec semantics.

All you have to do is pick a file descriptor, good canidates are 0 and
255 and make it a convention that that file descriptor is used for
fexecve.  At least when you want to support scripts.  Otherwise you can
set close-on-exec.

That results in no accumulation of file descriptors  because everyone
always uses the same file descriptor.

Regardless you don't have a patch and you aren't proposing code and the
code isn't actually broken so please go away.

Eric
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