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

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Date:	Fri, 9 Jan 2015 20:33:24 -0500
From:	Rich Felker <dalias@...ifal.cx>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
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@...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 07:17:41PM -0600, Eric W. Biederman wrote:
> Rich Felker <dalias@...ifal.cx> writes:
> 
> > I'm not proposing code because I'm a libc developer not a kernel
> > developer. I know what's needed for userspace to provide a conforming
> > fexecve to applications, not how to implement that on the kernel side,
> > although I'm trying to provide constructive ideas. The hostility is
> > really not necessary.
> 
> Conforming to what?
> 
> The open group fexecve says nothing about requiring a file descriptor
> passed to fexecve to have O_CLOEXEC.

It doesn't require it but it allows it, and in multithreaded programs
that might run child processes (or library code that might be used in
such situations), O_CLOEXEC is mandatory everywhere to avoid fd leaks.

> Further looking at open group specification of exec it seems to indicate
> the preferred way to handle this is for the kernel to return O_NOEXEC
> and then libc gets to figure out how to run the shell script.  Is that
> the kind of ``conforming'' implementation you are looking for?

This is a complex issue, and does not apply to native #! support
(which is a supported executable format and thus not ENOEXEC) but
rather standard POSIX shell scripts (which don't have a #! line at
all). In this case the behavior of fexecve is perhaps under-specified.
However, in cases where execve would succeed (without causing
ENOEXEC), I think it's at least undesirable, if not non-conforming,
for fexecve to fail.

Should we request clarification from the Austin Group?

Rich
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