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 16:31:18 -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 03:20:04PM -0600, Eric W. Biederman wrote:
> Rich Felker <dalias@...ifal.cx> writes:
> 
> > On Fri, Jan 09, 2015 at 08:56:26PM +0000, Al Viro wrote:
> >> On Fri, Jan 09, 2015 at 03:48:15PM -0500, Rich Felker wrote:
> >> > 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).
> >> 
> >> 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.
> 
> O_CLOEXEC with a #! intepreter can not work.  If the file descriptor is
> closed a #! interpreter can not open it.   So I don't know why or how
> you want that to work but it is nonsense.

The why is simple: fexecve always expects a close-on-exec file
descriptor. Otherwise the program being executed would need to take a
special option telling it to close the spurious fd it inherits. Most
programs don't have such an option, and there's no way to do it
without application-specific knowledge.

The how is difficult, but it can be done.

Rich
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