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

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Date:	Fri, 09 Jan 2015 15:20:04 -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 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.

This certainly does not break the intended usage for execveat.

Eric

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