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

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Message-ID: <CALCETrUpr5kqNm5M5z=RxM9T7DOQB3-Le2gxGM=D7+cpWVRQaQ@mail.gmail.com>
Date:	Fri, 9 Jan 2015 15:24:12 -0800
From:	Andy Lutomirski <luto@...capital.net>
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>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	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 9, 2015 at 3:12 PM, Rich Felker <dalias@...ifal.cx> wrote:
> On Fri, Jan 09, 2015 at 10:57:43PM +0000, Al Viro wrote:
>> On Fri, Jan 09, 2015 at 05:42:52PM -0500, Rich Felker wrote:
>>
>> > Here's a very simple way it could work -- it could put the O_PATH fd
>> > on a previously-unused fd number, and put a special flag on the fd,
>> > like FD_CLOEXEC, but that causes the kernel to close it whenever it's
>> > opened. The pathname passed could then simply be /dev/fd/%d or
>> > /proc/self/fd/%d, and although this is presently dependent on /proc
>> > being mounted, virtual /dev/fd/* could someday be something completely
>> > independent of procfs. The kernel keeps all the freedom to choose how
>> > to pass the name to the interpreter. I'm not proposing any kernel
>> > API/ABI lock-in and I'm with you in opposing such lock-in.
>>
>> Huh?  open() on procfs symlinks does *NOT* work the way - the symlink is
>> traversed and after that point there is no information whatsoever how we
>> got to that vfsmount/dentry pair.  I can imagine several kludges that would
>> work, but they are unspeakably ugly, and do_last() is already far too
>> convoluted as it is.
>
> I'm not sure where you're disagreeing with me. open of procfs symlinks
> does not resolve the symlink and open the resulting pathname. They are
> "magic symlinks" which are bound to the inode of the open file. I
> don't see why this action, which is already special for magic
> symlinks, can't check a flag on the magic symlink and possibly close
> the corresponding file descriptor as part of its action.
>
> In any case, whether/how fexecve works with interpreters is something
> the kernel can change without breaking userspace expectations. My goal
> is to avoid creating any new API/ABI requirement here.
>

I think that, if we really want to support clean fexecve on O_CLOEXEC
scripts some day, the right way to do it is to fix the script
interface for real.  Have a special flag in the headers of script
interpreters that support a new interface that says "when I'm a script
interpreter, I expect an auxv entry AT_SCRIPT_FD with an  open fd with
CLOEXEC set".  Then we can directly exec scripts by fd, even with
O_CLOEXEC set, without any races.

--Andy
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