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

[Andy Lutomirski]

On Fri, Jan 9, 2015 at 3:12 PM, Rich Felker <dalias@xxxxxxxxxx> 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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