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.