On Sat, Jan 10, 2015 at 04:27:23PM -0600, Eric W. Biederman wrote:

Rich Felker [off-list ref] writes:

quoted

On Sat, Jan 10, 2015 at 04:14:57AM +0000, Al Viro wrote:

quoted

quoted

Except that if your interpreter does stat(2) (or access(2), or getxattr(2),
etc.) before bothering with open(2), you'll get screwed.

Yes, but I think that would be very bad interpreter design.
stat/getxattr/access/whatever followed by open is always a TOCTOU
race. The correct sequence of actions is always open followed by
fstat/fgetxattr/...

Sigh.  I think everyone who has looked at this has been blind.

If userspace is reasonable all we have to do is fix /proc/self/exe
for shell scripts to point at the actual script,
and then pass /proc/self/exe on the shell scripts command line.

At a practical level we have to worry about backwards compability and
chroot jails.  But the existence of a clean implementation with
/proc/self/exe serves a proof of concept that it would not be too
difficult.  When someone cares enough to implement it.

Is /proc/self/exe a "magic symlink" that's bound to the inode, or just
a regular symlink? In the latter case it defeats the whole purpose of
using O_EXEC fds and fexecve rather than pathnames.

Rich