On Mon, Mar 25, 2019 at 8:34 PM Jann Horn [off-list ref] wrote:

 [...SNIP...]

Please don't do that. /proc/$pid/fd refers to the set of file
descriptors the process has open, and semantically doesn't have much
to do with the identity of the process. If you want to have a procfs
directory entry for getting a pidfd, please add a new entry. (Although
I don't see the point in adding a new procfs entry for this when you
could instead have an ioctl or syscall operating on the procfs
directory fd.)

There is no new entry. What I was saying (and I should have been
clearer) is that the existing entry for the fd when open'd with
O_DIRECTORY makes the kernel resolve the symlink to /proc/<PID> of the
process it maps to, so it would become:

  int dirfd = open("/proc/self/fd/3", O_DIRECTORY|O_CLOEXEC);

This also means you cannot cross the filesystem boundry, the said
process needs to have a visible entry (which would mean hidepid= and
gid= based access controls are honored), and you can only open the
dirfd of a process in the current ns (as the PID will not map to an
existent process if the pidfd maps to a process not in the same or
children pid ns, in fdinfo it lists -1 in the pid field (we might not
even need fdinfo anymore)).