On Sat, Mar 30, 2019 at 05:50:20PM +0000, Jonathan Kowalski wrote:

On Sat, Mar 30, 2019 at 5:24 PM Linus Torvalds
[off-list ref] wrote:

quoted

On Sat, Mar 30, 2019 at 10:12 AM Christian Brauner [off-list ref] wrote:

quoted

To clarify, what the Android guys really wanted to be part of the api is
a way to get race-free access to metadata associated with a given pidfd.
And the idea was that *if and only if procfs is mounted* you could do:

int pidfd = pidfd_open(1234, 0);

int procfd = open("/proc", O_RDONLY | O_CLOEXEC);
int procpidfd = ioctl(pidfd, PIDFD_TO_PROCFD, procfd);

And my claim is that this is three system calls - one of them very
hacky - to just do

    int pidfd = open("/proc/%d", O_PATH);

and you're done. It acts as the pidfd _and_ the way to get the
associated status files etc.

So there is absolutely zero advantage to going through pidfd_open().

No. No. No.

So the *only* reason for "pidfd_open()" is if you don't have /proc in
the first place. In which case the whole PIDFD_TO_PROCFD is bogus.

Yeah, yeah, if you want to avoid going through the pathname
translation, that's one thing, but if that's your aim, then you again
should also just admit that PIDFD_TO_PROCFD is disgusting and wrong,
and you're basically saying "ok, I'm not going to do /proc at all".

So I'm ok with the whole "simpler, faster, no-proc pidfd", but then it
really has to be *SIMPLER* and *NO PROCFS*.

(Resending because accidently it wasn't a reply-all)

If you go with pidfd_open, that should also mean you remove the
ability to be able to use /proc/<PID> dir fds in pidfd_send_signal.

Otherwise the semantics are hairy: I can only pidfd_open a task
reachable from my active namespace, but somehow also be able to open a

You can easily setns() to another pid namespace and get a pidfd there.
That's how most namespace interactions work right now. We already had
that discussion.