Why no pidfd's for threads?

[KJ Tsanaktsidis]

Hi all,

I've been working on some performance profiling tooling for Ruby with
eBPF/pref_event_open. I was hoping to be able to use pidfd's from my
profiler process to keep track of when threads from a Ruby program
being profiled had exited. Unfortunately, I was surprised to find out
that pidfd_open can't be used to get fd's for non-thread-group-leaders
(it just returns EINVAL).

The first version of the pidfd_open patch seemed to support this, but
the version finally merged in did not, and I couldn't see any
discussion about _why_ doing this for threads was blocked. It seemed
there were a few warts around pidfd_send_signal directed at threads,
but having the pidfd of a thread seems to be independently useful
without pidfd_send_signal (e.g. - I want to use it to be notified about
thread exits in my profiling tool in a race-free way).

Absent this functionality, I'll have to either live with the race
conditions or use ptrace. The profiler binary has CAP_HWMON and
CAP_BPF, but it having CAP_PTRACE too is substantially scarier!

I was mostly just curious to understand the rationale for why
pidfd_open can't work with threads?

Thanks,
KJ