On Thu, Oct 17, 2024 at 10:05:49PM GMT, Lorenzo Stoakes wrote:

The means by which a pid is determined from a pidfd is duplicated, with
some callers holding a reference to the (pid)fd, and others explicitly
pinning the pid.

Introduce __pidfd_get_pid() which narrows this to one approach of pinning
the pid, with an optional output parameters for file->f_flags to avoid the
need to hold onto a file to retrieve this.

Additionally, allow the ability to open a pidfd by opening a /proc/<pid>
directory, utilised by the pidfd_send_signal() system call, providing a
pidfd_get_pid_proc() helper function to do so.

Doing this allows us to eliminate open-coded pidfd pid lookup and to
consistently handle this in one place.

This lays the groundwork for a subsequent patch which adds a new sentinel
pidfd to explicitly reference the current process (i.e. thread group
leader) without the need for a pidfd.

Signed-off-by: Lorenzo Stoakes <redacted>

Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>