[RFC PATCH 0/3] introduce PIDFD_SELF

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

If you wish to utilise a pidfd interface to refer to the current process
(from the point of view of userland - from the kernel point of view - the
thread group leader), it is rather cumbersome, requiring something like:

	int pidfd = pidfd_open(getpid(), 0);

	...

	close(pidfd);

Or the equivalent call opening /proc/self. It is more convenient to use a
sentinel value to indicate to an interface that accepts a pidfd that we
simply wish to refer to the current process.

This series introduces such a sentinel, PIDFD_SELF, which can be passed as
the pidfd in this instance rather than having to establish a dummy fd for
this purpose.

The only pidfd interface where this is particularly useful is
process_madvise(), which provides the motivation for this series. However,
as this is a general interface, we ensure that all pidfd interfaces can
handle this correctly.

We ensure that pidfd_send_signal() and pidfd_getfd() work correctly, and
assert as much in selftests. All other interfaces except setns() will work
implicitly with this new interface, however it doesn't make sense to test
waitid(P_PIDFD, ...) as waiting on ourselves is a blocking operation.

In the case of setns() we explicitly disallow use of PIDFD_SELF as it
doesn't make sense to obtain the namespaces of our own process, and it
would require work to implement this functionality there that would be of
no use.

We also do not provide the ability to utilise PIDFD_SELF in ordinary fd
operations such as open() or poll(), as this would require extensive work
and be of no real use.

Lorenzo Stoakes (3):
  pidfd: refactor pidfd_get_pid/to_pid() and de-duplicate pid lookup
  pidfd: add PIDFD_SELF sentinel to refer to own process
  selftests: pidfd: add tests for PIDFD_SELF

 include/linux/pid.h                           | 43 +++++++++++-
 include/uapi/linux/pidfd.h                    |  3 +
 kernel/exit.c                                 |  3 +-
 kernel/nsproxy.c                              |  1 +
 kernel/pid.c                                  | 70 +++++++++++++------
 kernel/signal.c                               | 26 ++-----
 tools/testing/selftests/pidfd/pidfd.h         |  5 ++
 .../selftests/pidfd/pidfd_getfd_test.c        | 38 ++++++++++
 .../selftests/pidfd/pidfd_setns_test.c        |  6 ++
 tools/testing/selftests/pidfd/pidfd_test.c    | 13 ++++
 10 files changed, 165 insertions(+), 43 deletions(-)

-- 
2.46.2