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(-)