[PATCH v1 0/4] pid: add pidctl()

[Christian Brauner <christian@brauner.io>]

This is v1 of this patchset with various minor fixes which are listed in
the individual commits. Notably, pidfds are now O_CLOEXEC by default.

The pidctl() syscalls builds on, extends, and improves translate_pid()
[4] and serves as the natural connection between the pid-based and the
pidfd-based api.

I quote Konstantins original patchset first that has already been acked
and picked up by Eric before and whose functionality is preserved in
this syscall. Multiple people have asked when this patchset will be sent
in for merging (cf. [1], [2]). It has recently been revived by
Nagarathnam Muthusamy from Oracle [3].

The intention of the original translate_pid() syscall was twofold:
1. Provide translation of pids between pid namespaces especially for the
   case of deeply nested pid namespaces.
   The most obvious use-case is strace which has been waiting for this
   feature for a while.
2. Provide implicit pid namespace introspection

Both functionalities are preserved. The latter task has been improved
upon though. In the original version of the pachset passing pid as 1
would allow to deterimine the relationship between the pid namespaces.
This is inherhently racy. If pid 1 inside a pid namespace has died it
would report false negatives. For example, if pid 1 inside of the target
pid namespace already died, it would report that the target pid
namespace cannot be reached from the source pid namespace because it
couldn't find the pid inside of the target pid namespace and thus
falsely report to the user that the two pid namespaces are not related.
This problem is simple to avoid. In the new version we simply walk the
list of ancestors and check whether the namespace are related to each
other. By doing it this way we can reliably report what the relationship
between two pid namespace file descriptors looks like.

Additionally, this syscall has been extended to allow the retrieval of
pidfds independent of procfs. These pidfds can e.g. be used with the new
pidfd_send_signal() syscall we recently merged. The ability to retrieve
pidfds independent of procfs had already been requested in the
pidfd_send_signal patchset by e.g. Andrew [4] and later again by Alexey
[5]. A use-case where a kernel is compiled without procfs but where
pidfds are still useful has been outlined by Andy in [6]. Regular
anon-inode based file descriptors are used that stash a reference to
struct pid in file->private_data and drop that reference on close.

With this pidctl() has three closely related functionalities that
provide a natural connection between the pid-based and the pidfd-based
api. To clarify the semantics and to make it easier for userspace to use
the syscall it has a command argument and three commands clearly
reflecting the functionalities (PIDCMD_QUERY_PID, PIDCMD_QUERY_PIDNS,
PIDCMD_GET_PIDFD).

Embedding the retrieval of pidfds into this syscall has two main
advantages:
- pidctl provides a natural and clean connection between the traditional
  pid-based and the newer pidfd-based process API
- allows the retrieval of pidfds for other pid namespaces while
  enforcing that
  - the caller must have been given access to two file descriptors
    referring to target and source pid namespace
  - the source pid namespace must be an ancestor of the target pid
    namespace
  - the pid must be translatable from the source pid namespace into the
    target pid namespace

Note that this patchset also includes Al's and David's commit to make anon
inodes unconditional. The original intention is to make it possible to use
anon inodes in core vfs functions. pidctl() has the same requirement so
David suggested I sent this in alongside this patch. Both are informed of
this.

The syscall comes with extensive testing for all functionalities.

/* References */
[1]: https://lore.kernel.org/lkml/37b17950-b130-7933-99a1-4846c61c8555@oracle.com/
[2]: https://lore.kernel.org/lkml/20181109034919.GA21681@altlinux.org/
[3]: https://lore.kernel.org/lkml/37b17950-b130-7933-99a1-4846c61c8555@oracle.com/
[4]: 3eb39f47934f9d5a3027fe00d906a45fe3a15fad
[5]: https://lore.kernel.org/lkml/20190320203910.GA2842@avx2/
[6]: https://lore.kernel.org/lkml/CALCETrXO=V=+qEdLDVPf8eCgLZiB9bOTrUfe0V-U-tUZoeoRDA@mail.gmail.com/

Thanks!
Christian

Christian Brauner (3):
  pid: add pidctl()
  signal: support pidctl() with pidfd_send_signal()
  tests: add pidctl() tests

David Howells (1):
  Make anon_inodes unconditional

 arch/arm/kvm/Kconfig                        |   1 -
 arch/arm64/kvm/Kconfig                      |   1 -
 arch/mips/kvm/Kconfig                       |   1 -
 arch/powerpc/kvm/Kconfig                    |   1 -
 arch/s390/kvm/Kconfig                       |   1 -
 arch/x86/Kconfig                            |   1 -
 arch/x86/entry/syscalls/syscall_32.tbl      |   1 +
 arch/x86/entry/syscalls/syscall_64.tbl      |   1 +
 arch/x86/kvm/Kconfig                        |   1 -
 drivers/base/Kconfig                        |   1 -
 drivers/char/tpm/Kconfig                    |   1 -
 drivers/dma-buf/Kconfig                     |   1 -
 drivers/gpio/Kconfig                        |   1 -
 drivers/iio/Kconfig                         |   1 -
 drivers/infiniband/Kconfig                  |   1 -
 drivers/vfio/Kconfig                        |   1 -
 fs/Makefile                                 |   2 +-
 fs/notify/fanotify/Kconfig                  |   1 -
 fs/notify/inotify/Kconfig                   |   1 -
 include/linux/pid.h                         |   2 +
 include/linux/pid_namespace.h               |   8 +
 include/linux/syscalls.h                    |   2 +
 include/uapi/linux/wait.h                   |  14 +
 init/Kconfig                                |  10 -
 kernel/pid.c                                | 161 ++++++
 kernel/pid_namespace.c                      |  25 +
 kernel/signal.c                             |  29 +-
 kernel/sys_ni.c                             |   3 -
 tools/testing/selftests/pidfd/Makefile      |   2 +-
 tools/testing/selftests/pidfd/pidctl_test.c | 537 ++++++++++++++++++++
 30 files changed, 765 insertions(+), 48 deletions(-)
 create mode 100644 tools/testing/selftests/pidfd/pidctl_test.c

-- 
2.21.0