The pidctl() syscalls builds on, extends, and improves translate_pid() [4]. 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 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 translate_pid() has three closely related but still distinct functionalities. To clarify the semantics and to make it easier for userspace to use the syscall it has: - gained a command argument and three commands clearly reflecting the distinct functionalities (PIDCMD_QUERY_PID, PIDCMD_QUERY_PIDNS, PIDCMD_GET_PIDFD). - been renamed to pidctl() By gaining support for cleanly retrieving pidfds this syscall connects the traditional pid-based and the newer pidfd-based process API in a natural and clean way. Another advantage is that embedding this functionality into pidctl() let's us avoid adding another syscall just serving the single purpose of retrieving a pidfd. The flag argument allows to atomically set the cloexec when retrieving pidfds. 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@xxxxxxxxxx/ [2]: https://lore.kernel.org/lkml/20181109034919.GA21681@xxxxxxxxxxxx/ [3]: https://lore.kernel.org/lkml/37b17950-b130-7933-99a1-4846c61c8555@xxxxxxxxxx/ [4]: 3eb39f47934f9d5a3027fe00d906a45fe3a15fad [5]: https://lore.kernel.org/lkml/20190320203910.GA2842@avx2/ [6]: https://lore.kernel.org/lkml/CALCETrXO=V=+qEdLDVPf8eCgLZiB9bOTrUfe0V-U-tUZoeoRDA@xxxxxxxxxxxxxx/ 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 | 17 + init/Kconfig | 10 - kernel/pid.c | 162 ++++++ kernel/pid_namespace.c | 25 + kernel/signal.c | 20 +- kernel/sys_ni.c | 3 - tools/testing/selftests/pidfd/Makefile | 2 +- tools/testing/selftests/pidfd/pidctl_test.c | 553 ++++++++++++++++++++ 30 files changed, 782 insertions(+), 42 deletions(-) create mode 100644 tools/testing/selftests/pidfd/pidctl_test.c -- 2.21.0
