[CC += linux-api] On Wed, 22 Apr 2020 at 07:19, Josh Triplett <josh@xxxxxxxxxxxxxxxx> wrote: > > 5.8 material, not intended for 5.7. Now includes a patch for man-pages, > attached to this cover letter. > > Inspired by the X protocol's handling of XIDs, allow userspace to select > the file descriptor opened by a call like openat2, so that it can use > the resulting file descriptor in subsequent system calls without waiting > for the response to the initial openat2 syscall. > > The first patch is independent of the other two; it allows reserving > file descriptors below a certain minimum for userspace-selected fd > allocation only. > > The second patch implements userspace-selected fd allocation for > openat2, introducing a new O_SPECIFIC_FD flag and an fd field in struct > open_how. In io_uring, this allows sequences like openat2/read/close > without waiting for the openat2 to complete. Multiple such sequences can > overlap, as long as each uses a distinct file descriptor. > > The third patch adds userspace-selected fd allocation to pipe2 as well. > I did this partly as a demonstration of how simple it is to wire up > O_SPECIFIC_FD support for any fd-allocating system call, and partly in > the hopes that this may make it more useful to wire up io_uring support > for pipe2 in the future. > > v5: > > Rename padding field to __padding. > Add tests for non-zero __padding. > Include patch for man-pages. > > v4: > > Changed fd field to __u32. > Expanded and consolidated checks that return -EINVAL for invalid arguments. > Simplified and commented build_open_how. > Add documentation comment for fd field. > Add kselftests. > > Thanks to Aleksa Sarai for feedback. > > v3: > > This new version has an API to atomically increase the minimum fd and > return the previous minimum, rather than just getting and setting the > minimum; this makes it easier to allocate a range. (A library that might > initialize after the program has already opened other file descriptors > may need to check for existing open fds in the range after reserving it, > and reserve more fds if needed; this can be done entirely in userspace, > and we can't really do anything simpler in the kernel due to limitations > on file-descriptor semantics, so this patch series avoids introducing > any extra complexity in the kernel.) > > This new version also supports a __get_specific_unused_fd_flags call > which accepts the limit for RLIMIT_NOFILE as an argument, analogous to > __get_unused_fd_flags, since io_uring needs that to correctly handle > RLIMIT_NOFILE. > > Thanks to Jens Axboe for review and feedback. > > v2: > > Version 2 was a version incorporated into a larger patch series from Jens Axboe > on io_uring. > > Josh Triplett (3): > fs: Support setting a minimum fd for "lowest available fd" allocation > fs: openat2: Extend open_how to allow userspace-selected fds > fs: pipe2: Support O_SPECIFIC_FD > > fs/fcntl.c | 2 +- > fs/file.c | 62 +++++++++++++++++-- > fs/io_uring.c | 3 +- > fs/open.c | 8 ++- > fs/pipe.c | 16 +++-- > include/linux/fcntl.h | 5 +- > include/linux/fdtable.h | 1 + > include/linux/file.h | 4 ++ > include/uapi/asm-generic/fcntl.h | 4 ++ > include/uapi/linux/openat2.h | 3 + > include/uapi/linux/prctl.h | 3 + > kernel/sys.c | 5 ++ > tools/testing/selftests/openat2/helpers.c | 2 +- > tools/testing/selftests/openat2/helpers.h | 21 +++++-- > .../testing/selftests/openat2/openat2_test.c | 35 ++++++++++- > 15 files changed, 150 insertions(+), 24 deletions(-) > > -- > 2.26.2 > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/
