(Note: numbering this updated version v3, to avoid confusion with Jens' v2 that built on my v1. Jens, if you like this approach, please feel free to stack your additional patches from the io_uring-fd-select branch atop this series. 5.8 material, not intended for the current merge window.) 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. If this gets accepted, I'm happy to also write corresponding manpage patches. 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. 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 | 6 ++-- 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 | 2 ++ include/uapi/linux/prctl.h | 3 ++ kernel/sys.c | 5 +++ 12 files changed, 97 insertions(+), 16 deletions(-) -- 2.26.0
