[PATCH WIP 0/3] Support userspace-selected fds

[Josh Triplett <josh@joshtriplett.org>]

Prototype of the proposal I made at
https://lore.kernel.org/io-uring/20200211223235.GA25104@localhost/ . See
also the discussion at
https://twitter.com/josh_triplett/status/1235004009502494721 .

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 a new 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.

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                    |  2 +-
 fs/open.c                        |  6 ++--
 fs/pipe.c                        | 15 +++++---
 include/linux/fcntl.h            |  5 +--
 include/linux/fdtable.h          |  1 +
 include/linux/file.h             |  3 ++
 include/uapi/asm-generic/fcntl.h |  4 +++
 include/uapi/linux/openat2.h     |  2 ++
 include/uapi/linux/prctl.h       |  4 +++
 kernel/sys.c                     | 10 ++++++
 12 files changed, 100 insertions(+), 16 deletions(-)

-- 
2.25.1