Re: [PATCH v3 0/3] Support userspace-selected fds

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2020-04-07, Josh Triplett <josh@joshtriplett.org> wrote:
> (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

Aside from my specific comments and questions, the changes to openat2
deserve at least one or two selftests.

>  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(-)

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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