[PATCHSET RFC 0/6] add support for name_to, open_by_handle_at(2) to io_uring

[Thomas Bertschinger <tahbertschinger@gmail.com>]

This series adds support for name_to_handle_at() and open_by_handle_at()
to io_uring. The idea is for these opcodes to be useful for userspace
NFS servers that want to use io_uring.

name_to_handle_at()
===================

Support for name_to_handle_at() is added in patches 1 and 2.

In order to do a non-blocking name_to_handle_at(), a new helper
do_name_to_handle_at() is created that takes a lookup_flags argument.

This is to support non-blocking lookup when called with
IO_URING_F_NONBLOCK--user_path_at() will be called with LOOKUP_CACHED
in that case.

Aside from the lookup, I don't think there is anything else that
do_name_to_handle_at() does that would be a problem in the non-blocking
case. There is a GFP_KERNEL allocation:

do_name_to_handle_at()
  -> do_path_to_handle()
    -> kzalloc(..., GFP_KERNEL)

But I think that's OK? Let me know if there's anything else I'm
missing...

open_by_handle_at()
===================

Patch 3 is a fixup to fhandle.c:do_handle_open() that (I believe) fixes
a bug and can exist independently of this series, but it fits in with
these changes so I'm including it here.

Support for open_by_handle_at() is added in patches 4 - 6.

A helper __do_handle_open() is created that does the file open without
installing a file descriptor for it. This is needed because io_uring
needs to decide between using a file descriptor or a fixed file.

No attempt is made to support a non-blocking open_by_handle_at()--the
attempt is always immediately returned with -EAGAIN if
IO_URING_F_NONBLOCK is set.

This isn't ideal and it would be nice to add support for non-blocking
open by handle in the future. This would presumably require updates to
the ->encode_fh() implementation for filesystems that want to
support this.

I see that lack of support for non-blocking operation was a dealbreaker
for adding getdents to io_uring previously:

https://lore.kernel.org/io-uring/20230428050640.GA1969623@dread.disaster.area/

On the other hand, AFAICT, support for openat() was originally added in
15b71abe7b52 (io_uring: add support for IORING_OP_OPENAT) without a non-
blocking lookup, and the possibility of non-blocking lookup later added
in 3a81fd02045c (io_uring: enable LOOKUP_CACHED path resolution for
filename lookups).

(To be honest I'm a little confused by the history here. The commit
message of 15b71abe7b52 says

> For the normal case of a non-blocking path lookup this will complete
> inline. If we have to do IO to perform the open, it'll be done from
> async context.

but from the commit contents this would NOT appear to be the case: 

> +       if (force_nonblock) {
> +               req->work.flags |= IO_WQ_WORK_NEEDS_FILES;
> +               return -EAGAIN;
> +       }

until the support is really added in the later commit. Am I confused or
is the commit message wrong?)

In any event, based on my reading of the history, it would appear to be
OK to add open_by_handle_at() initially without support for inline
completion, and then later add that when the filesystem implementations
can be updated to support this.

Please let me know if I am wrong on my interpretation of the history or
if anyone disagrees with the conclusion.

Testing
=======

A liburing branch that includes support for the new opcodes, as well as
a test, is available at:

https://github.com/bertschingert/liburing/tree/open_by_handle_at

To run the test:

$ ./test/open_by_handle_at.t

Thomas Bertschinger (6):
  fhandle: create helper for name_to_handle_at(2)
  io_uring: add support for IORING_OP_NAME_TO_HANDLE_AT
  fhandle: do_handle_open() should get FD with user flags
  fhandle: create __do_handle_open() helper
  io_uring: add __io_open_prep() helper
  io_uring: add support for IORING_OP_OPEN_BY_HANDLE_AT

 fs/fhandle.c                  |  85 ++++++++++++---------
 fs/internal.h                 |   9 +++
 include/uapi/linux/io_uring.h |   2 +
 io_uring/opdef.c              |  14 ++++
 io_uring/openclose.c          | 137 +++++++++++++++++++++++++++++++---
 io_uring/openclose.h          |   5 ++
 6 files changed, 209 insertions(+), 43 deletions(-)

-- 
2.50.1