This adds custom filtering for IORING_OP_OPENAT and IORING_OP_OPENAT2,
where the open_how flags, mode, and resolve can be checked by filters.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

This works just like pipe2(2), except it also supports fixed file
descriptors. Used in a similar fashion as for other fd instantiating
opcodes (like accept, socket, open, etc), where sqe->file_slot is set
appropriately if two direct descriptors are desired rather than a set
of normal file descriptors.

sqe->addr must be set to a pointer to an array of 2 integers, which
is where the fixed/normal file descriptors are copied to.

sqe->pipe_flags contains flags, same as what is allowed for pipe2(2).

Future expansion of per-op private flags can go in sqe->ioprio,
like we do for other opcodes that take both a "syscall" flag set and
an io_uring opcode specific flag set.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring can currently open/close regular files or fixed/direct
descriptors. Or you can instantiate a fixed descriptor from a regular
one, and then close the regular descriptor. But you currently can't turn
a purely fixed/direct descriptor into a regular file descriptor.

IORING_OP_FIXED_FD_INSTALL adds support for installing a direct
descriptor into the normal file table, just like receiving a file
descriptor or opening a new file would do. This is all nicely abstracted
into receive_fd(), and hence adding support for this is truly trivial.

Since direct descriptors are only usable within io_uring itself, it can
be useful to turn them into real file descriptors if they ever need to
be accessed via normal syscalls. This can either be a transitory thing,
or just a permanent transition for a given direct descriptor.

By default, new fds are installed with O_CLOEXEC set. The application
can disable O_CLOEXEC by setting IORING_FIXED_FD_NO_CLOEXEC in the
sqe->install_fd_flags member.

Suggested-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Put it with the filetable code, which is where it belongs. While doing
so, have the helpers take a ctx rather than an io_kiocb. It doesn't make
sense to use a request, as it's not an operation on the request itself.
It applies to the ring itself.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Jens Axboe <axboe@kernel.dk>