Merge patch series "file: FD_{ADD,PREPARE}()"

Christian Brauner <brauner@kernel.org> says:

This now removes roughly double the code that it adds.

I've been playing with this to allow for moderately flexible usage of
the get_unused_fd_flags() + create file + fd_install() pattern that's
used quite extensively and requires cumbersome cleanup paths.

How callers allocate files is really heterogenous so it's not really
convenient to fold them into a single class. It's possibe to split them
into subclasses like for anon inodes. I think that's not necessarily
nice as well. This adds two primitives:

(1) FD_ADD() the simple cases a file is installed:

    fd = FD_ADD(O_CLOEXEC, vfio_device_open_file(device));
    if (fd < 0)
            vfio_device_put_registration(device);
    return fd;

(2) FD_PREPARE() that captures all the cases where access to fd or file
    or additional work before publishing the fd is needed:

    FD_PREPARE(fdf, O_CLOEXEC, sync_file->file);
    if (fdf.err) {
            fput(sync_file->file);
            return fdf.err;
    }

    data.fence = fd_prepare_fd(fdf);
    if (copy_to_user((void __user *)arg, &data, sizeof(data)))
            return -EFAULT;

    return fd_publish(fdf);

I've converted all of the easy cases over to it and it gets rid of an
aweful lot of convoluted cleanup logic. There are a bunch of other cases
that can also be converted after a bit of massaging.

It's centered around a simple struct. FD_PREPARE() encapsulates all of
allocation and cleanup logic and must be followed by a call to
fd_publish() which associates the fd with the file and installs it into
the callers fdtable. If fd_publish() isn't called both are deallocated.
FD_ADD() is a shorthand that does the fd_publish() and never exposes the
struct to the caller. That's often the case when they don't need access
to anything after installing the fd.

It mandates a specific order namely that first we allocate the fd and
then instantiate the file. But that shouldn't be a problem. Nearly
everyone I've converted used this order anyway.

There's a bunch of additional cases where it would be easy to convert
them to this pattern. For example, the whole sync file stuff in dma
currently returns the containing structure of the file instead of the
file itself even though it's only used to allocate files. Changing that
would make it fall into the FD_PREPARE() pattern easily. I've not done
that work yet.

There's room for extending this in a way that wed'd have subclasses for
some particularly often use patterns but as I said I'm not even sure
that's worth it.

* patches from https://patch.msgid.link/20251123-work-fd-prepare-v4-0-b6efa1706cfd@kernel.org: (47 commits)
  kvm: convert kvm_vcpu_ioctl_get_stats_fd() to FD_PREPARE()
  kvm: convert kvm_arch_supports_gmem_init_shared() to FD_PREPARE()
  io_uring: convert io_create_mock_file() to FD_PREPARE()
  file: convert replace_fd() to FD_PREPARE()
  vfio: convert vfio_group_ioctl_get_device_fd() to FD_PREPARE()
  tty: convert ptm_open_peer() to FD_PREPARE()
  ntsync: convert ntsync_obj_get_fd() to FD_PREPARE()
  media: convert media_request_alloc() to FD_PREPARE()
  hv: convert mshv_ioctl_create_partition() to FD_PREPARE()
  gpio: convert linehandle_create() to FD_PREPARE()
  dma: port sw_sync_ioctl_create_fence() to FD_PREPARE()
  pseries: port papr_rtas_setup_file_interface() to FD_PREPARE()
  pseries: convert papr_platform_dump_create_handle() to FD_PREPARE()
  spufs: convert spufs_gang_open() to FD_PREPARE()
  papr-hvpipe: convert papr_hvpipe_dev_create_handle() to FD_PREPARE()
  spufs: convert spufs_context_open() to FD_PREPARE()
  net/socket: convert __sys_accept4_file() to FD_PREPARE()
  net/socket: convert sock_map_fd() to FD_PREPARE()
  net/sctp: convert sctp_getsockopt_peeloff_common() to FD_PREPARE()
  net/kcm: convert kcm_ioctl() to FD_PREPARE()
  ...

Link: https://patch.msgid.link/20251123-work-fd-prepare-v4-0-b6efa1706cfd@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>