[GIT PULL 17/17 for v6.19] vfs fd prepare minimal

[Christian Brauner <brauner@kernel.org>]

Hey Linus,

/* Summary */
This is an alternative pull request for the FD_{ADD,PREPARE}() work containing
only parts of the conversion. Again, this work came late in the cycle but the
series is quite nice and worth doing. It removes roughly double the code that
it adds and eliminates a lot of convoluted cleanup logic across the kernel.

This adds the FD_ADD() and FD_PREPARE() primitive. They simplify the
common pattern of get_unused_fd_flags() + create file + fd_install()
that is used extensively throughout the kernel and currently requires
cumbersome cleanup paths.

FD_ADD() - For simple cases where a file is installed immediately:

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

FD_PREPARE() - For cases requiring access to the fd or file, or
additional work before publishing:

  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);

The primitives are centered around struct fd_prepare. FD_PREPARE()
encapsulates all 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 caller's fdtable. If fd_publish() isn't called, both are
deallocated automatically. FD_ADD() is a shorthand that does
fd_publish() immediately and never exposes the struct to the caller.

I've implemented this in a way that it's compatible with the cleanup
infrastructure while also being usable separately. IOW, it's centered
around struct fd_prepare which is aliased to class_fd_prepare_t and so
we can make use of all the basica guard infrastructure.

/* Testing */

gcc (Debian 14.2.0-19) 14.2.0
Debian clang version 19.1.7 (3+b1)

No build failures or warnings were observed.

/* Conflicts */

Merge conflicts with mainline or other vfs branches
===================================================

diff --cc include/linux/cleanup.h
index 19c7e475d3a4,361104bcfe92..b8bd2f15f91f
--- a/include/linux/cleanup.h
+++ b/include/linux/cleanup.h
@@@ -290,16 -294,18 +294,19 @@@ static inline class_##_name##_t class_#
  	class_##_name##_t var __cleanup(class_##_name##_destructor) =	\
  		class_##_name##_constructor
  
+ #define CLASS_INIT(_name, _var, _init_expr)                             \
+         class_##_name##_t _var __cleanup(class_##_name##_destructor) = (_init_expr)
+ 
 -#define scoped_class(_name, var, args)                          \
 -	for (CLASS(_name, var)(args);                           \
 -	     __guard_ptr(_name)(&var) || !__is_cond_ptr(_name); \
 -	     ({ goto _label; }))                                \
 -		if (0) {                                        \
 -_label:                                                         \
 -			break;                                  \
 +#define __scoped_class(_name, var, _label, args...)        \
 +	for (CLASS(_name, var)(args); ; ({ goto _label; })) \
 +		if (0) {                                   \
 +_label:                                                    \
 +			break;                             \
  		} else
  
 +#define scoped_class(_name, var, args...) \
 +	__scoped_class(_name, var, __UNIQUE_ID(label), args)
 +
  /*
   * DEFINE_GUARD(name, type, lock, unlock):
   *	trivial wrapper around DEFINE_CLASS() above specifically
diff --cc ipc/mqueue.c
index 83d9466710d6,d3a588d0dcf6..c118ca2c377a
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@@ -892,15 -892,36 +892,34 @@@ static int prepare_open(struct dentry *
  	return inode_permission(&nop_mnt_idmap, d_inode(dentry), acc);
  }
  
+ static struct file *mqueue_file_open(struct filename *name,
+ 				     struct vfsmount *mnt, int oflag, bool ro,
+ 				     umode_t mode, struct mq_attr *attr)
+ {
 -	struct path path __free(path_put) = {};
+ 	struct dentry *dentry;
++	struct file *file;
+ 	int ret;
+ 
 -	dentry = lookup_noperm(&QSTR(name->name), mnt->mnt_root);
++	dentry = start_creating_noperm(mnt->mnt_root, &QSTR(name->name));
+ 	if (IS_ERR(dentry))
+ 		return ERR_CAST(dentry);
+ 
 -	path.dentry = dentry;
 -	path.mnt = mntget(mnt);
 -
 -	ret = prepare_open(path.dentry, oflag, ro, mode, name, attr);
++	ret = prepare_open(dentry, oflag, ro, mode, name, attr);
+ 	if (ret)
 -		return ERR_PTR(ret);
 -
 -	return dentry_open(&path, oflag, current_cred());
++		file = ERR_PTR(ret);
++	else
++		file = dentry_open(&(const struct path){ .mnt = mnt, .dentry = dentry },
++				   oflag, current_cred());
++	end_creating(dentry);
++	return file;
+ }
+ 
  static int do_mq_open(const char __user *u_name, int oflag, umode_t mode,
  		      struct mq_attr *attr)
  {
+ 	struct filename *name __free(putname) = NULL;;
  	struct vfsmount *mnt = current->nsproxy->ipc_ns->mq_mnt;
--	struct dentry *root = mnt->mnt_root;
- 	struct filename *name;
- 	struct path path;
- 	int fd, error;
 -	int fd;
--	int ro;
++	int fd, ro;
  
  	audit_mq_open(oflag, mode, attr);
  
@@@ -908,35 -929,12 +927,10 @@@
  	if (IS_ERR(name))
  		return PTR_ERR(name);
  
- 	fd = get_unused_fd_flags(O_CLOEXEC);
- 	if (fd < 0)
- 		goto out_putname;
- 
  	ro = mnt_want_write(mnt);	/* we'll drop it in any case */
- 	path.dentry = start_creating_noperm(root, &QSTR(name->name));
- 	if (IS_ERR(path.dentry)) {
- 		error = PTR_ERR(path.dentry);
- 		goto out_putfd;
- 	}
- 	path.mnt = mnt;
- 	error = prepare_open(path.dentry, oflag, ro, mode, name, attr);
- 	if (!error) {
- 		struct file *file = dentry_open(&path, oflag, current_cred());
- 		if (!IS_ERR(file))
- 			fd_install(fd, file);
- 		else
- 			error = PTR_ERR(file);
- 	}
- out_putfd:
- 	if (error) {
- 		put_unused_fd(fd);
- 		fd = error;
- 	}
- 	end_creating(path.dentry);
 -	inode_lock(d_inode(root));
+ 	fd = FD_ADD(O_CLOEXEC, mqueue_file_open(name, mnt, oflag, ro, mode, attr));
 -	inode_unlock(d_inode(root));
  	if (!ro)
  		mnt_drop_write(mnt);
- out_putname:
- 	putname(name);
  	return fd;
  }
  

Merge conflicts with other trees
================================

[1]: https://lore.kernel.org/linux-next/20251125122934.36f75838@canb.auug.org.au

[2]: https://lore.kernel.org/linux-next/20251125171130.67ba74e1@canb.auug.org.au

The following changes since commit 3a8660878839faadb4f1a6dd72c3179c1df56787:

  Linux 6.18-rc1 (2025-10-12 13:42:36 -0700)

are available in the Git repository at:

  git@gitolite.kernel.org:pub/scm/linux/kernel/git/vfs/vfs tags/vfs-6.19-rc1.fd_prepare.fs

for you to fetch changes up to 0512bf9701f339c8fee2cc82b6fc35f0a8f6be7a:

  Merge patch series "file: FD_{ADD,PREPARE}()" (2025-11-28 12:42:36 +0100)

Please consider pulling these changes from the signed vfs-6.19-rc1.fd_prepare.fs tag.

Thanks!
Christian

----------------------------------------------------------------
vfs-6.19-rc1.fd_prepare.fs

----------------------------------------------------------------
Christian Brauner (43):
      file: add FD_{ADD,PREPARE}()
      anon_inodes: convert to FD_ADD()
      eventfd: convert do_eventfd() to FD_PREPARE()
      fhandle: convert do_handle_open() to FD_ADD()
      namespace: convert open_tree() to FD_ADD()
      namespace: convert open_tree_attr() to FD_PREPARE()
      namespace: convert fsmount() to FD_PREPARE()
      fanotify: convert fanotify_init() to FD_PREPARE()
      nsfs: convert open_namespace() to FD_PREPARE()
      nsfs: convert ns_ioctl() to FD_PREPARE()
      autofs: convert autofs_dev_ioctl_open_mountpoint() to FD_ADD()
      eventpoll: convert do_epoll_create() to FD_PREPARE()
      open: convert do_sys_openat2() to FD_ADD()
      signalfd: convert do_signalfd4() to FD_ADD()
      timerfd: convert timerfd_create() to FD_ADD()
      userfaultfd: convert new_userfaultfd() to FD_PREPARE()
      xfs: convert xfs_open_by_handle() to FD_PREPARE()
      dma: convert dma_buf_fd() to FD_ADD()
      af_unix: convert unix_file_open() to FD_ADD()
      exec: convert begin_new_exec() to FD_ADD()
      ipc: convert do_mq_open() to FD_ADD()
      bpf: convert bpf_iter_new_fd() to FD_PREPARE()
      bpf: convert bpf_token_create() to FD_PREPARE()
      memfd: convert memfd_create() to FD_ADD()
      secretmem: convert memfd_secret() to FD_ADD()
      net/handshake: convert handshake_nl_accept_doit() to FD_PREPARE()
      net/kcm: convert kcm_ioctl() to FD_PREPARE()
      net/socket: convert sock_map_fd() to FD_ADD()
      net/socket: convert __sys_accept4_file() to FD_ADD()
      spufs: convert spufs_context_open() to FD_PREPARE()
      papr-hvpipe: convert papr_hvpipe_dev_create_handle() to FD_PREPARE()
      spufs: convert spufs_gang_open() to FD_PREPARE()
      pseries: convert papr_platform_dump_create_handle() to FD_ADD()
      pseries: port papr_rtas_setup_file_interface() to FD_ADD()
      gpio: convert linehandle_create() to FD_PREPARE()
      hv: convert mshv_ioctl_create_partition() to FD_ADD()
      media: convert media_request_alloc() to FD_PREPARE()
      ntsync: convert ntsync_obj_get_fd() to FD_PREPARE()
      tty: convert ptm_open_peer() to FD_ADD()
      vfio: convert vfio_group_ioctl_get_device_fd() to FD_ADD()
      file: convert replace_fd() to FD_PREPARE()
      io_uring: convert io_create_mock_file() to FD_PREPARE()
      Merge patch series "file: FD_{ADD,PREPARE}()"

 arch/powerpc/platforms/cell/spufs/inode.c          |  42 ++-----
 arch/powerpc/platforms/pseries/papr-hvpipe.c       |  39 ++-----
 .../powerpc/platforms/pseries/papr-platform-dump.c |  30 ++---
 arch/powerpc/platforms/pseries/papr-rtas-common.c  |  27 +----
 drivers/dma-buf/dma-buf.c                          |  10 +-
 drivers/gpio/gpiolib-cdev.c                        |  66 ++++-------
 drivers/hv/mshv_root_main.c                        |  30 +----
 drivers/media/mc/mc-request.c                      |  34 ++----
 drivers/misc/ntsync.c                              |  21 +---
 drivers/tty/pty.c                                  |  51 +++------
 drivers/vfio/group.c                               |  28 +----
 fs/anon_inodes.c                                   |  23 +---
 fs/autofs/dev-ioctl.c                              |  30 +----
 fs/eventfd.c                                       |  31 ++---
 fs/eventpoll.c                                     |  32 ++----
 fs/exec.c                                          |   3 +-
 fs/fhandle.c                                       |  30 +++--
 fs/file.c                                          |  19 ++--
 fs/namespace.c                                     | 103 ++++++-----------
 fs/notify/fanotify/fanotify_user.c                 |  60 ++++------
 fs/nsfs.c                                          |  47 +++-----
 fs/open.c                                          |  17 +--
 fs/signalfd.c                                      |  29 ++---
 fs/timerfd.c                                       |  29 ++---
 fs/userfaultfd.c                                   |  30 ++---
 fs/xfs/xfs_handle.c                                |  56 +++------
 include/linux/cleanup.h                            |   7 ++
 include/linux/file.h                               | 126 +++++++++++++++++++++
 io_uring/mock_file.c                               |  43 +++----
 ipc/mqueue.c                                       |  54 ++++-----
 kernel/bpf/bpf_iter.c                              |  29 ++---
 kernel/bpf/token.c                                 |  47 +++-----
 mm/memfd.c                                         |  29 +----
 mm/secretmem.c                                     |  20 +---
 net/handshake/netlink.c                            |  38 +++----
 net/kcm/kcmsock.c                                  |  22 ++--
 net/socket.c                                       |  34 +-----
 net/unix/af_unix.c                                 |  16 +--
 38 files changed, 508 insertions(+), 874 deletions(-)