On Fri, Mar 21, 2025 at 8:49 PM Andrey Albershteyn [off-list ref] wrote:

From: Andrey Albershteyn <redacted>

Introduce getfsxattrat and setfsxattrat syscalls to manipulate inode
extended attributes/flags. The syscalls take parent directory fd and
path to the child together with struct fsxattr.

This is an alternative to FS_IOC_FSSETXATTR ioctl with a difference
that file don't need to be open as we can reference it with a path
instead of fd. By having this we can manipulated inode extended
attributes not only on regular files but also on special ones. This
is not possible with FS_IOC_FSSETXATTR ioctl as with special files
we can not call ioctl() directly on the filesystem inode using fd.

This patch adds two new syscalls which allows userspace to get/set
extended inode attributes on special files by using parent directory
and a path - *at() like syscall.

CC: linux-api@vger.kernel.org
CC: linux-fsdevel@vger.kernel.org
CC: linux-xfs@vger.kernel.org
Signed-off-by: Andrey Albershteyn <redacted>
Acked-by: Arnd Bergmann <arnd@arndb.de>
---

...

+SYSCALL_DEFINE5(setfsxattrat, int, dfd, const char __user *, filename,
+               struct fsxattr __user *, ufsx, size_t, usize,
+               unsigned int, at_flags)
+{
+       struct fileattr fa;
+       struct path filepath;
+       int error;
+       unsigned int lookup_flags = 0;
+       struct filename *name;
+       struct mnt_idmap *idmap;.

+       struct dentry *dentry;
+       struct vfsmount *mnt;
+       struct fsxattr fsx = {};
+
+       BUILD_BUG_ON(sizeof(struct fsxattr) < FSXATTR_SIZE_VER0);
+       BUILD_BUG_ON(sizeof(struct fsxattr) != FSXATTR_SIZE_LATEST);
+
+       if ((at_flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH)) != 0)
+               return -EINVAL;
+
+       if (!(at_flags & AT_SYMLINK_NOFOLLOW))
+               lookup_flags |= LOOKUP_FOLLOW;
+
+       if (at_flags & AT_EMPTY_PATH)
+               lookup_flags |= LOOKUP_EMPTY;
+
+       if (usize > PAGE_SIZE)
+               return -E2BIG;
+
+       if (usize < FSXATTR_SIZE_VER0)
+               return -EINVAL;
+
+       error = copy_struct_from_user(&fsx, sizeof(struct fsxattr), ufsx, usize);
+       if (error)
+               return error;
+
+       fsxattr_to_fileattr(&fsx, &fa);
+
+       name = getname_maybe_null(filename, at_flags);
+       if (!name) {
+               CLASS(fd, f)(dfd);
+
+               if (fd_empty(f))
+                       return -EBADF;
+
+               idmap = file_mnt_idmap(fd_file(f));
+               dentry = file_dentry(fd_file(f));
+               mnt = fd_file(f)->f_path.mnt;
+       } else {
+               error = filename_lookup(dfd, name, lookup_flags, &filepath,
+                                       NULL);
+               if (error)
+                       return error;
+
+               idmap = mnt_idmap(filepath.mnt);
+               dentry = filepath.dentry;
+               mnt = filepath.mnt;
+       }
+
+       error = mnt_want_write(mnt);
+       if (!error) {
+               error = vfs_fileattr_set(idmap, dentry, &fa);
+               if (error == -ENOIOCTLCMD)
+                       error = -EOPNOTSUPP;

This is awkward.
vfs_fileattr_set() should return -EOPNOTSUPP.
ioctl_setflags() could maybe convert it to -ENOIOCTLCMD,
but looking at similar cases ioctl_fiemap(), ioctl_fsfreeze() the
ioctl returns -EOPNOTSUPP.

I don't think it is necessarily a bad idea to start returning
 -EOPNOTSUPP instead of -ENOIOCTLCMD for the ioctl
because that really reflects the fact that the ioctl is now implemented
in vfs and not in the specific fs.

and I think it would not be a bad idea at all to make that change
together with the merge of the syscalls as a sort of hint to userspace
that uses the ioctl, that the sycalls API exists.

Thanks,
Amir.