Re: [PATCH v4 3/3] fs: introduce getfsxattrat and setfsxattrat syscalls

[Amir Goldstein <amir73il@gmail.com>]

On Fri, Mar 21, 2025 at 8:49 PM Andrey Albershteyn <aalbersh@redhat.com> wrote:
>
> From: Andrey Albershteyn <aalbersh@redhat.com>
>
> 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 <aalbersh@redhat.com>
> 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.