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

[Andrey Albershteyn]

On 2025-03-23 09:56:25, Amir Goldstein wrote:
> On Fri, Mar 21, 2025 at 8:49 PM Andrey Albershteyn <aalbersh@xxxxxxxxxx> wrote:
> >
> > From: Andrey Albershteyn <aalbersh@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxxxx
> > CC: linux-fsdevel@xxxxxxxxxxxxxxx
> > CC: linux-xfs@xxxxxxxxxxxxxxx
> > Signed-off-by: Andrey Albershteyn <aalbersh@xxxxxxxxxx>
> > Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
> > ---
> ...
> > +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.
> 

Hmm, not sure what you're suggesting here. I see it as:
- get/setfsxattrat should return EOPNOTSUPP as it make more sense
  than ENOIOCTLCMD
- ioctl_setflags returns ENOIOCTLCMD which also expected

Don't really see a reason to change what vfs_fileattr_set() returns
and then copying this if() to other places or start returning
EOPNOTSUPP.

-- 
- Andrey