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

[Christian Brauner <brauner@kernel.org>]

On Wed, Apr 23, 2025 at 11:53:25AM +0200, Jan Kara wrote:
> On Tue 22-04-25 16:59:02, Christian Brauner wrote:
> > On Fri, Mar 21, 2025 at 08:48:42PM +0100, Andrey Albershteyn 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>
> ...
> > > +		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 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;
> > > +
> > > +	name = getname_maybe_null(filename, at_flags);
> > > +	if (!name) {
> > 
> > This is broken as it doesn't handle AT_FDCWD correctly. You need:
> > 
> >         name = getname_maybe_null(filename, at_flags);
> >         if (IS_ERR(name))
> >                 return PTR_ERR(name);
> > 
> >         if (!name && dfd >= 0) {
> > 		CLASS(fd, f)(dfd);
> 
> Ah, you're indeed right that if dfd == AT_FDCWD and filename == NULL, the
> we should operate on cwd but we'd bail with error here. I've missed that
> during my review. But as far as I've checked the same bug is there in
> path_setxattrat() and path_getxattrat() so we should fix this there as
> well?

Yes, please!