NFSv4: xattr handlers should check for absent nfs filehandles

JIRA: https://issues.redhat.com/browse/RHEL-79731

commit 6e9a2f8
Author: Scott Mayhew <smayhew@redhat.com>
Date:   Wed Apr 16 11:23:38 2025 -0400

    NFSv4: xattr handlers should check for absent nfs filehandles

    The nfs inodes for referral anchors that have not yet been followed have
    their filehandles zeroed out.

    Attempting to call getxattr() on one of these will cause the nfs client
    to send a GETATTR to the nfs server with the preceding PUTFH sans
    filehandle.  The server will reply NFS4ERR_NOFILEHANDLE, leading to -EIO
    being returned to the application.

    For example:

    $ strace -e trace=getxattr getfattr -n system.nfs4_acl /mnt/t/ref
    getxattr("/mnt/t/ref", "system.nfs4_acl", NULL, 0) = -1 EIO (Input/output error)
    /mnt/t/ref: system.nfs4_acl: Input/output error
    +++ exited with 1 +++

    Have the xattr handlers return -ENODATA instead.

    Signed-off-by: Scott Mayhew <smayhew@redhat.com>
    Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>

Signed-off-by: Scott Mayhew <smayhew@redhat.com>

Author: scottmayhew

fs/nfs/nfs4proc.c

@@ -6166,6 +6166,8 @@ static ssize_t nfs4_proc_get_acl(struct inode *inode, void *buf, size_t buflen,
 	struct nfs_server *server = NFS_SERVER(inode);
 	int ret;
 
+	if (unlikely(NFS_FH(inode)->size == 0))
+		return -ENODATA;
 	if (!nfs4_server_supports_acls(server, type))
 		return -EOPNOTSUPP;
 	ret = nfs_revalidate_inode(inode, NFS_INO_INVALID_CHANGE);
@@ -6240,6 +6242,9 @@ static int nfs4_proc_set_acl(struct inode *inode, const void *buf,
 {
 	struct nfs4_exception exception = { };
 	int err;
+
+	if (unlikely(NFS_FH(inode)->size == 0))
+		return -ENODATA;
 	do {
 		err = __nfs4_proc_set_acl(inode, buf, buflen, type);
 		trace_nfs4_set_acl(inode, err);