nfsd4: Handle the NFSv4 READDIR 'dircount' hint being zero

BugLink: https://bugs.launchpad.net/bugs/1951643

commit f2e717d upstream.

RFC3530 notes that the 'dircount' field may be zero, in which case the
recommendation is to ignore it, and only enforce the 'maxcount' field.
In RFC5661, this recommendation to ignore a zero valued field becomes a
requirement.

Fixes: aee3776 ("nfsd4: fix rd_dircount enforcement")
Cc: <stable@vger.kernel.org>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

Author: Trond Myklebust

fs/nfsd/nfs4xdr.c

@@ -3542,15 +3542,18 @@ nfsd4_encode_dirent(void *ccdv, const char *name, int namlen,
 		goto fail;
 	cd->rd_maxcount -= entry_bytes;
 	/*
-	 * RFC 3530 14.2.24 describes rd_dircount as only a "hint", so
-	 * let's always let through the first entry, at least:
+	 * RFC 3530 14.2.24 describes rd_dircount as only a "hint", and
+	 * notes that it could be zero. If it is zero, then the server
+	 * should enforce only the rd_maxcount value.
 	 */
-	if (!cd->rd_dircount)
-		goto fail;
-	name_and_cookie = 4 + 4 * XDR_QUADLEN(namlen) + 8;
-	if (name_and_cookie > cd->rd_dircount && cd->cookie_offset)
-		goto fail;
-	cd->rd_dircount -= min(cd->rd_dircount, name_and_cookie);
+	if (cd->rd_dircount) {
+		name_and_cookie = 4 + 4 * XDR_QUADLEN(namlen) + 8;
+		if (name_and_cookie > cd->rd_dircount && cd->cookie_offset)
+			goto fail;
+		cd->rd_dircount -= min(cd->rd_dircount, name_and_cookie);
+		if (!cd->rd_dircount)
+			cd->rd_maxcount = 0;
+	}
 
 	cd->cookie_offset = cookie_offset;
 skip_entry: