nfscl: Fix a crash when a readdir entry has nul in it

Commit 026cdaa3b3 added a check for a nul or "/" in a file
name in a readdir reply.  Unfortunately, the minimal testing
done on it did not detect a bug that can cause the client
to crash.

This patch fixes the code so that it does not crash.

Note that a NFS server will not normally return a file
name in a readdir reply that has a nul or "/" in it,
so the crash is unlikely.

PR:	283965
Reported by:	asomers
Tested by:	asomers
MFC after:	2 weeks

sys/fs/nfsclient/nfs_clrpcops.c

@@ -3397,6 +3397,7 @@ nfsrpc_readdir(vnode_t vp, struct uio *uiop, nfsuint64 *cookiep,
 	nfsattrbit_t attrbits, dattrbits;
 	u_int32_t rderr, *tl2 = NULL;
 	size_t tresid;
+	bool validentry;
 
 	KASSERT(uiop->uio_iovcnt == 1 &&
 	    (uiop->uio_resid & (DIRBLKSIZ - 1)) == 0,
@@ -3622,6 +3623,7 @@ nfsrpc_readdir(vnode_t vp, struct uio *uiop, nfsuint64 *cookiep,
 
 		/* loop through the dir entries, doctoring them to 4bsd form */
 		while (more_dirs && bigenough) {
+			validentry = true;
 			if (nd->nd_flag & ND_NFSV4) {
 				NFSM_DISSECT(tl, u_int32_t *, 3*NFSX_UNSIGNED);
 				ncookie.lval[0] = *tl++;
@@ -3701,6 +3703,7 @@ nfsrpc_readdir(vnode_t vp, struct uio *uiop, nfsuint64 *cookiep,
 					uiop->uio_offset = savoff;
 					uiop->uio_resid = savresid;
 					blksiz = savblksiz;
+					validentry = false;
 				} else {
 					cp = uiop->uio_iov->iov_base;
 					tlen -= len;
@@ -3738,7 +3741,7 @@ nfsrpc_readdir(vnode_t vp, struct uio *uiop, nfsuint64 *cookiep,
 				ncookie.lval[0] = 0;
 				ncookie.lval[1] = *tl++;
 			}
-			if (bigenough) {
+			if (bigenough && validentry) {
 			    if (nd->nd_flag & ND_NFSV4) {
 				if (rderr) {
 				    dp->d_fileno = 0;
@@ -3875,7 +3878,7 @@ nfsrpc_readdirplus(vnode_t vp, struct uio *uiop, nfsuint64 *cookiep,
 	size_t tresid;
 	u_int32_t *tl2 = NULL, rderr;
 	struct timespec dctime, ts;
-	bool attr_ok;
+	bool attr_ok, validentry;
 
 	KASSERT(uiop->uio_iovcnt == 1 &&
 	    (uiop->uio_resid & (DIRBLKSIZ - 1)) == 0,
@@ -4086,6 +4089,7 @@ nfsrpc_readdirplus(vnode_t vp, struct uio *uiop, nfsuint64 *cookiep,
 
 		/* loop through the dir entries, doctoring them to 4bsd form */
 		while (more_dirs && bigenough) {
+			validentry = true;
 			NFSM_DISSECT(tl, u_int32_t *, 3 * NFSX_UNSIGNED);
 			if (nd->nd_flag & ND_NFSV4) {
 				ncookie.lval[0] = *tl++;
@@ -4161,6 +4165,7 @@ nfsrpc_readdirplus(vnode_t vp, struct uio *uiop, nfsuint64 *cookiep,
 					uiop->uio_offset = savoff;
 					uiop->uio_resid = savresid;
 					blksiz = savblksiz;
+					validentry = false;
 				} else {
 					cp = uiop->uio_iov->iov_base;
 					tlen -= len;
@@ -4217,7 +4222,7 @@ nfsrpc_readdirplus(vnode_t vp, struct uio *uiop, nfsuint64 *cookiep,
 					goto nfsmout;
 			}
 
-			if (bigenough) {
+			if (bigenough && validentry) {
 			    if (nd->nd_flag & ND_NFSV4) {
 				if (rderr) {
 				    dp->d_fileno = 0;