SUNRPC: improve error response to over-size gss credential

When the NFS server receives a large gss (kerberos) credential and tries
to pass it up to rpc.svcgssd (which is deprecated), it triggers an
infinite loop in cache_read().

cache_request() always returns -EAGAIN, and this causes a "goto again".

This patch:
 - changes the error to -E2BIG to avoid the infinite loop, and
 - generates a WARN_ONCE when rsi_request first sees an over-sized
   credential.  The warning suggests switching to gssproxy.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=196583
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/auth_gss/svcauth_gss.c

@@ -194,6 +194,8 @@ static void rsi_request(struct cache_detail *cd,
 	qword_addhex(bpp, blen, rsii->in_handle.data, rsii->in_handle.len);
 	qword_addhex(bpp, blen, rsii->in_token.data, rsii->in_token.len);
 	(*bpp)[-1] = '\n';
+	WARN_ONCE(*blen < 0,
+		  "RPCSEC/GSS credential too large - please use gssproxy\n");
 }
 
 static int rsi_parse(struct cache_detail *cd,

net/sunrpc/cache.c

@@ -803,7 +803,7 @@ static int cache_request(struct cache_detail *detail,
 
 	detail->cache_request(detail, crq->item, &bp, &len);
 	if (len < 0)
-		return -EAGAIN;
+		return -E2BIG;
 	return PAGE_SIZE - len;
 }