tcp: warn if offset reach the maxlen limit when using snprintf

snprintf returns the number of chars that would be written, not number
of chars that were actually written. As such, 'offs' may get larger than
'tbl.maxlen', causing the 'tbl.maxlen - offs' being < 0, and since the
parameter is size_t, it would overflow.

Since using scnprintf may hide the limit error, while the buffer is still
enough now, let's just add a WARN_ON_ONCE in case it reach the limit
in future.

v2: Use WARN_ON_ONCE as Jiri and Eric suggested.
Suggested-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/sysctl_net_ipv4.c

@@ -340,6 +340,10 @@ static int proc_tcp_fastopen_key(struct ctl_table *table, int write,
 				user_key[i * 4 + 1],
 				user_key[i * 4 + 2],
 				user_key[i * 4 + 3]);
+
+		if (WARN_ON_ONCE(off >= tbl.maxlen - 1))
+			break;
+
 		if (i + 1 < n_keys)
 			off += snprintf(tbl.data + off, tbl.maxlen - off, ",");
 	}

net/ipv4/tcp_cong.c

@@ -256,6 +256,9 @@ void tcp_get_available_congestion_control(char *buf, size_t maxlen)
 		offs += snprintf(buf + offs, maxlen - offs,
 				 "%s%s",
 				 offs == 0 ? "" : " ", ca->name);
+
+		if (WARN_ON_ONCE(offs >= maxlen))
+			break;
 	}
 	rcu_read_unlock();
 }
@@ -285,6 +288,9 @@ void tcp_get_allowed_congestion_control(char *buf, size_t maxlen)
 		offs += snprintf(buf + offs, maxlen - offs,
 				 "%s%s",
 				 offs == 0 ? "" : " ", ca->name);
+
+		if (WARN_ON_ONCE(offs >= maxlen))
+			break;
 	}
 	rcu_read_unlock();
 }

net/ipv4/tcp_ulp.c

@@ -92,6 +92,9 @@ void tcp_get_available_ulp(char *buf, size_t maxlen)
 		offs += snprintf(buf + offs, maxlen - offs,
 				 "%s%s",
 				 offs == 0 ? "" : " ", ulp_ops->name);
+
+		if (WARN_ON_ONCE(offs >= maxlen))
+			break;
 	}
 	rcu_read_unlock();
 }