[PATCHv2 net-next] tcp: warn if offset reach the maxlen limit when using snprintf

[Hangbin Liu <liuhangbin@gmail.com>]

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>
---
 net/ipv4/sysctl_net_ipv4.c | 4 ++++
 net/ipv4/tcp_cong.c        | 6 ++++++
 net/ipv4/tcp_ulp.c         | 3 +++
 3 files changed, 13 insertions(+)

diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 59ded25acd04..c9eaf924df63 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/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, ",");
 	}
diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
index c445a81d144e..3737ec096650 100644
--- a/net/ipv4/tcp_cong.c
+++ b/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();
 }
diff --git a/net/ipv4/tcp_ulp.c b/net/ipv4/tcp_ulp.c
index 4849edb62d52..12ab5db2b71c 100644
--- a/net/ipv4/tcp_ulp.c
+++ b/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();
 }
-- 
2.19.2