tcp: Work around gcc 12 bogus warning in tcp_rtt_dst_check()

gcc 12.1.x (e.g. current OpenSUSE Tumbleweed, x86_64 only,
gcc-12-1.4.x86_64) reports:

tcp.c: In function ‘tcp_send_flag’:
tcp.c:1014:9: warning: writing 16 bytes into a region of size 0 [-Wstringop-overflow=]
 1014 |         memcpy(low_rtt_dst + hole++, &conn->a.a6, sizeof(conn->a.a6));
      |         ^
tcp.c:559:24: note: at offset -16 into destination object ‘low_rtt_dst’ of size 128
  559 | static struct in6_addr low_rtt_dst[LOW_RTT_TABLE_SIZE];
      |

but 'hole' can't be -1, because the low_rtt_dst table is guaranteed
to have a hole: if we happened to write to the last entry, we'll go
back to index 0 and clear that one.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/tcp.c b/tcp.c
index e68409a..53af3db 100644
--- a/tcp.c
+++ b/tcp.c
@@ -1011,6 +1011,12 @@ static void tcp_rtt_dst_check(const struct tcp_conn *conn,
 			hole = i;
 	}
 
+	/* Keep gcc 12 happy: this won't actually happen because the table is
+	 * guaranteed to have a hole, see the second memcpy() below.
+	 */
+	if (hole == -1)
+		return;
+
 	memcpy(low_rtt_dst + hole++, &conn->a.a6, sizeof(conn->a.a6));
 	if (hole == LOW_RTT_TABLE_SIZE)
 		hole = 0;