selftests/bpf: Fix invalid use of strncat in test_sockmap

strncat()'s third argument is how many bytes will be added *in addition* to
already existing bytes in destination. Plus extra zero byte will be added
after that. So existing use in test_sockmap has many opportunities to overflow
the string and cause memory corruptions. And in this case, GCC complains for
a good reason.

Fixes: 16962b24 ("bpf: sockmap, add selftests")
Fixes: 73563aa3 ("selftests/bpf: test_sockmap, print additional test options")
Fixes: 1ade9aba ("bpf: test_sockmap, add options for msg_pop_data() helper")
Fixes: 463bac5f ("bpf, selftests: Add test for ktls with skb bpf ingress policy")
Fixes: e9dd9047 ("bpf: add tls support for testing in test_sockmap")
Fixes: 753fb2ee ("bpf: sockmap, add msg_peek tests to test_sockmap")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201203235440.2302137-2-andrii@kernel.org

tools/testing/selftests/bpf/test_sockmap.c

@@ -1273,6 +1273,16 @@ static char *test_to_str(int test)
 	return "unknown";
 }
 
+static void append_str(char *dst, const char *src, size_t dst_cap)
+{
+	size_t avail = dst_cap - strlen(dst);
+
+	if (avail <= 1) /* just zero byte could be written */
+		return;
+
+	strncat(dst, src, avail - 1); /* strncat() adds + 1 for zero byte */
+}
+
 #define OPTSTRING 60
 static void test_options(char *options)
 {
@@ -1281,42 +1291,42 @@ static void test_options(char *options)
 	memset(options, 0, OPTSTRING);
 
 	if (txmsg_pass)
-		strncat(options, "pass,", OPTSTRING);
+		append_str(options, "pass,", OPTSTRING);
 	if (txmsg_redir)
-		strncat(options, "redir,", OPTSTRING);
+		append_str(options, "redir,", OPTSTRING);
 	if (txmsg_drop)
-		strncat(options, "drop,", OPTSTRING);
+		append_str(options, "drop,", OPTSTRING);
 	if (txmsg_apply) {
 		snprintf(tstr, OPTSTRING, "apply %d,", txmsg_apply);
-		strncat(options, tstr, OPTSTRING);
+		append_str(options, tstr, OPTSTRING);
 	}
 	if (txmsg_cork) {
 		snprintf(tstr, OPTSTRING, "cork %d,", txmsg_cork);
-		strncat(options, tstr, OPTSTRING);
+		append_str(options, tstr, OPTSTRING);
 	}
 	if (txmsg_start) {
 		snprintf(tstr, OPTSTRING, "start %d,", txmsg_start);
-		strncat(options, tstr, OPTSTRING);
+		append_str(options, tstr, OPTSTRING);
 	}
 	if (txmsg_end) {
 		snprintf(tstr, OPTSTRING, "end %d,", txmsg_end);
-		strncat(options, tstr, OPTSTRING);
+		append_str(options, tstr, OPTSTRING);
 	}
 	if (txmsg_start_pop) {
 		snprintf(tstr, OPTSTRING, "pop (%d,%d),",
 			 txmsg_start_pop, txmsg_start_pop + txmsg_pop);
-		strncat(options, tstr, OPTSTRING);
+		append_str(options, tstr, OPTSTRING);
 	}
 	if (txmsg_ingress)
-		strncat(options, "ingress,", OPTSTRING);
+		append_str(options, "ingress,", OPTSTRING);
 	if (txmsg_redir_skb)
-		strncat(options, "redir_skb,", OPTSTRING);
+		append_str(options, "redir_skb,", OPTSTRING);
 	if (txmsg_ktls_skb)
-		strncat(options, "ktls_skb,", OPTSTRING);
+		append_str(options, "ktls_skb,", OPTSTRING);
 	if (ktls)
-		strncat(options, "ktls,", OPTSTRING);
+		append_str(options, "ktls,", OPTSTRING);
 	if (peek_flag)
-		strncat(options, "peek,", OPTSTRING);
+		append_str(options, "peek,", OPTSTRING);
 }
 
 static int __test_exec(int cgrp, int test, struct sockmap_options *opt)