selftest/bpf: Check invalid length in test_xdp_update_frags

Update test_xdp_update_frags adding a test for a buffer size
set to (MAX_SKB_FRAGS + 2) * PAGE_SIZE. The kernel is supposed
to return -ENOMEM.
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/3e4afa0ee4976854b2f0296998fe6754a80b62e5.1644366736.git.lorenzo@kernel.org

tools/testing/selftests/bpf/prog_tests/xdp_adjust_frags.c

@@ -5,11 +5,12 @@
 static void test_xdp_update_frags(void)
 {
 	const char *file = "./test_xdp_update_frags.o";
+	int err, prog_fd, max_skb_frags, buf_size, num;
 	struct bpf_program *prog;
 	struct bpf_object *obj;
-	int err, prog_fd;
 	__u32 *offset;
 	__u8 *buf;
+	FILE *f;
 	LIBBPF_OPTS(bpf_test_run_opts, topts);
 
 	obj = bpf_object__open(file);
@@ -99,6 +100,41 @@ static void test_xdp_update_frags(void)
 	ASSERT_EQ(buf[7621], 0xbb, "xdp_update_frag buf[7621]");
 
 	free(buf);
+
+	/* test_xdp_update_frags: unsupported buffer size */
+	f = fopen("/proc/sys/net/core/max_skb_frags", "r");
+	if (!ASSERT_OK_PTR(f, "max_skb_frag file pointer"))
+		goto out;
+
+	num = fscanf(f, "%d", &max_skb_frags);
+	fclose(f);
+
+	if (!ASSERT_EQ(num, 1, "max_skb_frags read failed"))
+		goto out;
+
+	/* xdp_buff linear area size is always set to 4096 in the
+	 * bpf_prog_test_run_xdp routine.
+	 */
+	buf_size = 4096 + (max_skb_frags + 1) * sysconf(_SC_PAGE_SIZE);
+	buf = malloc(buf_size);
+	if (!ASSERT_OK_PTR(buf, "alloc buf"))
+		goto out;
+
+	memset(buf, 0, buf_size);
+	offset = (__u32 *)buf;
+	*offset = 16;
+	buf[*offset] = 0xaa;
+	buf[*offset + 15] = 0xaa;
+
+	topts.data_in = buf;
+	topts.data_out = buf;
+	topts.data_size_in = buf_size;
+	topts.data_size_out = buf_size;
+
+	err = bpf_prog_test_run_opts(prog_fd, &topts);
+	ASSERT_EQ(err, -ENOMEM,
+		  "unsupported buf size, possible non-default /proc/sys/net/core/max_skb_flags?");
+	free(buf);
 out:
 	bpf_object__close(obj);
 }