selftests/bpf: Cover skb metadata access after bpf_skb_adjust_room

Add a test to verify that skb metadata remains accessible after calling
bpf_skb_adjust_room(), which modifies the packet headroom and can trigger
head reallocation.

The helper expects an Ethernet frame carrying an IP packet so switch test
packet identification by source MAC address since we can no longer rely on
Ethernet proto being set to zero.

Signed-off-by: Jakub Sitnicki <[email protected]>

Author: jsitnicki

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

@@ -124,10 +124,10 @@ static int send_test_packet(int ifindex)
 	int n, sock = -1;
 	__u8 packet[sizeof(struct ethhdr) + TEST_PAYLOAD_LEN];
 
-	/* The ethernet header is not relevant for this test and doesn't need to
-	 * be meaningful.
-	 */
-	struct ethhdr eth = { 0 };
+	/* We use the Ethernet header only to identify the test packet */
+	struct ethhdr eth = {
+		.h_source = { 0x12, 0x34, 0xDE, 0xAD, 0xBE, 0xEF },
+	};
 
 	memcpy(packet, &eth, sizeof(eth));
 	memcpy(packet + sizeof(eth), test_payload, TEST_PAYLOAD_LEN);
@@ -160,8 +160,16 @@ static int write_test_packet(int tap_fd)
 	__u8 packet[sizeof(struct ethhdr) + TEST_PAYLOAD_LEN];
 	int n;
 
-	/* The ethernet header doesn't need to be valid for this test */
-	memset(packet, 0, sizeof(struct ethhdr));
+	/* The Ethernet header is mostly not relevant. We use it to identify the
+	 * test packet and some BPF helpers we exercise expect to operate on
+	 * Ethernet frames carrying IP packets. Pretend that's the case.
+	 */
+	struct ethhdr eth = {
+		.h_source = { 0x12, 0x34, 0xDE, 0xAD, 0xBE, 0xEF },
+		.h_proto = htons(ETH_P_IP),
+	};
+
+	memcpy(packet, &eth, sizeof(eth));
 	memcpy(packet + sizeof(struct ethhdr), test_payload, TEST_PAYLOAD_LEN);
 
 	n = write(tap_fd, packet, sizeof(packet));
@@ -484,6 +492,11 @@ void test_xdp_context_tuntap(void)
 			    skel->progs.helper_skb_vlan_push_pop,
 			    NULL, /* tc prio 2 */
 			    &skel->bss->test_pass);
+	if (test__start_subtest("helper_skb_adjust_room"))
+		test_tuntap(skel->progs.ing_xdp,
+			    skel->progs.helper_skb_adjust_room,
+			    NULL, /* tc prio 2 */
+			    &skel->bss->test_pass);
 
 	test_xdp_meta__destroy(skel);
 }

tools/testing/selftests/bpf/progs/test_xdp_meta.c

@@ -20,13 +20,22 @@
 
 bool test_pass;
 
+static const __u8 smac_want[ETH_ALEN] = {
+	0x12, 0x34, 0xDE, 0xAD, 0xBE, 0xEF,
+};
+
 static const __u8 meta_want[META_SIZE] = {
 	0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08,
 	0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18,
 	0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28,
 	0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38,
 };
 
+static bool check_smac(const struct ethhdr *eth)
+{
+	return !__builtin_memcmp(eth->h_source, smac_want, ETH_ALEN);
+}
+
 static bool check_metadata(const char *file, int line, __u8 *meta_have)
 {
 	if (!__builtin_memcmp(meta_have, meta_want, META_SIZE))
@@ -281,7 +290,7 @@ int ing_xdp_zalloc_meta(struct xdp_md *ctx)
 	/* Drop any non-test packets */
 	if (eth + 1 > ctx_ptr(ctx, data_end))
 		return XDP_DROP;
-	if (eth->h_proto != 0)
+	if (!check_smac(eth))
 		return XDP_DROP;
 
 	ret = bpf_xdp_adjust_meta(ctx, -META_SIZE);
@@ -321,9 +330,9 @@ int ing_xdp(struct xdp_md *ctx)
 
 	/* The Linux networking stack may send other packets on the test
 	 * interface that interfere with the test. Just drop them.
-	 * The test packets can be recognized by their ethertype of zero.
+	 * The test packets can be recognized by their source MAC address.
 	 */
-	if (eth->h_proto != 0)
+	if (!check_smac(eth))
 		return XDP_DROP;
 
 	__builtin_memcpy(data_meta, payload, META_SIZE);
@@ -343,7 +352,7 @@ int clone_data_meta_survives_data_write(struct __sk_buff *ctx)
 	if (eth + 1 > ctx_ptr(ctx, data_end))
 		goto out;
 	/* Ignore non-test packets */
-	if (eth->h_proto != 0)
+	if (!check_smac(eth))
 		goto out;
 
 	if (meta_have + META_SIZE > eth)
@@ -373,7 +382,7 @@ int clone_data_meta_survives_meta_write(struct __sk_buff *ctx)
 	if (eth + 1 > ctx_ptr(ctx, data_end))
 		goto out;
 	/* Ignore non-test packets */
-	if (eth->h_proto != 0)
+	if (!check_smac(eth))
 		goto out;
 
 	if (meta_have + META_SIZE > eth)
@@ -406,7 +415,7 @@ int clone_meta_dynptr_survives_data_slice_write(struct __sk_buff *ctx)
 	if (!eth)
 		goto out;
 	/* Ignore non-test packets */
-	if (eth->h_proto != 0)
+	if (!check_smac(eth))
 		goto out;
 
 	bpf_dynptr_from_skb_meta(ctx, 0, &meta);
@@ -435,7 +444,7 @@ int clone_meta_dynptr_survives_meta_slice_write(struct __sk_buff *ctx)
 	if (!eth)
 		goto out;
 	/* Ignore non-test packets */
-	if (eth->h_proto != 0)
+	if (!check_smac(eth))
 		goto out;
 
 	bpf_dynptr_from_skb_meta(ctx, 0, &meta);
@@ -469,7 +478,7 @@ int clone_meta_dynptr_rw_before_data_dynptr_write(struct __sk_buff *ctx)
 	if (!eth)
 		goto out;
 	/* Ignore non-test packets */
-	if (eth->h_proto != 0)
+	if (!check_smac(eth))
 		goto out;
 
 	/* Expect read-write metadata before unclone */
@@ -511,7 +520,7 @@ int clone_meta_dynptr_rw_before_meta_dynptr_write(struct __sk_buff *ctx)
 	if (!eth)
 		goto out;
 	/* Ignore non-test packets */
-	if (eth->h_proto != 0)
+	if (!check_smac(eth))
 		goto out;
 
 	/* Expect read-write metadata before unclone */
@@ -568,4 +577,38 @@ int helper_skb_vlan_push_pop(struct __sk_buff *ctx)
 	return TC_ACT_SHOT;
 }
 
+SEC("tc")
+int helper_skb_adjust_room(struct __sk_buff *ctx)
+{
+	int err;
+
+	/* Grow a 1 byte hole after the MAC header */
+	err = bpf_skb_adjust_room(ctx, 1, BPF_ADJ_ROOM_MAC, 0);
+	if (err)
+		goto out;
+
+	if (!check_skb_metadata(ctx))
+		goto out;
+
+	/* Shrink a 1 byte hole after the MAC header */
+	err = bpf_skb_adjust_room(ctx, -1, BPF_ADJ_ROOM_MAC, 0);
+	if (err)
+		goto out;
+
+	if (!check_skb_metadata(ctx))
+		goto out;
+
+	/* Grow a 256 byte hole to trigger head reallocation */
+	err = bpf_skb_adjust_room(ctx, 256, BPF_ADJ_ROOM_MAC, 0);
+	if (err)
+		goto out;
+
+	if (!check_skb_metadata(ctx))
+		goto out;
+
+	test_pass = true;
+out:
+	return TC_ACT_SHOT;
+}
+
 char _license[] SEC("license") = "GPL";