selftests/bpf: Add test for freplace program with write access

This adds a selftest that tests the behavior when a freplace target program
attempts to make a write access on a packet. The expectation is that the read or write
access is granted based on the program type of the linked program and
not itself (which is of type, for e.g., BPF_PROG_TYPE_EXT).

This test fails without the associated patch on the verifier.

Signed-off-by: Udip Pant <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c

···
 		"freplace/get_skb_len",
 		"freplace/get_skb_ifindex",
 		"freplace/get_constant",
+		"freplace/test_pkt_write_access_subprog",
 	};
 	test_fexit_bpf2bpf_common("./fexit_bpf2bpf.o",
 				  "./test_pkt_access.o",

tools/testing/selftests/bpf/progs/fexit_bpf2bpf.c

···
 // SPDX-License-Identifier: GPL-2.0
 /* Copyright (c) 2019 Facebook */
 #include <linux/stddef.h>
+#include <linux/if_ether.h>
 #include <linux/ipv6.h>
 #include <linux/bpf.h>
+#include <linux/tcp.h>
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_endian.h>
 #include <bpf/bpf_tracing.h>
···
 	test_get_constant = 1;
 	return test_get_constant; /* original get_constant() returns val - 122 */
 }
+
+__u64 test_pkt_write_access_subprog = 0;
+SEC("freplace/test_pkt_write_access_subprog")
+int new_test_pkt_write_access_subprog(struct __sk_buff *skb, __u32 off)
+{
+
+	void *data = (void *)(long)skb->data;
+	void *data_end = (void *)(long)skb->data_end;
+	struct tcphdr *tcp;
+
+	if (off > sizeof(struct ethhdr) + sizeof(struct ipv6hdr))
+		return -1;
+
+	tcp = data + off;
+	if (tcp + 1 > data_end)
+		return -1;
+
+	/* make modifications to the packet data */
+	tcp->check++;
+	tcp->syn = 0;
+
+	test_pkt_write_access_subprog = 1;
+	return 0;
+}
+
 char _license[] SEC("license") = "GPL";

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

···
 	return skb->ifindex * val * var;
 }
 
+__attribute__ ((noinline))
+int test_pkt_write_access_subprog(struct __sk_buff *skb, __u32 off)
+{
+	void *data = (void *)(long)skb->data;
+	void *data_end = (void *)(long)skb->data_end;
+	struct tcphdr *tcp = NULL;
+
+	if (off > sizeof(struct ethhdr) + sizeof(struct ipv6hdr))
+		return -1;
+
+	tcp = data + off;
+	if (tcp + 1 > data_end)
+		return -1;
+	/* make modification to the packet data */
+	tcp->check++;
+	return 0;
+}
+
 SEC("classifier/test_pkt_access")
 int test_pkt_access(struct __sk_buff *skb)
 {
···
 	if (test_pkt_access_subprog3(3, skb) != skb->len * 3 * skb->ifindex)
 		return TC_ACT_SHOT;
 	if (tcp) {
+		if (test_pkt_write_access_subprog(skb, (void *)tcp - data))
+			return TC_ACT_SHOT;
 		if (((void *)(tcp) + 20) > data_end || proto != 6)
 			return TC_ACT_SHOT;
 		barrier(); /* to force ordering of checks */