selftests/bpf: Add tests for program extensions

Add program extension tests that build on top of fexit_bpf2bpf tests.
Replace three global functions in previously loaded test_pkt_access.c program
with three new implementations:
int get_skb_len(struct __sk_buff *skb);
int get_constant(long val);
int get_skb_ifindex(int val, struct __sk_buff *skb, int var);
New function return the same results as original only if arguments match.

new_get_skb_ifindex() demonstrates that 'skb' argument doesn't have to be first
and only argument of BPF program. All normal skb based accesses are available.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20200121005348.2769920-4-ast@kernel.org

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

@@ -26,7 +26,7 @@ static void test_fexit_bpf2bpf_common(const char *obj_file,
 
 	link = calloc(sizeof(struct bpf_link *), prog_cnt);
 	prog = calloc(sizeof(struct bpf_program *), prog_cnt);
-	result = malloc(prog_cnt * sizeof(u64));
+	result = malloc((prog_cnt + 32 /* spare */) * sizeof(u64));
 	if (CHECK(!link || !prog || !result, "alloc_memory",
 		  "failed to alloc memory"))
 		goto close_prog;
@@ -106,8 +106,26 @@ static void test_target_yes_callees(void)
 				  prog_name);
 }
 
+static void test_func_replace(void)
+{
+	const char *prog_name[] = {
+		"fexit/test_pkt_access",
+		"fexit/test_pkt_access_subprog1",
+		"fexit/test_pkt_access_subprog2",
+		"fexit/test_pkt_access_subprog3",
+		"freplace/get_skb_len",
+		"freplace/get_skb_ifindex",
+		"freplace/get_constant",
+	};
+	test_fexit_bpf2bpf_common("./fexit_bpf2bpf.o",
+				  "./test_pkt_access.o",
+				  ARRAY_SIZE(prog_name),
+				  prog_name);
+}
+
 void test_fexit_bpf2bpf(void)
 {
 	test_target_no_callees();
 	test_target_yes_callees();
+	test_func_replace();
 }

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

@@ -1,7 +1,10 @@
 // SPDX-License-Identifier: GPL-2.0
 /* Copyright (c) 2019 Facebook */
+#include <linux/stddef.h>
+#include <linux/ipv6.h>
 #include <linux/bpf.h>
 #include <bpf/bpf_helpers.h>
+#include <bpf/bpf_endian.h>
 #include "bpf_trace_helpers.h"
 
 struct sk_buff {
@@ -94,4 +97,58 @@ int BPF_PROG(test_subprog3, int val, struct sk_buff *skb, int ret)
 	test_result_subprog3 = 1;
 	return 0;
 }
+
+__u64 test_get_skb_len = 0;
+SEC("freplace/get_skb_len")
+int new_get_skb_len(struct __sk_buff *skb)
+{
+	int len = skb->len;
+
+	if (len != 74)
+		return 0;
+	test_get_skb_len = 1;
+	return 74; /* original get_skb_len() returns skb->len */
+}
+
+__u64 test_get_skb_ifindex = 0;
+SEC("freplace/get_skb_ifindex")
+int new_get_skb_ifindex(int val, struct __sk_buff *skb, int var)
+{
+	void *data_end = (void *)(long)skb->data_end;
+	void *data = (void *)(long)skb->data;
+	struct ipv6hdr ip6, *ip6p;
+	int ifindex = skb->ifindex;
+	__u32 eth_proto;
+	__u32 nh_off;
+
+	/* check that BPF extension can read packet via direct packet access */
+	if (data + 14 + sizeof(ip6) > data_end)
+		return 0;
+	ip6p = data + 14;
+
+	if (ip6p->nexthdr != 6 || ip6p->payload_len != __bpf_constant_htons(123))
+		return 0;
+
+	/* check that legacy packet access helper works too */
+	if (bpf_skb_load_bytes(skb, 14, &ip6, sizeof(ip6)) < 0)
+		return 0;
+	ip6p = &ip6;
+	if (ip6p->nexthdr != 6 || ip6p->payload_len != __bpf_constant_htons(123))
+		return 0;
+
+	if (ifindex != 1 || val != 3 || var != 1)
+		return 0;
+	test_get_skb_ifindex = 1;
+	return 3; /* original get_skb_ifindex() returns val * ifindex * var */
+}
+
+volatile __u64 test_get_constant = 0;
+SEC("freplace/get_constant")
+int new_get_constant(long val)
+{
+	if (val != 123)
+		return 0;
+	test_get_constant = 1;
+	return test_get_constant; /* original get_constant() returns val - 122 */
+}
 char _license[] SEC("license") = "GPL";

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

@@ -57,12 +57,18 @@ int get_skb_len(struct __sk_buff *skb)
 	return skb->len;
 }
 
+__attribute__ ((noinline))
+int get_constant(long val)
+{
+	return val - 122;
+}
+
 int get_skb_ifindex(int, struct __sk_buff *skb, int);
 
 __attribute__ ((noinline))
 int test_pkt_access_subprog3(int val, struct __sk_buff *skb)
 {
-	return get_skb_len(skb) * get_skb_ifindex(val, skb, 1);
+	return get_skb_len(skb) * get_skb_ifindex(val, skb, get_constant(123));
 }
 
 __attribute__ ((noinline))