samples/bpf: syscall_tp_user: Fix array out-of-bound access

Commit 06744f24 ("samples/bpf: Add openat2() enter/exit tracepoint
to syscall_tp sample") added two more eBPF programs to support the
openat2() syscall. However, it did not increase the size of the array
that holds the corresponding bpf_links. This leads to an out-of-bound
access on that array in the bpf_object__for_each_program loop and could
corrupt other variables on the stack. On our testing QEMU, it corrupts
the map1_fds array and causes the sample to fail:

  # ./syscall_tp
  prog #0: map ids 4 5
  verify map:4 val: 5
  map_lookup failed: Bad file descriptor

Dynamically allocate the array based on the number of programs reported
by libbpf to prevent similar inconsistencies in the future

Fixes: 06744f24 ("samples/bpf: Add openat2() enter/exit tracepoint to syscall_tp sample")
Signed-off-by: Jinghao Jia <jinghao@linux.ibm.com>
Signed-off-by: Ruowen Qin <ruowenq2@illinois.edu>
Signed-off-by: Jinghao Jia <jinghao7@illinois.edu>
Link: https://lore.kernel.org/r/20230917214220.637721-4-jinghao7@illinois.eduSigned-off-by: Alexei Starovoitov <ast@kernel.org>

samples/bpf/syscall_tp_user.c

@@ -48,7 +48,7 @@ static void verify_map(int map_id)
 static int test(char *filename, int nr_tests)
 {
 	int map0_fds[nr_tests], map1_fds[nr_tests], fd, i, j = 0;
-	struct bpf_link *links[nr_tests * 4];
+	struct bpf_link **links = NULL;
 	struct bpf_object *objs[nr_tests];
 	struct bpf_program *prog;
 
@@ -60,6 +60,19 @@ static int test(char *filename, int nr_tests)
 			goto cleanup;
 		}
 
+		/* One-time initialization */
+		if (!links) {
+			int nr_progs = 0;
+
+			bpf_object__for_each_program(prog, objs[i])
+				nr_progs += 1;
+
+			links = calloc(nr_progs * nr_tests, sizeof(struct bpf_link *));
+
+			if (!links)
+				goto cleanup;
+		}
+
 		/* load BPF program */
 		if (bpf_object__load(objs[i])) {
 			fprintf(stderr, "loading BPF object file failed\n");
@@ -107,8 +120,12 @@ static int test(char *filename, int nr_tests)
 	}
 
 cleanup:
-	for (j--; j >= 0; j--)
-		bpf_link__destroy(links[j]);
+	if (links) {
+		for (j--; j >= 0; j--)
+			bpf_link__destroy(links[j]);
+
+		free(links);
+	}
 
 	for (i--; i >= 0; i--)
 		bpf_object__close(objs[i]);