selftests/bpf: Fix invalid pointer check in get_xlated_program()

Dan Carpenter reported invalid check for calloc() result in
test_verifier.c:get_xlated_program():

  ./tools/testing/selftests/bpf/test_verifier.c:1365 get_xlated_program()
  warn: variable dereferenced before check 'buf' (see line 1364)

  ./tools/testing/selftests/bpf/test_verifier.c
    1363		*cnt = xlated_prog_len / buf_element_size;
    1364		*buf = calloc(*cnt, buf_element_size);
    1365		if (!buf) {

  This should be if (!*buf) {

    1366			perror("can't allocate xlated program buffer");
    1367			return -ENOMEM;

This commit refactors the get_xlated_program() to avoid using double
pointer type.

Fixes: 933ff531 ("selftests/bpf: specify expected instructions in test_verifier tests")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Closes: https://lore.kernel.org/bpf/ZH7u0hEGVB4MjGZq@moroto/
Link: https://lore.kernel.org/bpf/20230609221637.2631800-1-eddyz87@gmail.com

tools/testing/selftests/bpf/test_verifier.c

@@ -1341,45 +1341,46 @@ static bool cmp_str_seq(const char *log, const char *exp)
 	return true;
 }
 
-static int get_xlated_program(int fd_prog, struct bpf_insn **buf, int *cnt)
+static struct bpf_insn *get_xlated_program(int fd_prog, int *cnt)
 {
+	__u32 buf_element_size = sizeof(struct bpf_insn);
 	struct bpf_prog_info info = {};
 	__u32 info_len = sizeof(info);
 	__u32 xlated_prog_len;
-	__u32 buf_element_size = sizeof(struct bpf_insn);
+	struct bpf_insn *buf;
 
 	if (bpf_prog_get_info_by_fd(fd_prog, &info, &info_len)) {
 		perror("bpf_prog_get_info_by_fd failed");
-		return -1;
+		return NULL;
 	}
 
 	xlated_prog_len = info.xlated_prog_len;
 	if (xlated_prog_len % buf_element_size) {
 		printf("Program length %d is not multiple of %d\n",
 		       xlated_prog_len, buf_element_size);
-		return -1;
+		return NULL;
 	}
 
 	*cnt = xlated_prog_len / buf_element_size;
-	*buf = calloc(*cnt, buf_element_size);
+	buf = calloc(*cnt, buf_element_size);
 	if (!buf) {
 		perror("can't allocate xlated program buffer");
-		return -ENOMEM;
+		return NULL;
 	}
 
 	bzero(&info, sizeof(info));
 	info.xlated_prog_len = xlated_prog_len;
-	info.xlated_prog_insns = (__u64)(unsigned long)*buf;
+	info.xlated_prog_insns = (__u64)(unsigned long)buf;
 	if (bpf_prog_get_info_by_fd(fd_prog, &info, &info_len)) {
 		perror("second bpf_prog_get_info_by_fd failed");
 		goto out_free_buf;
 	}
 
-	return 0;
+	return buf;
 
 out_free_buf:
-	free(*buf);
-	return -1;
+	free(buf);
+	return NULL;
 }
 
 static bool is_null_insn(struct bpf_insn *insn)
@@ -1512,7 +1513,8 @@ static bool check_xlated_program(struct bpf_test *test, int fd_prog)
 	if (!check_expected && !check_unexpected)
 		goto out;
 
-	if (get_xlated_program(fd_prog, &buf, &cnt)) {
+	buf = get_xlated_program(fd_prog, &cnt);
+	if (!buf) {
 		printf("FAIL: can't get xlated program\n");
 		result = false;
 		goto out;