[PATCH] selftests/mm:fix test_prctl_fork_exec return failure

[aigourensheng <shechenglong001@xxxxxxxxx>]

After calling fork() in test_prctl_fork_exec(), the global variable
ksm_full_scans_fd is initialized to 0 in the child process upon entering
the main function of ./ksm_functional_tests.

In the function call chain test_child_ksm() -> __mmap_and_merge_range ->
ksm_merge-> ksm_get_full_scans, start_scans = ksm_get_full_scans() will
return an error.  Therefore, the value of ksm_full_scans_fd needs to be
initialized before calling test_child_ksm in the child process.

Link: https://lkml.kernel.org/r/20240617052934.5834-1-shechenglong001@xxxxxxxxx
Signed-off-by: aigourensheng <shechenglong001@xxxxxxxxx>
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
Cc: Shuah Khan <shuah@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>

diff --git a/tools/testing/selftests/mm/ksm_functional_tests.c b/tools/testing/selftests/mm/ksm_functional_tests.c
index 37de82da9be7..b61803e36d1c 100644
--- a/tools/testing/selftests/mm/ksm_functional_tests.c
+++ b/tools/testing/selftests/mm/ksm_functional_tests.c
@@ -656,24 +656,8 @@ static void test_prot_none(void)
 	munmap(map, size);
 }
 
-int main(int argc, char **argv)
+static void init_global_file_handles(void)
 {
-	unsigned int tests = 8;
-	int err;
-
-	if (argc > 1 && !strcmp(argv[1], FORK_EXEC_CHILD_PRG_NAME)) {
-		exit(test_child_ksm());
-	}
-
-#ifdef __NR_userfaultfd
-	tests++;
-#endif
-
-	ksft_print_header();
-	ksft_set_plan(tests);
-
-	pagesize = getpagesize();
-
 	mem_fd = open("/proc/self/mem", O_RDWR);
 	if (mem_fd < 0)
 		ksft_exit_fail_msg("opening /proc/self/mem failed\n");
@@ -688,8 +672,30 @@ int main(int argc, char **argv)
 		ksft_exit_skip("open(\"/proc/self/pagemap\") failed\n");
 	proc_self_ksm_stat_fd = open("/proc/self/ksm_stat", O_RDONLY);
 	proc_self_ksm_merging_pages_fd = open("/proc/self/ksm_merging_pages",
-					      O_RDONLY);
+						O_RDONLY);
 	ksm_use_zero_pages_fd = open("/sys/kernel/mm/ksm/use_zero_pages", O_RDWR);
+}
+
+int main(int argc, char **argv)
+{
+	unsigned int tests = 8;
+	int err;
+
+	if (argc > 1 && !strcmp(argv[1], FORK_EXEC_CHILD_PRG_NAME)) {
+		init_global_file_handles();
+		exit(test_child_ksm());
+	}
+
+#ifdef __NR_userfaultfd
+	tests++;
+#endif
+
+	ksft_print_header();
+	ksft_set_plan(tests);
+
+	pagesize = getpagesize();
+
+	init_global_file_handles();
 
 	test_unmerge();
 	test_unmerge_zero_pages();