selftests/mm:fix test_prctl_fork_exec return failure

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@gmail.com
Signed-off-by: aigourensheng <shechenglong001@gmail.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: aigourensheng

tools/testing/selftests/mm/ksm_functional_tests.c

@@ -656,12 +656,33 @@ static void test_prot_none(void)
 	munmap(map, size);
 }
 
+static void init_global_file_handles(void)
+{
+	mem_fd = open("/proc/self/mem", O_RDWR);
+	if (mem_fd < 0)
+		ksft_exit_fail_msg("opening /proc/self/mem failed\n");
+	ksm_fd = open("/sys/kernel/mm/ksm/run", O_RDWR);
+	if (ksm_fd < 0)
+		ksft_exit_skip("open(\"/sys/kernel/mm/ksm/run\") failed\n");
+	ksm_full_scans_fd = open("/sys/kernel/mm/ksm/full_scans", O_RDONLY);
+	if (ksm_full_scans_fd < 0)
+		ksft_exit_skip("open(\"/sys/kernel/mm/ksm/full_scans\") failed\n");
+	pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
+	if (pagemap_fd < 0)
+		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);
+	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());
 	}
 
@@ -674,22 +695,7 @@ int main(int argc, char **argv)
 
 	pagesize = getpagesize();
 
-	mem_fd = open("/proc/self/mem", O_RDWR);
-	if (mem_fd < 0)
-		ksft_exit_fail_msg("opening /proc/self/mem failed\n");
-	ksm_fd = open("/sys/kernel/mm/ksm/run", O_RDWR);
-	if (ksm_fd < 0)
-		ksft_exit_skip("open(\"/sys/kernel/mm/ksm/run\") failed\n");
-	ksm_full_scans_fd = open("/sys/kernel/mm/ksm/full_scans", O_RDONLY);
-	if (ksm_full_scans_fd < 0)
-		ksft_exit_skip("open(\"/sys/kernel/mm/ksm/full_scans\") failed\n");
-	pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
-	if (pagemap_fd < 0)
-		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);
-	ksm_use_zero_pages_fd = open("/sys/kernel/mm/ksm/use_zero_pages", O_RDWR);
+	init_global_file_handles();
 
 	test_unmerge();
 	test_unmerge_zero_pages();