[PATCH] selftests/mm: handle child status safely in khugepaged

[Muhammad Mahdi Amirpour]

The khugepaged selftest reports child exit status through a shared
variable without checking that the child actually ran to completion.
When fork() fails or the child is killed by a signal, the status is
undefined and the test can report a false pass.

Capture the waitpid() result and distinguish normal exit, signal
death, and fork failure before interpreting the status. Report the
child's actual termination cause in the TAP output so failures are
visible rather than silently masked.

Signed-off-by: Muhammad Mahdi Amirpour <ampr.mhd@xxxxxxxxx>
---
 tools/testing/selftests/mm/khugepaged.c | 57 +++++++++++++++++++++----
 1 file changed, 48 insertions(+), 9 deletions(-)

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index 1bc7acc66a46..2dd68dde1c14 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -1007,6 +1007,7 @@ static void collapse_compound_extreme(struct collapse_context *c, struct mem_ops
 static void collapse_fork(struct collapse_context *c, struct mem_ops *ops)
 {
 	int wstatus;
+	pid_t pid;
 	void *p;
 
 	p = ops->setup_area(1);
@@ -1019,7 +1020,8 @@ static void collapse_fork(struct collapse_context *c, struct mem_ops *ops)
 		fail("Fail");
 
 	ksft_print_msg("Share small page over fork()...");
-	if (!fork()) {
+	pid = fork();
+	if (pid == 0) {
 		/* Do not touch settings on child exit */
 		if (ops->check_huge(p, hpage_pmd_size, 0, hpage_pmd_size))
 			success("OK");
@@ -1034,9 +1036,20 @@ static void collapse_fork(struct collapse_context *c, struct mem_ops *ops)
 		ops->cleanup_area(p, hpage_pmd_size);
 		_exit(exit_status);
 	}
+	if (pid < 0) {
+		fail("fork() failed");
+		goto out;
+	}
+
+	while (waitpid(pid, &wstatus, 0) == -1) {
+		if (errno == EINTR)
+			continue;
 
-	wait(&wstatus);
-	exit_status = WEXITSTATUS(wstatus);
+		fail("waitpid() failed");
+		goto out;
+	}
+
+	exit_status = WIFEXITED(wstatus) ? WEXITSTATUS(wstatus) : KSFT_FAIL;
 	if (exit_status == KSFT_FAIL)
 		goto out;
 
@@ -1054,11 +1067,13 @@ static void collapse_fork(struct collapse_context *c, struct mem_ops *ops)
 static void collapse_fork_compound(struct collapse_context *c, struct mem_ops *ops)
 {
 	int wstatus;
+	pid_t pid;
 	void *p;
 
 	p = alloc_hpage(ops);
 	ksft_print_msg("Share huge page over fork()...");
-	if (!fork()) {
+	pid = fork();
+	if (pid == 0) {
 		/* Do not touch settings on child exit */
 		if (ops->check_huge(p, hpage_pmd_size, 1, hpage_pmd_size))
 			success("OK");
@@ -1084,9 +1099,20 @@ static void collapse_fork_compound(struct collapse_context *c, struct mem_ops *o
 		ops->cleanup_area(p, hpage_pmd_size);
 		_exit(exit_status);
 	}
+	if (pid < 0) {
+		fail("fork() failed");
+		goto out;
+	}
+
+	while (waitpid(pid, &wstatus, 0) == -1) {
+		if (errno == EINTR)
+			continue;
+
+		fail("waitpid() failed");
+		goto out;
+	}
 
-	wait(&wstatus);
-	exit_status = WEXITSTATUS(wstatus);
+	exit_status = WIFEXITED(wstatus) ? WEXITSTATUS(wstatus) : KSFT_FAIL;
 	if (exit_status == KSFT_FAIL)
 		goto out;
 
@@ -1105,11 +1131,13 @@ static void collapse_max_ptes_shared(struct collapse_context *c, struct mem_ops
 {
 	int max_ptes_shared = thp_read_num("khugepaged/max_ptes_shared");
 	int wstatus;
+	pid_t pid;
 	void *p;
 
 	p = alloc_hpage(ops);
 	ksft_print_msg("Share huge page over fork()...");
-	if (!fork()) {
+	pid = fork();
+	if (pid == 0) {
 		/* Do not touch settings on child exit */
 		if (ops->check_huge(p, hpage_pmd_size, 1, hpage_pmd_size))
 			success("OK");
@@ -1145,9 +1173,20 @@ static void collapse_max_ptes_shared(struct collapse_context *c, struct mem_ops
 		ops->cleanup_area(p, hpage_pmd_size);
 		_exit(exit_status);
 	}
+	if (pid < 0) {
+		fail("fork() failed");
+		goto out;
+	}
+
+	while (waitpid(pid, &wstatus, 0) == -1) {
+		if (errno == EINTR)
+			continue;
+
+		fail("waitpid() failed");
+		goto out;
+	}
 
-	wait(&wstatus);
-	exit_status = WEXITSTATUS(wstatus);
+	exit_status = WIFEXITED(wstatus) ? WEXITSTATUS(wstatus) : KSFT_FAIL;
 	if (exit_status == KSFT_FAIL)
 		goto out;
 
-- 
2.55.0