[PATCH v3] selftests/mm: ksm-functional-tests: fix partial write handling

[Vineet Agarwal <agarwal.vineet2006@gmail.com>]

Update write() checks to properly detect and handle partial writes.

Previously, partial writes (ret > 0 && ret != len) could be treated
as success because write() does not set errno in this case and the
code returned -errno. This could result in returning 0 and
incorrectly signaling success.

Fix this by verifying that write() returns the full expected length
and treating any mismatch as failure.

Signed-off-by: Vineet Agarwal <agarwal.vineet2006@gmail.com>

Changes in v3:
- Simplify error handling as suggested by reviewer
- Return -1 when write() does not complete fully
- Rebase patch against base tree
---
 .../selftests/mm/ksm_functional_tests.c       | 27 +++++++++++++------
 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/tools/testing/selftests/mm/ksm_functional_tests.c b/tools/testing/selftests/mm/ksm_functional_tests.c
index 8d874c4754f3..4abd98be3f59 100644
--- a/tools/testing/selftests/mm/ksm_functional_tests.c
+++ b/tools/testing/selftests/mm/ksm_functional_tests.c
@@ -498,6 +498,8 @@ static void test_prctl_fork(void)
 static int start_ksmd_and_set_frequency(char *pages_to_scan, char *sleep_ms)
 {
 	int ksm_fd;
+	ssize_t ret;
+	size_t len, sleep_len;
 
 	ksm_fd = open("/sys/kernel/mm/ksm/run", O_RDWR);
 	if (ksm_fd < 0)
@@ -506,11 +508,17 @@ static int start_ksmd_and_set_frequency(char *pages_to_scan, char *sleep_ms)
 	if (write(ksm_fd, "1", 1) != 1)
 		return -errno;
 
-	if (write(pages_to_scan_fd, pages_to_scan, strlen(pages_to_scan)) <= 0)
-		return -errno;
+	len = strlen(pages_to_scan);
 
-	if (write(sleep_millisecs_fd, sleep_ms, strlen(sleep_ms)) <= 0)
-		return -errno;
+	ret = write(pages_to_scan_fd, pages_to_scan, len);
+	if (ret != len)
+		return -1;
+
+	sleep_len = strlen(sleep_ms);
+
+	ret = write(sleep_millisecs_fd, sleep_ms, sleep_len);
+	if (ret != sleep_len)
+		return -1;
 
 	return 0;
 }
@@ -518,6 +526,7 @@ static int start_ksmd_and_set_frequency(char *pages_to_scan, char *sleep_ms)
 static int stop_ksmd_and_restore_frequency(void)
 {
 	int ksm_fd;
+	ssize_t ret;
 
 	ksm_fd = open("/sys/kernel/mm/ksm/run", O_RDWR);
 	if (ksm_fd < 0)
@@ -526,11 +535,13 @@ static int stop_ksmd_and_restore_frequency(void)
 	if (write(ksm_fd, "2", 1) != 1)
 		return -errno;
 
-	if (write(pages_to_scan_fd, "100", 3) <= 0)
-		return -errno;
+	ret = write(pages_to_scan_fd, "100", 3);
+	if (ret != 3)
+		return -1;
 
-	if (write(sleep_millisecs_fd, "20", 2) <= 0)
-		return -errno;
+	ret = write(sleep_millisecs_fd, "20", 2);
+	if (ret != 2)
+		return -1;
 
 	return 0;
 }
-- 
2.54.0