[PATCH] selftests/mm: ksm-functional-tests: fix partial writes

Vineet Agarwal
2026-05-02T00:57:17Z

Some write() calls only check for <= 0, which fails to detect
partial writes. A positive return value smaller than the requested
length is currently treated as success, potentially resulting in
truncated writes to KSM sysfs interfaces.

Other write() usages in the same file already verify that the full
length is written, making the current checks inconsistent.

Fix by verifying that write() returns the full expected length.

This also aligns with recent improvements in vm_util write_file(),
which enforce strict validation of write() results.

Signed-off-by: Vineet Agarwal <agarwal.vineet2006@gmail.com>
---
tools/testing/selftests/mm/ksm_functional_tests.c | 14 ++++++++++----
1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/mm/ksm_functional_tests.c b/tools/testing/selftests/mm/ksm_functional_tests.c
index 8d874c4754f3..9a8c852acd68 100644
--- a/tools/testing/selftests/mm/ksm_functional_tests.c
+++ b/tools/testing/selftests/mm/ksm_functional_tests.c
@@ -498,6 +498,7 @@ 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;

ksm_fd = open("/sys/kernel/mm/ksm/run", O_RDWR);
if (ksm_fd < 0)
@@ -506,10 +507,12 @@ 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)
+ ret = write(pages_to_scan_fd, pages_to_scan, strlen(pages_to_scan));
+ if (ret < 0 || ret != strlen(pages_to_scan))
return -errno;

- if (write(sleep_millisecs_fd, sleep_ms, strlen(sleep_ms)) <= 0)
+ ret = write(sleep_millisecs_fd, sleep_ms, strlen(sleep_ms));
+ if (ret < 0 || ret != strlen(sleep_ms))
return -errno;

return 0;
@@ -518,6 +521,7 @@ 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)
@@ -526,10 +530,12 @@ 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)
+ ret = write(pages_to_scan_fd, "100", 3);
+ if (ret < 0 || ret != 3)
return -errno;

- if (write(sleep_millisecs_fd, "20", 2) <= 0)
+ ret = write(sleep_millisecs_fd, "20", 2);
+ if (ret < 0 || ret != 2)
return -errno;

return 0;
--
2.54.0