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

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

Previously, partial writes (ret > 0 && ret != len) would return
-errno, but write() does not set errno in this case. This could
result in returning 0 and incorrectly signaling success.

Fix this by:
  - returning -errno only on actual failures (ret < 0)
  - returning -EIO when a partial write is detected

This ensures partial writes are treated as errors and prevents
false success reporting in tests.

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

Changes in v2:
- Fix incorrect use of -errno on partial writes
- Return -EIO when write() completes partially
---
 tools/testing/selftests/mm/ksm_functional_tests.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/mm/ksm_functional_tests.c b/tools/testing/selftests/mm/ksm_functional_tests.c
index 9a8c852acd68..c80254cda926 100644
--- a/tools/testing/selftests/mm/ksm_functional_tests.c
+++ b/tools/testing/selftests/mm/ksm_functional_tests.c
@@ -507,9 +507,13 @@ static int start_ksmd_and_set_frequency(char *pages_to_scan, char *sleep_ms)
 	if (write(ksm_fd, "1", 1) != 1)
 		return -errno;
 
-	ret = write(pages_to_scan_fd, pages_to_scan, strlen(pages_to_scan));
-	if (ret < 0 || ret != strlen(pages_to_scan))
+	ssize_t len = strlen(pages_to_scan);
+
+	ret = write(pages_to_scan_fd, pages_to_scan, len);
+	if (ret < 0)
 		return -errno;
+	if (ret != len)
+		return -EIO;
 
 	ret = write(sleep_millisecs_fd, sleep_ms, strlen(sleep_ms));
 	if (ret < 0 || ret != strlen(sleep_ms))
@@ -531,8 +535,10 @@ static int stop_ksmd_and_restore_frequency(void)
 		return -errno;
 
 	ret = write(pages_to_scan_fd, "100", 3);
-	if (ret < 0 || ret != 3)
+	if (ret < 0)
 		return -errno;
+	if (ret != 3)
+		return -EIO;
 
 	ret = write(sleep_millisecs_fd, "20", 2);
 	if (ret < 0 || ret != 2)
-- 
2.54.0

On Sun, May 03, 2026 at 11:30:30AM +0530, Vineet Agarwal wrote:
> Update write() checks to properly detect and handle partial writes.
> 
> Previously, partial writes (ret > 0 && ret != len) would return
> -errno, but write() does not set errno in this case. This could
> result in returning 0 and incorrectly signaling success.
> 
> Fix this by:
>   - returning -errno only on actual failures (ret < 0)
>   - returning -EIO when a partial write is detected
> 
> This ensures partial writes are treated as errors and prevents
> false success reporting in tests.
> 
> Signed-off-by: Vineet Agarwal <agarwal.vineet2006@gmail.com>
> 
> Changes in v2:
> - Fix incorrect use of -errno on partial writes
> - Return -EIO when write() completes partially

Please start a new thread for a new version of the patch.

> ---
>  tools/testing/selftests/mm/ksm_functional_tests.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/ksm_functional_tests.c b/tools/testing/selftests/mm/ksm_functional_tests.c
> index 9a8c852acd68..c80254cda926 100644
> --- a/tools/testing/selftests/mm/ksm_functional_tests.c
> +++ b/tools/testing/selftests/mm/ksm_functional_tests.c
> @@ -507,9 +507,13 @@ static int start_ksmd_and_set_frequency(char *pages_to_scan, char *sleep_ms)
>  	if (write(ksm_fd, "1", 1) != 1)
>  		return -errno;
>  
> -	ret = write(pages_to_scan_fd, pages_to_scan, strlen(pages_to_scan));
> -	if (ret < 0 || ret != strlen(pages_to_scan))
> +	ssize_t len = strlen(pages_to_scan);

The v2 diff should be against the base, not against v1.

> +
> +	ret = write(pages_to_scan_fd, pages_to_scan, len);
> +	if (ret < 0)
>  		return -errno;
> +	if (ret != len)
> +		return -EIO;

Just return -1 if write() != strlen(), the actual error code does not
matter anyway.

>  	ret = write(sleep_millisecs_fd, sleep_ms, strlen(sleep_ms));
>  	if (ret < 0 || ret != strlen(sleep_ms))
> @@ -531,8 +535,10 @@ static int stop_ksmd_and_restore_frequency(void)
>  		return -errno;
>  
>  	ret = write(pages_to_scan_fd, "100", 3);
> -	if (ret < 0 || ret != 3)
> +	if (ret < 0)
>  		return -errno;
> +	if (ret != 3)
> +		return -EIO;
>  
>  	ret = write(sleep_millisecs_fd, "20", 2);
>  	if (ret < 0 || ret != 2)
> -- 
> 2.54.0
> 

-- 
Sincerely yours,
Mike.