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

[Mike Rapoport <rppt@kernel.org>]

Hi,

On Sun, May 03, 2026 at 03:38:40PM +0530, Vineet Agarwal wrote:
> 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;

ret and len should be of the same type and one len variable is enough.
  
>  	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 (write(pages_to_scan_fd, pages_to_scan, len) != len)
		return -1;

would be enough here and below.

> +	if (ret != len)
> +		return -1;
> +

-- 
Sincerely yours,
Mike.