Re: [PATCH v8 4/6] selftests/mm/vm_util: robust write_file()

[Lorenzo Stoakes <ljs@kernel.org>]

On Thu, Apr 02, 2026 at 09:45:41AM +0800, Chunyu Hu wrote:
> Add three more checks for buflen and numwritten. The buflen should be at
> least two, that means at least one char and the null-end. The error case
> check is added by checking numwriten < 0 instead of numwritten < 1. And the
> truncate case is checked. The test will exit if any of these conditions
> aren't met.
>
> Additionally, add more print information when a write failure occurs or
> a truncated write happens, providing clearer diagnostics.
>
> Signed-off-by: Chunyu Hu <chuhu@redhat.com>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>

> ---
> Changes in v8:
>   - go back to ksft_exit_fail_msg in the two error exit path as no clean
>     ksft_exit_fail_perror that supports va_list
> Changes in v7:
>   - add an empty line before the 'buflen < 1' line.  And change to
>    'buflen < 2 to limit the buflen at least to be 2, so at least one char
>     is allowed.
>   - handle the numwriten < 0 case, which is the error path.
>   - use ksft_exit_fail_perror in error exit path.
>   - use %.*s to print the buf when error happened
> Changes in v6:
>   - save/restore errno before and after the close(), so the log could show
>     the correct error info on failure. Suggested by AI.
> Chagnes in v5:
>   - new patch for making improve on write_file. Add more safety checks and
>     diagnostics info in log
> ---
>  tools/testing/selftests/mm/vm_util.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
> index ad96d19d1b85..db94564f4431 100644
> --- a/tools/testing/selftests/mm/vm_util.c
> +++ b/tools/testing/selftests/mm/vm_util.c
> @@ -767,15 +767,24 @@ int unpoison_memory(unsigned long pfn)
>
>  void write_file(const char *path, const char *buf, size_t buflen)
>  {
> -	int fd;
> +	int fd, saved_errno;
>  	ssize_t numwritten;
>
> +	if (buflen < 2)
> +		ksft_exit_fail_msg("Incorrect buffer len: %zu\n", buflen);
> +
>  	fd = open(path, O_WRONLY);
>  	if (fd == -1)
>  		ksft_exit_fail_msg("%s open failed: %s\n", path, strerror(errno));
>
>  	numwritten = write(fd, buf, buflen - 1);
> +	saved_errno = errno;
>  	close(fd);
> -	if (numwritten < 1)
> -		ksft_exit_fail_msg("Write failed\n");
> +	errno = saved_errno;
> +	if (numwritten < 0)
> +		ksft_exit_fail_msg("%s write(%.*s) failed: %s\n", path, (int)(buflen - 1),
> +				buf, strerror(errno));
> +	if (numwritten != buflen - 1)
> +		ksft_exit_fail_msg("%s write(%.*s) is truncated, expected %zu bytes, got %zd bytes\n",
> +				path, (int)(buflen - 1), buf, buflen - 1, numwritten);
>  }
> --
> 2.53.0
>