Re: [PATCH v6 4/6] selftests/mm/vm_util: robust write_file()

[Mike Rapoport <rppt@kernel.org>]

On Tue, Mar 24, 2026 at 09:33:14AM +0800, Chunyu Hu wrote:
> Add two more checks for buflen and numwritten. The buflen should be at
> least one, otherwise the 'buflen - 1' could underflow and cause trouble.
> The numwritten should be equal to 'buflen - 1'. 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>
> ---
> 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 | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
> index ad96d19d1b85..572ccb99de8e 100644
> --- a/tools/testing/selftests/mm/vm_util.c
> +++ b/tools/testing/selftests/mm/vm_util.c
> @@ -767,15 +767,23 @@ 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 < 1)
> +		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));

We have ksft_exit_fail_perror(), fits great here

>  
>  	numwritten = write(fd, buf, buflen - 1);
> +	saved_errno = errno;
>  	close(fd);
> +	errno = saved_errno;
>  	if (numwritten < 1)
> -		ksft_exit_fail_msg("Write failed\n");
> +		ksft_exit_fail_msg("%s write(%s) failed: %s\n", path, buf,
> +				strerror(errno));

and here.

> +	if (numwritten != buflen - 1)
> +		ksft_exit_fail_msg("%s write(%s) is truncated, expected %zu bytes, got %zd bytes\n",
> +				path, buf, buflen - 1, numwritten);
>  }
> -- 
> 2.53.0
> 

-- 
Sincerely yours,
Mike.