[PATCH i-g-t v2] lib/igt_sysfs: make sure to write empty strings

[Kamil Konieczny]

Hi Lucas,
On 2024-01-31 at 19:11:56 -0800, Lucas De Marchi wrote:
> 	echo -n "" > /sys/module/<modulename>/parameters/<param>
> 
> doesn't really work as it will just create a open() + close() expecting
> the file to be truncated. The same issue happens with igt as it will
> stop writing when there are 0 chars to write. Special case the empty
> string so it always write a '\0' and make sure igt_sysfs_set() accounts
> for the extra null char.
> 
> Shell example:
> 	# echo -n "/foo" > /sys/module/firmware_class/parameters/path
> 	# cat /sys/module/firmware_class/parameters/path
> 	/foo
> 	# echo -n "" > /sys/module/firmware_class/parameters/path
> 	/foo
> 	# # make sure to actually write a \0:
> 	echo -ne "\0" > /sys/module/firmware_class/parameters/path
> 	# cat /sys/module/firmware_class/parameters/path
> 	/foo
> 
> Same thing happens when testing igt_sysfs_set():
>        int dirfd = open("/sys/module/firmware_class/parameters", O_RDONLY);
>        igt_sysfs_set(dirfd, "path", "");
> 
> Previously it was not really setting the param.
> 
> Signed-off-by: Lucas De Marchi <lucas.demarchi at intel.com>
> ---
>  lib/igt_sysfs.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/igt_sysfs.c b/lib/igt_sysfs.c
> index 567b4f6d5..64c2aaabb 100644
> --- a/lib/igt_sysfs.c
> +++ b/lib/igt_sysfs.c
> @@ -406,7 +406,7 @@ int igt_sysfs_read(int dir, const char *attr, void *data, int len)
>   */
>  bool igt_sysfs_set(int dir, const char *attr, const char *value)
>  {
> -	int len = strlen(value);
> +	int len = strlen(value) + 1;
>  	return igt_sysfs_write(dir, attr, value, len) == len;

Why not adding write "\n" at end?

>  }
>  
> @@ -513,8 +513,16 @@ int igt_sysfs_vprintf(int dir, const char *attr, const char *fmt, va_list ap)
>  		return -errno;
>  
>  	va_copy(tmp, ap);
> -	ret = vsnprintf(buf, sizeof(stack), fmt, tmp);
> +	ret = vsnprintf(stack, sizeof(stack), fmt, tmp);
------------------- ^^^^^
Nice catch and looks like a separate bug fix, please send
as separate patch.

Regards,
Kamil

>  	va_end(tmp);
> +
> +	/*
> +	 * make sure to always issue a write() syscall, even if writing an empty string,
> +	 * otherwise values in sysfs like module parameters don't really get overwritten
> +	 */
> +	if (igt_debug_on(ret == 0))
> +		return igt_writen(fd, "", 1);
> +
>  	if (igt_debug_on(ret < 0))
>  		return -EINVAL;
>  
> -- 
> 2.43.0
>