Re: [PATCH v2 1/2] selftests/resctrl: Fix schemata write error check

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

On Mon, 28 Aug 2023, Wieczor-Retman, Maciej wrote:

> Writing bitmasks to the schemata can fail when the bitmask doesn't
> adhere to some constraints defined by what a particular CPU supports.
> Some example of constraints are max length or having contiguous bits.
> The driver should properly return errors when any rule concerning
> bitmask format is broken.
> 
> Resctrl FS returns error codes from fprintf() only when fclose() is
> called. Current error checking scheme allows invalid bitmasks to be
> written into schemata file and the selftest doesn't notice because the
> fclose() error code isn't checked.
> 
> Substitute fopen(), flose() and fprintf() with open(), close() and
> write() to avoid error code buffering between fprintf() and fclose().
> 
> Add newline to the end of the schema string so it satisfies rdt
> schemata writing requirements.
> 
> Remove newline character from the schemat string after writing it to
> the schemata file so it prints correctly before function return.
> 
> Pass the string generated with strerror() to the "reason" buffer so
> the error message is more verbose. Extend "reason" buffer so it can hold
> longer messages.
> 
> Changelog v2:
> - Rewrite patch message.
> - Double "reason" buffer size to fit longer error explanation.
> - Redo file interactions with syscalls instead of stdio functions.
> 
> Signed-off-by: Wieczor-Retman, Maciej <maciej.wieczor-retman@intel.com>
> ---
>  tools/testing/selftests/resctrl/resctrlfs.c | 24 +++++++++++----------
>  1 file changed, 13 insertions(+), 11 deletions(-)
> 
> diff --git a/tools/testing/selftests/resctrl/resctrlfs.c b/tools/testing/selftests/resctrl/resctrlfs.c
> index bd36ee206602..0f9644e5a25e 100644
> --- a/tools/testing/selftests/resctrl/resctrlfs.c
> +++ b/tools/testing/selftests/resctrl/resctrlfs.c
> @@ -488,9 +488,8 @@ int write_bm_pid_to_resctrl(pid_t bm_pid, char *ctrlgrp, char *mongrp,
>   */
>  int write_schemata(char *ctrlgrp, char *schemata, int cpu_no, char *resctrl_val)
>  {
> -	char controlgroup[1024], schema[1024], reason[64];
> -	int resource_id, ret = 0;
> -	FILE *fp;
> +	char controlgroup[1024], schema[1024], reason[128];
> +	int resource_id, fp, ret = 0;

fp -> fd to follow the usual convention.

>  
>  	if (strncmp(resctrl_val, MBA_STR, sizeof(MBA_STR)) &&
>  	    strncmp(resctrl_val, MBM_STR, sizeof(MBM_STR)) &&
> @@ -518,27 +517,30 @@ int write_schemata(char *ctrlgrp, char *schemata, int cpu_no, char *resctrl_val)
>  
>  	if (!strncmp(resctrl_val, CAT_STR, sizeof(CAT_STR)) ||
>  	    !strncmp(resctrl_val, CMT_STR, sizeof(CMT_STR)))
> -		sprintf(schema, "%s%d%c%s", "L3:", resource_id, '=', schemata);
> +		snprintf(schema, sizeof(schema), "%s%d%c%s\n", "L3:",
> +			 resource_id, '=', schemata);
>  	if (!strncmp(resctrl_val, MBA_STR, sizeof(MBA_STR)) ||
>  	    !strncmp(resctrl_val, MBM_STR, sizeof(MBM_STR)))
> -		sprintf(schema, "%s%d%c%s", "MB:", resource_id, '=', schemata);
> +		snprintf(schema, sizeof(schema), "%s%d%c%s\n", "MB:",
> +			 resource_id, '=', schemata);
>  
> -	fp = fopen(controlgroup, "w");
> +	fp = open(controlgroup, O_WRONLY);
>  	if (!fp) {
>  		sprintf(reason, "Failed to open control group");
>  		ret = -1;
>  
>  		goto out;
>  	}
> -
> -	if (fprintf(fp, "%s\n", schema) < 0) {
> -		sprintf(reason, "Failed to write schemata in control group");
> -		fclose(fp);
> +	if (write(fp, schema, strlen(schema)) < 0) {

snprintf() returns you the length, just store the return value and there's 
no need to calculate it here.

> +		snprintf(reason, sizeof(reason),
> +			 "write() failed : %s", strerror(errno));
> +		close(fp);
>  		ret = -1;
>  
>  		goto out;
>  	}
> -	fclose(fp);
> +	close(fp);
> +	schema[strcspn(schema, "\n")] = 0;

Here too you can use the known length/index instead of strcspr().


-- 
 i.