Re: [PATCH v3] string.h: Add str_has_prefix() helper

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 21 Dec 2018 18:25:07 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Fri, 21 Dec 2018 15:19:33 -0800
> Joe Perches <joe@perches.com> wrote:
> 
> > I believe this should be bool.
> > 
> > I don't find a use for non-zero assigned len value in the kernel
> > for strncmp and I believe the function should simply be:
> > 
> > static inline bool str_has_prefix(const char *str, const char prefix[])
> > {
> > 	return !strncmp(str, prefix, strlen(prefix));
> > }  
> 
> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
> index 3bb2b3351e35..e4566b9c2553 100644
> --- a/drivers/scsi/sd.c
> +++ b/drivers/scsi/sd.c
> @@ -172,8 +172,8 @@ cache_type_store(struct device *dev, struct device_attribute *attr,
>  		 * it's not worth the risk */
>  		return -EINVAL;
>  
> -	if (strncmp(buf, temp, sizeof(temp) - 1) == 0) {
> -		buf += sizeof(temp) - 1;
> +	if ((len = str_has_prefix(buf, temp))) {
> +		buf += len;
>  		sdkp->cache_override = 1;
>  	} else {
>  		sdkp->cache_override = 0;
> 
> And there's more places like this.
>

Heck, even the tracing code would like this. From the code that started
this entire discussion:

-- Steve

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 9d590138f870..632410c6e6c0 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -4342,12 +4342,13 @@ static int parse_actions(struct hist_trigger_data *hist_data)
 	unsigned int i;
 	int ret = 0;
 	char *str;
+	int len;
 
 	for (i = 0; i < hist_data->attrs->n_actions; i++) {
 		str = hist_data->attrs->action_str[i];
 
-		if (str_has_prefix(str, "onmatch(")) {
-			char *action_str = str + sizeof("onmatch(") - 1;
+		if ((len = str_has_prefix(str, "onmatch("))) {
+			char *action_str = str + len;
 
 			data = onmatch_parse(tr, action_str);
 			if (IS_ERR(data)) {
@@ -4355,8 +4356,8 @@ static int parse_actions(struct hist_trigger_data *hist_data)
 				break;
 			}
 			data->fn = action_trace;
-		} else if (str_has_prefix(str, "onmax(")) {
-			char *action_str = str + sizeof("onmax(") - 1;
+		} else if ((len = str_has_prefix(str, "onmax("))) {
+			char *action_str = str + len;
 
 			data = onmax_parse(action_str);
 			if (IS_ERR(data)) {