printk: Replace strncmp() with str_has_prefix()

strncmp(str, const, len) is error-prone because len is easy to have typo.
An example is the hard-coded len has counting error or sizeof(const)
forgets - 1.

So we prefer using newly introduced str_has_prefix() to substitute
such strncmp() to make code better.

Link: http://lkml.kernel.org/r/20190809071034.17279-1-hslester96@gmail.com
Cc: "Steven Rostedt" <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Reviewed-by:  Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
[pmladek@suse.com: Slightly updated and reformatted the commit message.]
Signed-off-by: Petr Mladek <pmladek@suse.com>

kernel/printk/braille.c

@@ -11,11 +11,18 @@
 
 int _braille_console_setup(char **str, char **brl_options)
 {
-	if (!strncmp(*str, "brl,", 4)) {
+	size_t len;
+
+	len = str_has_prefix(*str, "brl,");
+	if (len) {
 		*brl_options = "";
-		*str += 4;
-	} else if (!strncmp(*str, "brl=", 4)) {
-		*brl_options = *str + 4;
+		*str += len;
+		return 0;
+	}
+
+	len = str_has_prefix(*str, "brl=");
+	if (len) {
+		*brl_options = *str + len;
 		*str = strchr(*brl_options, ',');
 		if (!*str) {
 			pr_err("need port name after brl=\n");

kernel/printk/printk.c

@@ -118,19 +118,29 @@ static unsigned int __read_mostly devkmsg_log = DEVKMSG_LOG_MASK_DEFAULT;
 
 static int __control_devkmsg(char *str)
 {
+	size_t len;
+
 	if (!str)
 		return -EINVAL;
 
-	if (!strncmp(str, "on", 2)) {
+	len = str_has_prefix(str, "on");
+	if (len) {
 		devkmsg_log = DEVKMSG_LOG_MASK_ON;
-		return 2;
-	} else if (!strncmp(str, "off", 3)) {
-		devkmsg_log = DEVKMSG_LOG_MASK_OFF;
-		return 3;
-	} else if (!strncmp(str, "ratelimit", 9)) {
-		devkmsg_log = DEVKMSG_LOG_MASK_DEFAULT;
-		return 9;
+		return len;
 	}
+
+	len = str_has_prefix(str, "off");
+	if (len) {
+		devkmsg_log = DEVKMSG_LOG_MASK_OFF;
+		return len;
+	}
+
+	len = str_has_prefix(str, "ratelimit");
+	if (len) {
+		devkmsg_log = DEVKMSG_LOG_MASK_DEFAULT;
+		return len;
+	}
+
 	return -EINVAL;
 }