[PATCH] string: implement str_has_prefix

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

In addition to strstarts which we already have, Linux also defines
str_has_prefix(), which returns the length of the prefix on success and
zero otherwise. This allows writing straight-forward code to skip over
a prefix in case it exists. Port it over.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 include/linux/string.h | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/include/linux/string.h b/include/linux/string.h
index ae5e5bca8d24..0c79d3e5cf3e 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -155,4 +155,25 @@ static inline bool strstarts(const char *str, const char *prefix)
 	return strncmp(str, prefix, strlen(prefix)) == 0;
 }
 
+/**
+ * str_has_prefix - Test if a string has a given prefix
+ * @str: The string to test
+ * @prefix: The string to see if @str starts with
+ *
+ * A common way to test a prefix of a string is to do:
+ *  strncmp(str, prefix, sizeof(prefix) - 1)
+ *
+ * But this can lead to bugs due to typos, or if prefix is a pointer
+ * and not a constant. Instead use str_has_prefix().
+ *
+ * Returns:
+ * * strlen(@prefix) if @str starts with @prefix
+ * * 0 if @str does not start with @prefix
+ */
+static __always_inline size_t str_has_prefix(const char *str, const char *prefix)
+{
+	size_t len = strlen(prefix);
+	return strncmp(str, prefix, len) == 0 ? len : 0;
+}
+
 #endif /* _LINUX_STRING_H_ */
-- 
2.30.2