As Bartosz reported, str_has_prefix() has a bug: it can't differentiate
between an empty prefix matching an empty string from a non-matching
prefix.  This is a fundamental design flaw in this API, and not
something that can be fixed.

As always, this is a consequence of choosing sizes instead of pointers.
Sizes are often the wrong choice.  In this specific case, pointers are
more capable of differentiating these cases, as NULL is a special
sentinel value that doesn't need to overload a valid value.

Let's implement strprefix(), a function that returns a pointer
right after the prefix if found, and a null pointer if not found.

We'll slowly migrate existing code from str_has_prefix() to strprefix().

Reported-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Cc: Andreas Schwab <schwab@linux-m68k.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Andy Shevchenko <andy@kernel.org>
Cc: Bartosz Golaszewski <brgl@kernel.org>
Cc: Daniel Scally <djrscally@gmail.com>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Cc: Joe Perches <joe@perches.com>
Cc: Johan Hovold <johan@kernel.org>
Cc: Kees Cook <kees@kernel.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Len Brown <lenb@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Linus Walleij <linusw@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
Cc: Saravana Kannan <saravanak@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Tom Zanussi <zanussi@kernel.org
Cc: linux-acpi@vger.kernel.org
Cc: driver-core@lists.linux.dev
Cc: linux-kernel@vger.kernel.org
Cc: linux-hardening@vger.kernel.org
Cc: devicetree@vger.kernel.org
Cc: linux-gpio@vger.kernel.org
Signed-off-by: Alejandro Colomar <alx@kernel.org>
---
 include/linux/string.h | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/include/linux/string.h b/include/linux/string.h
index 6cb5cdd01158..0397e630bdc8 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -545,16 +545,33 @@ void memcpy_and_pad(void *dest, size_t dest_len, const void *src, size_t count,
 	       sizeof(*(obj)) - offsetof(typeof(*(obj)), member));	\
 })
 
+/**
+ * strprefix - string prefix
+ * @s: The string to be tested
+ * @prefix: The string to see if @s starts with
+ *
+ * Returns:
+ * *  @s+strlen(@prefix) if @s starts with @prefix
+ * *  NULL if @s does not start with @prefix
+ */
+static __always_inline char *strprefix(const char *s, const char *prefix)
+{
+	size_t len = strlen(prefix);
+	return strncmp(s, prefix, len) == 0 ? (char *) s + len : NULL;
+}
+
 /**
  * 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
  *
+ * DEPRECATED: Use strprefix()
+ *
  * 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().
+ * and not a constant. Instead use strprefix().
  *
  * Returns:
  * * strlen(@prefix) if @str starts with @prefix
-- 
2.55.0