[PATCH v5 2/7] string: Warn if the prefix is the empty string

[Alejandro Colomar <alx@kernel.org>]

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This corner case is mishandled by this function.
Warn if it happens.  Also, document that if such corner case
is needed, strprefix() is preferable.

We need to move str_has_prefix() to "lib/string.h" to be able to use
WARN_ON_ONCE().

Reported-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Link: https://lore.kernel.org/linux-hardening/20261104-fwnode-for-each-prop-v1-1-6e28fe1c72c4@oss.qualcomm.com/
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
---
 include/linux/string.h | 19 +------------------
 lib/string.c           | 24 ++++++++++++++++++++++++
 2 files changed, 25 insertions(+), 18 deletions(-)

diff --git a/include/linux/string.h b/include/linux/string.h
index 04e8a8c0f05a..dcda1b7801bb 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -194,6 +194,7 @@ extern char * strstr(const char *, const char *);
 #ifndef __HAVE_ARCH_STRNSTR
 extern char * strnstr(const char *, const char *, size_t);
 #endif
+extern size_t str_has_prefix(const char *str, const char *prefix);
 #ifndef __HAVE_ARCH_STRLEN
 extern __kernel_size_t strlen(const char *);
 #endif
@@ -566,24 +567,6 @@ static inline char *strprefix(const char *s, const char *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
- *
- * This is similar to strprefix(), but returns an offset instead of
- * a pointer.
- *
- * 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;
-}
-
 /**
  * strstarts - does @str start with @prefix?
  * @str: string to examine
diff --git a/lib/string.c b/lib/string.c
index a3778d5aab4a..61a1e79c1b87 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -774,6 +774,30 @@ char *strnstr(const char *s1, const char *s2, size_t len)
 EXPORT_SYMBOL(strnstr);
 #endif
 
+/**
+ * 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
+ *
+ * This is similar to strprefix(), but returns an offset instead of
+ * a pointer.
+ *
+ * Unlike for strprefix(), if the prefix is an empty string, the
+ * behavior is undefined.  If you need to support arbitrary prefixes,
+ * use strprefix().
+ *
+ * Returns:
+ * * strlen(@prefix) if @str starts with @prefix
+ * * 0 if @str does not start with @prefix
+ */
+size_t str_has_prefix(const char *str, const char *prefix)
+{
+	size_t len = strlen(prefix);
+	WARN_ON_ONCE(len == 0);
+	return strncmp(str, prefix, len) == 0 ? len : 0;
+}
+EXPORT_SYMBOL(str_has_prefix);
+
 #ifndef __HAVE_ARCH_MEMCHR
 /**
  * memchr - Find a character in an area of memory.
-- 
2.55.0


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