linux-kernel - [PATCH 2/2] lib/string: Improve str_has

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20250407-imp_str_perf-v1-2-ed95d52964a4@quicinc.com>
Date: Mon, 07 Apr 2025 21:15:05 +0800
From: Zijun Hu <zijun_hu@...oud.com>
To: Kees Cook <kees@...nel.org>, Andy Shevchenko <andy@...nel.org>, 
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Zijun Hu <zijun_hu@...oud.com>, linux-hardening@...r.kernel.org, 
 linux-kernel@...r.kernel.org, Zijun Hu <quic_zijuhu@...cinc.com>, 
 "Rob Herring (Arm)" <robh@...nel.org>
Subject: [PATCH 2/2] lib/string: Improve str_has_prefix() performance

From: Zijun Hu <quic_zijuhu@...cinc.com>

str_has_prefix() is frequently invoked to return length of the prefix
string if a string has another string as prefix, but its performance
is degraded by the strlen() loop contained.

Improve its performance by eliminating the strlen() loop.

Link: https://lore.kernel.org/all/20250113234643.GA3631169-robh@kernel.org
Signed-off-by: Zijun Hu <quic_zijuhu@...cinc.com>
Cc: Rob Herring (Arm) <robh@...nel.org>
---
 include/linux/string.h | 21 +--------------------
 lib/string.c           | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 20 deletions(-)

diff --git a/include/linux/string.h b/include/linux/string.h
index e5f7defa277572719e1dbfdd264f3de6ef8544f1..394b76666ece0924c50264aaca39784d5630a2fe 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -533,25 +533,6 @@ void memcpy_and_pad(void *dest, size_t dest_len, const void *src, size_t count,
 	       sizeof(*(obj)) - offsetof(typeof(*(obj)), member));	\
 })
 
-/**
- * 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;
-}
+size_t str_has_prefix(const char *str, const char *prefix);
 
 #endif /* _LINUX_STRING_H_ */
diff --git a/lib/string.c b/lib/string.c
index ea52c8509328358e436766b1186a82419d45089d..17f1a070b190debc3eaeff6d1ae45b55bae69b29 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -332,6 +332,39 @@ int strncmp(const char *cs, const char *ct, size_t count)
 EXPORT_SYMBOL(strncmp);
 #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
+ *
+ * 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
+ */
+size_t str_has_prefix(const char *str, const char *prefix)
+{
+	const char *p = prefix;
+	unsigned char c1, c2;
+
+	do {
+		c1 = *str++;
+		c2 = *p++;
+
+		if (c1 != c2)
+			return c2 == '\0' ? p - 1 - prefix : 0;
+
+	} while (c2 != '\0');
+
+	return p - 1 - prefix;
+}
+EXPORT_SYMBOL(str_has_prefix);
+
 #ifndef __HAVE_ARCH_STRCHR
 /**
  * strchr - Find the first occurrence of a character in a string

-- 
2.34.1