linux-kernel - [PATCH 1/2] lib/string: Improve strstarts() performance

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Message-Id: <20250407-imp_str_perf-v1-1-ed95d52964a4@quicinc.com>
Date: Mon, 07 Apr 2025 21:15:04 +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 1/2] lib/string: Improve strstarts() performance

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

strstarts() is frequently invoked to test 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 | 10 +---------
 lib/string.c           | 22 ++++++++++++++++++++++
 2 files changed, 23 insertions(+), 9 deletions(-)

diff --git a/include/linux/string.h b/include/linux/string.h
index 01621ad0f598ccdfae819b1a5058607e79d8a751..e5f7defa277572719e1dbfdd264f3de6ef8544f1 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -345,15 +345,7 @@ extern ssize_t memory_read_from_buffer(void *to, size_t count, loff_t *ppos,
 
 int ptr_to_hashval(const void *ptr, unsigned long *hashval_out);
 
-/**
- * strstarts - does @str start with @prefix?
- * @str: string to examine
- * @prefix: prefix to look for.
- */
-static inline bool strstarts(const char *str, const char *prefix)
-{
-	return strncmp(str, prefix, strlen(prefix)) == 0;
-}
+bool strstarts(const char *str, const char *prefix);
 
 size_t memweight(const void *ptr, size_t bytes);
 
diff --git a/lib/string.c b/lib/string.c
index eb4486ed40d259e43dfb63d76c2bd57ef74e2fd1..ea52c8509328358e436766b1186a82419d45089d 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -285,6 +285,28 @@ int strcmp(const char *cs, const char *ct)
 EXPORT_SYMBOL(strcmp);
 #endif
 
+/**
+ * strstarts - does @str start with @prefix?
+ * @str: string to examine
+ * @prefix: prefix to look for.
+ */
+bool strstarts(const char *str, const char *prefix)
+{
+	unsigned char c1, c2;
+
+	do {
+		c1 = *str++;
+		c2 = *prefix++;
+
+		if (c1 != c2)
+			return c2 == '\0';
+
+	} while (c2 != '\0');
+
+	return true;
+}
+EXPORT_SYMBOL(strstarts);
+
 #ifndef __HAVE_ARCH_STRNCMP
 /**
  * strncmp - Compare two length-limited strings

-- 
2.34.1