linux-hardening - [PATCH v1 1/1] lib/string_helpers: Don't copy a tail in kstrdup_and

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Message-Id: <20230913094557.451463-1-andriy.shevchenko@linux.intel.com>
Date:   Wed, 13 Sep 2023 12:45:57 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Kees Cook <keescook@...omium.org>,
        Andy Shevchenko <andy@...nel.org>
Subject: [PATCH v1 1/1] lib/string_helpers: Don't copy a tail in kstrdup_and_replace() if 'new' is \0

The kstrdup_and_replace() takes two characters, old and new, to replace
former with latter after the copying of the original string. But in case
when new is a NUL, there is no point to copy the rest of the string,
the contract with the callers is that that the function returns a
NUL-terminated string and not a buffer of the size filled with a given
data. With this we can optimize the memory consumption by copying only
meaningful part of the original string and drop the rest.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
---

The first user of this is pending:
https://lore.kernel.org/platform-driver-x86/20230913092701.440959-1-andriy.shevchenko@linux.intel.com/

 lib/string_helpers.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/lib/string_helpers.c b/lib/string_helpers.c
index 7713f73e66b0..e385bf3cc2de 100644
--- a/lib/string_helpers.c
+++ b/lib/string_helpers.c
@@ -723,11 +723,17 @@ EXPORT_SYMBOL_GPL(kstrdup_quotable_file);
 
 /*
  * Returns duplicate string in which the @old characters are replaced by @new.
+ *
+ * If @new is NUL, copy the string up to the first occurrence of @old, which
+ * will be replaced by a NUL.
  */
 char *kstrdup_and_replace(const char *src, char old, char new, gfp_t gfp)
 {
 	char *dst;
 
+	if (new == '\0')
+		return kmemdup_nul(src, strchrnul(src, old) - src, gfp);
+
 	dst = kstrdup(src, gfp);
 	if (!dst)
 		return NULL;
-- 
2.40.0.1.gaa8946217a0b