[PATCH v1 1/1] lib/string_helpers: Don't copy a tail in kstrdup_and_replace() if 'new' is \0

[PATCH v1 1/1] lib/string_helpers: Don't copy a tail in kstrdup_and_replace() if 'new' is \0

[Andy Shevchenko]

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@linux.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

Re: [PATCH v1 1/1] lib/string_helpers: Don't copy a tail in kstrdup_and_replace() if 'new' is \0

[Andy Shevchenko]

On Wed, Sep 13, 2023 at 12:45:57PM +0300, Andy Shevchenko wrote:
> 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.

Thinking about this more, I self NAK this.
If the caller knows the size of the original message it can be handy to make
a copy and replace all occurrences of old by NUL. This will be an optimized
implementation of strsep(str, "$OLD").

-- 
With Best Regards,
Andy Shevchenko