On Mon, Aug 26, 2024 at 5:25 PM Alejandro Colomar [off-list ref] wrote:

Hi Yafang,

On Sat, Aug 17, 2024 at 10:58:02AM GMT, Alejandro Colomar wrote:

quoted

Hi Yafang,

On Sat, Aug 17, 2024 at 10:56:22AM GMT, Yafang Shao wrote:

quoted

These three functions follow the same pattern. To deduplicate the code,
let's introduce a common helper __kmemdup_nul().

Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Yafang Shao <redacted>
Cc: Simon Horman <horms@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
---
 mm/util.c | 67 +++++++++++++++++++++----------------------------------
 1 file changed, 26 insertions(+), 41 deletions(-)

diff --git a/mm/util.c b/mm/util.c
index 4542d8a800d9..310c7735c617 100644
--- a/mm/util.c
+++ b/mm/util.c

@@ -45,33 +45,40 @@ void kfree_const(const void *x)
 EXPORT_SYMBOL(kfree_const);

 /**
- * kstrdup - allocate space for and copy an existing string
- * @s: the string to duplicate
+ * __kmemdup_nul - Create a NUL-terminated string from @s, which might be unterminated.
+ * @s: The data to copy
+ * @len: The size of the data, including the null terminator
  * @gfp: the GFP mask used in the kmalloc() call when allocating memory
  *
- * Return: newly allocated copy of @s or %NULL in case of error
+ * Return: newly allocated copy of @s with NUL-termination or %NULL in
+ * case of error
  */
-noinline
-char *kstrdup(const char *s, gfp_t gfp)
+static __always_inline char *__kmemdup_nul(const char *s, size_t len, gfp_t gfp)
 {
-   size_t len;
    char *buf;

-   if (!s)
+   buf = kmalloc_track_caller(len, gfp);
+   if (!buf)
            return NULL;

-   len = strlen(s) + 1;
-   buf = kmalloc_track_caller(len, gfp);
-   if (buf) {
-           memcpy(buf, s, len);
-           /* During memcpy(), the string might be updated to a new value,
-            * which could be longer than the string when strlen() is
-            * called. Therefore, we need to add a null termimator.
-            */
-           buf[len - 1] = '\0';
-   }
+   memcpy(buf, s, len);
+   /* Ensure the buf is always NUL-terminated, regardless of @s. */
+   buf[len - 1] = '\0';
    return buf;
 }
+
+/**
+ * kstrdup - allocate space for and copy an existing string
+ * @s: the string to duplicate
+ * @gfp: the GFP mask used in the kmalloc() call when allocating memory
+ *
+ * Return: newly allocated copy of @s or %NULL in case of error
+ */
+noinline
+char *kstrdup(const char *s, gfp_t gfp)
+{
+   return s ? __kmemdup_nul(s, strlen(s) + 1, gfp) : NULL;
+}
 EXPORT_SYMBOL(kstrdup);

 /**

@@ -106,19 +113,7 @@ EXPORT_SYMBOL(kstrdup_const);
  */
 char *kstrndup(const char *s, size_t max, gfp_t gfp)
 {
-   size_t len;
-   char *buf;
-
-   if (!s)
-           return NULL;
-
-   len = strnlen(s, max);
-   buf = kmalloc_track_caller(len+1, gfp);
-   if (buf) {
-           memcpy(buf, s, len);
-           buf[len] = '\0';
-   }
-   return buf;
+   return s ? __kmemdup_nul(s, strnlen(s, max) + 1, gfp) : NULL;
 }
 EXPORT_SYMBOL(kstrndup);

@@ -192,17 +187,7 @@ EXPORT_SYMBOL(kvmemdup);
  */
 char *kmemdup_nul(const char *s, size_t len, gfp_t gfp)
 {
-   char *buf;
-
-   if (!s)
-           return NULL;
-
-   buf = kmalloc_track_caller(len + 1, gfp);
-   if (buf) {
-           memcpy(buf, s, len);
-           buf[len] = '\0';
-   }
-   return buf;
+   return s ? __kmemdup_nul(s, len + 1, gfp) : NULL;
 }
 EXPORT_SYMBOL(kmemdup_nul);

I like the idea of the patch, but it's plagued with all those +1 and -1.
I think that's due to a bad choice of value being passed by.  If you
pass the actual length of the string (as suggested in my reply to the
previous patch) you should end up with a cleaner set of APIs.

The only remaining +1 is for kmalloc_track_caller(), which I ignore what
it does.

      char *
      __kmemdup_nul(const char *s, size_t len, gfp_t gfp)
      {
              char *buf;

              buf = kmalloc_track_caller(len + 1, gfp);
              if (!buf)
                      return NULL;

              strcpy(mempcpy(buf, s, len), "");

Changing these strcpy(, "") to the usual; ='\0' or =0, but I'd still
recommend the rest of the changes, that is, changing the value passed in
len, to remove several +1 and -1s.

What do you think?

I will update it. Thanks for your suggestion.

-- 
Regards
Yafang