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* [PATCH 1/2] kmemdup: introduce (updated)
@ 2006-09-12  0:37 Alexey Dobriyan
  0 siblings, 0 replies; only message in thread
From: Alexey Dobriyan @ 2006-09-12  0:37 UTC (permalink / raw)
  To: Andrew Morton; +Cc: linux-kernel

signed-off-by + updated changelog. second patch remains as is.
One of idiomatic ways to duplicate a region of memory is

	dst = kmalloc(len, GFP_KERNEL);
	if (!dst)
		return -ENOMEM;
	memcpy(dst, src, len);

which is neat code except a programmer needs to write size twice.  Which
sometimes leads to mistakes.  If len passed to kmalloc is smaller that len
passed to memcpy, it's straight overwrite-beyond-end.  If len passed to
memcpy is smaller than len passed to kmalloc, it's either a) legit
behaviour ;-), or b) cloned buffer will contain garbage in second half.

Slight trolling of commit lists shows several duplications bugs
done exactly because of diverged lenghts:

		[CRYPTO]: Fix memcpy/memset args.
		[PATCH] memcpy/memset fixes
		kerberosV/src/lib/asn1: der_copy.c:1.4

If programmer is given only one place to play with lengths, I believe, such
mistakes could be avoided.

With kmemdup, the snippet above will be rewritten as:

	dst = kmemdup(src, len, GFP_KERNEL);
	if (!dst)
		return -ENOMEM;

This also leads to smaller code (kzalloc effect). Quick grep shows
200+ places where kmemdup() can be used.

Signed-off-by: Alexey Dobriyan <>

 include/linux/string.h |    1 +
 mm/util.c              |   18 ++++++++++++++++++
 2 files changed, 19 insertions(+)

--- a/include/linux/string.h~kmemdup-introduce
+++ a/include/linux/string.h
@@ -99,6 +99,7 @@ extern void * memchr(const void *,int,__
 extern char *kstrdup(const char *s, gfp_t gfp);
+extern void *kmemdup(const void *src, size_t len, gfp_t gfp);
 #ifdef __cplusplus
diff -puN mm/util.c~kmemdup-introduce mm/util.c
--- a/mm/util.c~kmemdup-introduce
+++ a/mm/util.c
@@ -40,6 +40,24 @@ char *kstrdup(const char *s, gfp_t gfp)
+ * kmemdup - duplicate region of memory
+ *
+ * @src: memory region to duplicate
+ * @len: memory region length
+ * @gfp: GFP mask to use
+ */
+void *kmemdup(const void *src, size_t len, gfp_t gfp)
+	void *p;
+	p = ____kmalloc(len, gfp);
+	if (p)
+		memcpy(p, src, len);
+	return p;
  * strndup_user - duplicate an existing string from user space

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2006-09-12  0:37 [PATCH 1/2] kmemdup: introduce (updated) Alexey Dobriyan

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