[PATCH 1/5] mm/util: add kstrdup_const

Andrzej Hajda
2015-01-12T09:20:05Z

The patch adds alternative version of kstrdup which returns pointer
to constant char array. The function checks if input string is in
persistent and read-only memory section, if yes it returns the input string,
otherwise it fallbacks to kstrdup.
kstrdup_const is accompanied by kfree_const performing conditional memory
deallocation of the string.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
---
include/linux/string.h | 3 +++
mm/util.c | 38 ++++++++++++++++++++++++++++++++++++++
2 files changed, 41 insertions(+)

diff --git a/include/linux/string.h b/include/linux/string.h
index 2e22a2e..b11ed1e 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -115,7 +115,10 @@ extern void * memchr(const void *,int,__kernel_size_t);
#endif
void *memchr_inv(const void *s, int c, size_t n);

+extern void kfree_const(const void *x);
+
extern char *kstrdup(const char *s, gfp_t gfp);
+extern const char *kstrdup_const(const char *s, gfp_t gfp);
extern char *kstrndup(const char *s, size_t len, gfp_t gfp);
extern void *kmemdup(const void *src, size_t len, gfp_t gfp);

diff --git a/mm/util.c b/mm/util.c
index fec39d4..7c62128 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -12,10 +12,30 @@
#include <linux/hugetlb.h>
#include <linux/vmalloc.h>

+#include <asm/sections.h>
#include <asm/uaccess.h>

#include "internal.h"

+static inline int is_kernel_rodata(unsigned long addr)
+{
+ return addr >= (unsigned long)__start_rodata &&
+ addr < (unsigned long)__end_rodata;
+}
+
+/**
+ * kfree_const - conditionally free memory
+ * @x: pointer to the memory
+ *
+ * Function calls kfree only if @x is not in .rodata section.
+ */
+void kfree_const(const void *x)
+{
+ if (!is_kernel_rodata((unsigned long)x))
+ kfree(x);
+}
+EXPORT_SYMBOL(kfree_const);
+
/**
* kstrdup - allocate space for and copy an existing string
* @s: the string to duplicate
@@ -38,6 +58,24 @@ char *kstrdup(const char *s, gfp_t gfp)
EXPORT_SYMBOL(kstrdup);

/**
+ * kstrdup_const - conditionally duplicate an existing const string
+ * @s: the string to duplicate
+ * @gfp: the GFP mask used in the kmalloc() call when allocating memory
+ *
+ * Function returns source string if it is in .rodata section otherwise it
+ * fallbacks to kstrdup.
+ * Strings allocated by kstrdup_const should be freed by kfree_const.
+ */
+const char *kstrdup_const(const char *s, gfp_t gfp)
+{
+ if (is_kernel_rodata((unsigned long)s))
+ return s;
+
+ return kstrdup(s, gfp);
+}
+EXPORT_SYMBOL(kstrdup_const);
+
+/**
* kstrndup - allocate space for and copy an existing string
* @s: the string to duplicate
* @max: read at most @max chars from @s
--
1.9.1