[PATCH] strndup_user()

This patch series creates a strndup_user() function to easy copying C strings
from userspace.  Also we avoid common pitfalls like userspace modifying the
final \0 after the strlen_user().
Signed-off-by: Davi Arnaut <davi.arnaut@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/string.h

@@ -18,6 +18,8 @@ extern char * strsep(char **,const char *);
 extern __kernel_size_t strspn(const char *,const char *);
 extern __kernel_size_t strcspn(const char *,const char *);
 
+extern char *strndup_user(const char __user *, long);
+
 /*
  * Include machine specific inline routines
  */

mm/util.c

 #include <linux/slab.h>
 #include <linux/string.h>
 #include <linux/module.h>
+#include <linux/err.h>
+#include <asm/uaccess.h>
 
 /**
  * kzalloc - allocate memory. The memory is set to zero.
@@ -37,3 +39,38 @@ char *kstrdup(const char *s, gfp_t gfp)
 	return buf;
 }
 EXPORT_SYMBOL(kstrdup);
+
+/*
+ * strndup_user - duplicate an existing string from user space
+ *
+ * @s: The string to duplicate
+ * @n: Maximum number of bytes to copy, including the trailing NUL.
+ */
+char *strndup_user(const char __user *s, long n)
+{
+	char *p;
+	long length;
+
+	length = strnlen_user(s, n);
+
+	if (!length)
+		return ERR_PTR(-EFAULT);
+
+	if (length > n)
+		return ERR_PTR(-EINVAL);
+
+	p = kmalloc(length, GFP_KERNEL);
+
+	if (!p)
+		return ERR_PTR(-ENOMEM);
+
+	if (copy_from_user(p, s, length)) {
+		kfree(p);
+		return ERR_PTR(-EFAULT);
+	}
+
+	p[length - 1] = '\0';
+
+	return p;
+}
+EXPORT_SYMBOL(strndup_user);