new helper: memdup_user_nul()

Similar to memdup_user(), except that allocated buffer is one byte
longer and '\0' is stored after the copied data.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/string.h

@@ -10,6 +10,7 @@
 
 extern char *strndup_user(const char __user *, long);
 extern void *memdup_user(const void __user *, size_t);
+extern void *memdup_user_nul(const void __user *, size_t);
 
 /*
  * Include machine specific inline routines

mm/util.c

@@ -176,6 +176,37 @@ char *strndup_user(const char __user *s, long n)
 }
 EXPORT_SYMBOL(strndup_user);
 
+/**
+ * memdup_user_nul - duplicate memory region from user space and NUL-terminate
+ *
+ * @src: source address in user space
+ * @len: number of bytes to copy
+ *
+ * Returns an ERR_PTR() on failure.
+ */
+void *memdup_user_nul(const void __user *src, size_t len)
+{
+	char *p;
+
+	/*
+	 * Always use GFP_KERNEL, since copy_from_user() can sleep and
+	 * cause pagefault, which makes it pointless to use GFP_NOFS
+	 * or GFP_ATOMIC.
+	 */
+	p = kmalloc_track_caller(len + 1, GFP_KERNEL);
+	if (!p)
+		return ERR_PTR(-ENOMEM);
+
+	if (copy_from_user(p, src, len)) {
+		kfree(p);
+		return ERR_PTR(-EFAULT);
+	}
+	p[len] = '\0';
+
+	return p;
+}
+EXPORT_SYMBOL(memdup_user_nul);
+
 void __vma_link_list(struct mm_struct *mm, struct vm_area_struct *vma,
 		struct vm_area_struct *prev, struct rb_node *rb_parent)
 {