memdup_user(): introduce

I notice there are many places doing copy_from_user() which follows
kmalloc():

        dst = kmalloc(len, GFP_KERNEL);
        if (!dst)
                return -ENOMEM;
        if (copy_from_user(dst, src, len)) {
		kfree(dst);
		return -EFAULT
	}

memdup_user() is a wrapper of the above code.  With this new function, we
don't have to write 'len' twice, which can lead to typos/mistakes.  It
also produces smaller code and kernel text.

A quick grep shows 250+ places where memdup_user() *may* be used.  I'll
prepare a patchset to do this conversion.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Americo Wang <xiyou.wangcong@gmail.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 31 insertions, 0 deletions

diff --git a/include/linux/string.h b/include/linux/string.h
index d18fc198aa2f..8852739f36df 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -12,6 +12,7 @@
 #include <linux/stddef.h>       /* for NULL */
 
 extern char *strndup_user(const char __user *, long);
+extern void *memdup_user(const void __user *, size_t);
 
 /*
  * Include machine specific inline routines
diff --git a/mm/util.c b/mm/util.c
index 37eaccdf3054..7c122e49f769 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -70,6 +70,36 @@ void *kmemdup(const void *src, size_t len, gfp_t gfp)
 EXPORT_SYMBOL(kmemdup);
 
 /**
+ * memdup_user - duplicate memory region from user space
+ *
+ * @src: source address in user space
+ * @len: number of bytes to copy
+ *
+ * Returns an ERR_PTR() on failure.
+ */
+void *memdup_user(const void __user *src, size_t len)
+{
+        void *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, GFP_KERNEL);
+        if (!p)
+                return ERR_PTR(-ENOMEM);
+
+        if (copy_from_user(p, src, len)) {
+                kfree(p);
+                return ERR_PTR(-EFAULT);
+        }
+
+        return p;
+}
+EXPORT_SYMBOL(memdup_user);
+
+/**
  * __krealloc - like krealloc() but don't free @p.
  * @p: object to reallocate memory for.
  * @new_size: how many bytes of memory are required.