1 files changed, 55 insertions, 0 deletions

diff --git a/lib/usercopy.c b/lib/usercopy.c
index c2bfbcaeb3dc..cbb4d9ec00f2 100644
--- a/lib/usercopy.c
+++ b/lib/usercopy.c
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/uaccess.h>
+#include <linux/bitops.h>
 
 /* out-of-line parts */
 
@@ -31,3 +32,57 @@ unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n)
 }
 EXPORT_SYMBOL(_copy_to_user);
 #endif
+
+/**
+ * check_zeroed_user: check if a userspace buffer only contains zero bytes
+ * @from: Source address, in userspace.
+ * @size: Size of buffer.
+ *
+ * This is effectively shorthand for "memchr_inv(from, 0, size) == NULL" for
+ * userspace addresses (and is more efficient because we don't care where the
+ * first non-zero byte is).
+ *
+ * Returns:
+ *  * 0: There were non-zero bytes present in the buffer.
+ *  * 1: The buffer was full of zero bytes.
+ *  * -EFAULT: access to userspace failed.
+ */
+int check_zeroed_user(const void __user *from, size_t size)
+{
+        unsigned long val;
+        uintptr_t align = (uintptr_t) from % sizeof(unsigned long);
+
+        if (unlikely(size == 0))
+                return 1;
+
+        from -= align;
+        size += align;
+
+        if (!user_access_begin(from, size))
+                return -EFAULT;
+
+        unsafe_get_user(val, (unsigned long __user *) from, err_fault);
+        if (align)
+                val &= ~aligned_byte_mask(align);
+
+        while (size > sizeof(unsigned long)) {
+                if (unlikely(val))
+                        goto done;
+
+                from += sizeof(unsigned long);
+                size -= sizeof(unsigned long);
+
+                unsafe_get_user(val, (unsigned long __user *) from, err_fault);
+        }
+
+        if (size < sizeof(unsigned long))
+                val &= aligned_byte_mask(size);
+
+done:
+        user_access_end();
+        return (val == 0);
+err_fault:
+        user_access_end();
+        return -EFAULT;
+}
+EXPORT_SYMBOL(check_zeroed_user);