[PATCH 22/24] uaccess: add memzero_user

[Paolo Bonzini]

This will be used from KVM.  Add it to lib/ so that everyone can use it.

Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 include/linux/uaccess.h |  1 +
 lib/usercopy.c          | 63 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 64 insertions(+)

diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 67f016010aad..bd8c85b50e67 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -232,6 +232,7 @@ __copy_from_user_inatomic_nocache(void *to, const void __user *from,
 #endif		/* ARCH_HAS_NOCACHE_UACCESS */
 
 extern __must_check int check_zeroed_user(const void __user *from, size_t size);
+extern __must_check int memzero_user(void __user *from, size_t size);
 
 /**
  * copy_struct_from_user: copy a struct from userspace
diff --git a/lib/usercopy.c b/lib/usercopy.c
index cbb4d9ec00f2..82997862bf02 100644
--- a/lib/usercopy.c
+++ b/lib/usercopy.c
@@ -33,6 +33,69 @@ unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n)
 EXPORT_SYMBOL(_copy_to_user);
 #endif
 
+/**
+ * memzero_user: write zero bytes to a userspace buffer
+ * @from: Source address, in userspace.
+ * @size: Size of buffer.
+ *
+ * This is effectively shorthand for "memset(from, 0, size)" for
+ * userspace addresses.
+ *
+ * Returns:
+ *  * 0: zeroes have been written to the buffer
+ *  * -EFAULT: access to userspace failed.
+ */
+int memzero_user(void __user *from, size_t size)
+{
+	unsigned long val = 0;
+	unsigned long mask = 0;
+	uintptr_t align = (uintptr_t) from % sizeof(unsigned long);
+
+	if (unlikely(size == 0))
+		return 0;
+
+	from -= align;
+	size += align;
+
+	if (!user_access_begin(from, ALIGN_UP(size, sizeof(unsigned long))))
+		return -EFAULT;
+
+	if (align) {
+		unsafe_get_user(val, (unsigned long __user *) from, err_fault);
+		/* Prepare a mask to keep the first "align" bytes.  */
+		mask = aligned_byte_mask(align);
+	}
+
+	if (size >= sizeof(unsigned long)) {
+		/* The mask only applies to the first full word.  */
+		val &= mask;
+		mask = 0;
+		do {
+			unsafe_put_user(val, (unsigned long __user *) from, err_fault);
+			from += sizeof(unsigned long);
+			size -= sizeof(unsigned long);
+			val = 0;
+		} while (size >= sizeof(unsigned long));
+
+		if (!size)
+			goto done;
+		unsafe_get_user(val, (unsigned long __user *) from, err_fault);
+	}
+
+	/* Bytes after the first "size" have to be kept too. */
+	mask |= ~aligned_byte_mask(size);
+	val &= mask;
+	unsafe_put_user(val, (unsigned long __user *) from, err_fault);
+
+done:
+	user_access_end();
+	return 0;
+err_fault:
+	user_access_end();
+	return -EFAULT;
+}
+EXPORT_SYMBOL(memzero_user);
+
 /**
  * check_zeroed_user: check if a userspace buffer only contains zero bytes
  * @from: Source address, in userspace.
-- 
2.18.2