1 files changed, 55 insertions, 0 deletions
diff --git a/arch/arm64/lib/clear_user.S b/arch/arm64/lib/clear_user.S
new file mode 100644
index 000000000..a5a5f5b97
--- /dev/null
+++ b/arch/arm64/lib/clear_user.S
@@ -0,0 +1,55 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2021 Arm Ltd.
+ */
+
+#include <linux/linkage.h>
+#include <asm/asm-uaccess.h>
+
+	.text
+
+/* Prototype: int __arch_clear_user(void *addr, size_t sz)
+ * Purpose  : clear some user memory
+ * Params   : addr - user memory address to clear
+ *          : sz   - number of bytes to clear
+ * Returns  : number of bytes NOT cleared
+ *
+ * Alignment fixed up by hardware.
+ */
+
+	.p2align 4
+	// Alignment is for the loop, but since the prologue (including BTI)
+	// is also 16 bytes we can keep any padding outside the function
+SYM_FUNC_START(__arch_clear_user)
+	add	x2, x0, x1
+	subs	x1, x1, #8
+	b.mi	2f
+1:
+USER(9f, sttr	xzr, [x0])
+	add	x0, x0, #8
+	subs	x1, x1, #8
+	b.hi	1b
+USER(9f, sttr	xzr, [x2, #-8])
+	mov	x0, #0
+	ret
+
+2:	tbz	x1, #2, 3f
+USER(9f, sttr	wzr, [x0])
+USER(8f, sttr	wzr, [x2, #-4])
+	mov	x0, #0
+	ret
+
+3:	tbz	x1, #1, 4f
+USER(9f, sttrh	wzr, [x0])
+4:	tbz	x1, #0, 5f
+USER(7f, sttrb	wzr, [x2, #-1])
+5:	mov	x0, #0
+	ret
+
+	// Exception fixups
+7:	sub	x0, x2, #5	// Adjust for faulting on the final byte...
+8:	add	x0, x0, #4	// ...or the second word of the 4-7 byte case
+9:	sub	x0, x2, x0
+	ret
+SYM_FUNC_END(__arch_clear_user)
+EXPORT_SYMBOL(__arch_clear_user)