[PATCH v15 04/10] arm64: Kprobes with single stepping support

On Thu, Jul 21, 2016 at 12:33:36PM -0400, David Long wrote:

On 07/20/2016 12:09 PM, Marc Zyngier wrote:

quoted

On 08/07/16 17:35, David Long wrote:

quoted

diff --git a/arch/arm64/include/asm/kprobes.h b/arch/arm64/include/asm/kprobes.h
new file mode 100644
index 0000000..79c9511
--- /dev/null
+++ b/arch/arm64/include/asm/kprobes.h

@@ -0,0 +1,60 @@
+/*
+ * arch/arm64/include/asm/kprobes.h
+ *
+ * Copyright (C) 2013 Linaro Limited
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ */
+
+#ifndef _ARM_KPROBES_H
+#define _ARM_KPROBES_H
+
+#include <linux/types.h>
+#include <linux/ptrace.h>
+#include <linux/percpu.h>
+
+#define __ARCH_WANT_KPROBES_INSN_SLOT
+#define MAX_INSN_SIZE			1
+#define MAX_STACK_SIZE			128

Where is that value coming from? Because even on my 6502, I have a 256
byte stack.

Although I don't claim to know the original author's thoughts I would guess
it is based on the seven other existing implementations for kprobes on
various architectures, all of which appear to use either 64 or 128 for
MAX_STACK_SIZE.  The code is not trying to duplicate the whole stack.

This seems to be some random choice in the hope that arguments passed on
the stack cannot exceed MAX_STACK_SIZE.

I'll put a patch together which checks the previous stack frame and
limit the copying. If the fp information or if the size to be copied
exceeds MAX_STACK_SIZE, just skip the probe hook (return 0 in
setjmp_pre_handler).

-- 
Catalin