Re: [PATCH RFC 3/6] arm64: Kprobes instruction simulation support

[Will Deacon]

On Thu, Oct 17, 2013 at 12:17:48PM +0100, Sandeepa Prabhu wrote:
> Add support for AArch64 instruction simulation in kprobes.
> 
> Kprobes need simulation of instructions that cannot be stepped
> right-away from different memory location. i.e. those instructions
> that uses PC-relative addressing. In simulation, the behaviour
> of the instruction is implemented using copy of pt_regs.
> 
> Following instruction catagories are simulated:
>  - All branching instructions(conditional, register, and immediate)
>  - Literal access instructions(load-literal, adr/adrp)
> 
> conditional execution are limited to branching instructions in
> ARM v8. If conditions at PSTATE does not match the condition fields
> of opcode, the instruction is effectively NOP. Kprobes consider
> this case as 'miss'.

[...]

> diff --git a/arch/arm64/kernel/kprobes-arm64.c b/arch/arm64/kernel/kprobes-arm64.c
> index 30d1c14..c690be3 100644
> --- a/arch/arm64/kernel/kprobes-arm64.c
> +++ b/arch/arm64/kernel/kprobes-arm64.c
> @@ -20,6 +20,101 @@
> 
>  #include "probes-decode.h"
>  #include "kprobes-arm64.h"
> +#include "simulate-insn.h"
> +
> +/*
> + * condition check functions for kprobes simulation
> + */
> +static unsigned long __kprobes
> +__check_pstate(struct kprobe *p, struct pt_regs *regs)
> +{
> +       struct arch_specific_insn *asi = &p->ainsn;
> +       unsigned long pstate = regs->pstate & 0xffffffff;
> +
> +       return asi->pstate_cc(pstate);
> +}
> +
> +static unsigned long __kprobes
> +__check_cbz(struct kprobe *p, struct pt_regs *regs)
> +{
> +       return check_cbz((u32)p->opcode, regs);

Isn't p->opcode already a u32? (by your definition of kprobe_opcode_t).

> diff --git a/arch/arm64/kernel/simulate-insn.c b/arch/arm64/kernel/simulate-insn.c
> new file mode 100644
> index 0000000..10173cf
> --- /dev/null
> +++ b/arch/arm64/kernel/simulate-insn.c
> @@ -0,0 +1,184 @@
> +/*
> + * arch/arm64/kernel/simulate-insn.c
> + *
> + * 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.
> + */
> +
> +#include <linux/kernel.h>
> +#include <linux/kprobes.h>
> +#include <linux/module.h>
> +
> +#include "simulate-insn.h"
> +
> +#define sign_extend(x, signbit)                \
> +       ((x) | (0 - ((x) & (1 << (signbit)))))
> +
> +#define bbl_displacement(insn)         \
> +       sign_extend(((insn) & 0x3ffffff) << 2, 27)
> +
> +#define bcond_displacement(insn)       \
> +       sign_extend(((insn >> 5) & 0xfffff) << 2, 21)
> +
> +#define cbz_displacement(insn) \
> +       sign_extend(((insn >> 5) & 0xfffff) << 2, 21)
> +
> +#define tbz_displacement(insn) \
> +       sign_extend(((insn >> 5) & 0x3fff) << 2, 15)
> +
> +#define ldr_displacement(insn) \
> +       sign_extend(((insn >> 5) & 0xfffff) << 2, 21)

The mask, shift and signbit position are all related here, so you could
rework the definition of sign_extend to avoid having three magic numbers.

Will
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