Re: [PATCH] x86: Introduce REX prefix helper for kprobes

[Masami Hiramatsu <mhiramat@redhat.com>]

Hi Harvey,

Harvey Harrison wrote:
> Fold some small ifdefs into a helper function.
> 
> Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
> ---
> Masami, Ingo, I had this left in some unsent kprobes unification
> work.  Depends on your tastes, but does reduce ifdefs and is a bit
> better about self-documenting the REX prefix on X86_64.

Basically, I think it is good idea.
Could you use a macro same as the stack_addr() macro, like as below?

#defile is_REX_prefix(insn) ((insn & 0xf0) == 0x40))

This is just a bit checker, so I think a macro is better to do that.

> If I find places that could also use this I'll try to find a suitable
> header any stick a static inline there instead.  Otherwise static to
> kprobes.c is probably more appropriate for now.
> 
>  arch/x86/kernel/kprobes.c |   27 +++++++++++++++++++--------
>  1 files changed, 19 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kernel/kprobes.c b/arch/x86/kernel/kprobes.c
> index 4e33329..b1804e4 100644
> --- a/arch/x86/kernel/kprobes.c
> +++ b/arch/x86/kernel/kprobes.c
> @@ -171,6 +171,19 @@ static void __kprobes set_jmp_op(void *from, void *to)
>  }
>  
>  /*
> + * Check for the REX prefix which can only exist on X86_64
> + * X86_32 always returns 0
> + */
> +static int __kprobes is_REX_prefix(kprobe_opcode_t *insn)
> +{
> +#ifdef CONFIG_X86_64
> +	if ((*insn & 0xf0) == 0x40)
> +		return 1;
> +#endif
> +	return 0;
> +}
> +
> +/*
>   * Returns non-zero if opcode is boostable.
>   * RIP relative instructions are adjusted at copying time in 64 bits mode
>   */
> @@ -239,14 +252,14 @@ static int __kprobes is_IF_modifier(kprobe_opcode_t *insn)
>  	case 0x9d:		/* popf/popfd */
>  		return 1;
>  	}
> -#ifdef CONFIG_X86_64
> +
>  	/*
> -	 * on 64 bit x86, 0x40-0x4f are prefixes so we need to look
> +	 * on X86_64, 0x40-0x4f are REX prefixes so we need to look
>  	 * at the next byte instead.. but of course not recurse infinitely
>  	 */
> -	if (*insn  >= 0x40 && *insn <= 0x4f)
> +	if (is_REX_prefix(insn))
>  		return is_IF_modifier(++insn);
> -#endif
> +
>  	return 0;
>  }
>  
> @@ -284,7 +297,7 @@ static void __kprobes fix_riprel(struct kprobe *p)
>  	}
>  
>  	/* Skip REX instruction prefix.  */
> -	if ((*insn & 0xf0) == 0x40)
> +	if (is_REX_prefix(insn))
>  		++insn;
>  
>  	if (*insn == 0x0f) {
> @@ -748,11 +761,9 @@ static void __kprobes resume_execution(struct kprobe *p,
>  	unsigned long orig_ip = (unsigned long)p->addr;
>  	kprobe_opcode_t *insn = p->ainsn.insn;
>  
> -#ifdef CONFIG_X86_64
>  	/*skip the REX prefix*/
> -	if (*insn >= 0x40 && *insn <= 0x4f)
> +	if (is_REX_prefix(insn))
>  		insn++;
> -#endif
>  
>  	regs->flags &= ~X86_EFLAGS_TF;
>  	switch (*insn) {

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@redhat.com, masami.hiramatsu.pt@hitachi.com