Re: [PATCH 08/15] x86/alternatives: Teach text_poke_bp() to emulate instructions

[Masami Hiramatsu <mhiramat@kernel.org>]

On Fri, 7 Jun 2019 10:20:13 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Fri, Jun 07, 2019 at 05:41:42AM +0000, Nadav Amit wrote:
> 
> > > int poke_int3_handler(struct pt_regs *regs)
> > > {
> > > +	long ip = regs->ip - INT3_INSN_SIZE + CALL_INSN_SIZE;
> > > +	struct opcode {
> > > +		u8 insn;
> > > +		s32 rel;
> > > +	} __packed opcode;
> > > +
> > > 	/*
> > > 	 * Having observed our INT3 instruction, we now must observe
> > > 	 * bp_patching_in_progress.
> > > 	 *
> > > -	 * 	in_progress = TRUE		INT3
> > > -	 * 	WMB				RMB
> > > -	 * 	write INT3			if (in_progress)
> > > +	 *	in_progress = TRUE		INT3
> > > +	 *	WMB				RMB
> > > +	 *	write INT3			if (in_progress)
> > 
> > I don’t see what has changed in this chunk… Whitespaces?
> 
> Yep, my editor kept marking that stuff red (space before tab), which
> annoyed me enough to fix it.
> 
> > > 	 *
> > > -	 * Idem for bp_int3_handler.
> > > +	 * Idem for bp_int3_opcode.
> > > 	 */
> > > 	smp_rmb();
> > > 
> > > @@ -943,8 +949,21 @@ int poke_int3_handler(struct pt_regs *re
> > > 	if (user_mode(regs) || regs->ip != (unsigned long)bp_int3_addr)
> > > 		return 0;
> > > 
> > > -	/* set up the specified breakpoint handler */
> > > -	regs->ip = (unsigned long) bp_int3_handler;
> > > +	opcode = *(struct opcode *)bp_int3_opcode;
> > > +
> > > +	switch (opcode.insn) {
> > > +	case 0xE8: /* CALL */
> > > +		int3_emulate_call(regs, ip + opcode.rel);
> > > +		break;
> > > +
> > > +	case 0xE9: /* JMP */
> > > +		int3_emulate_jmp(regs, ip + opcode.rel);
> > > +		break;
> > 
> > Consider using RELATIVECALL_OPCODE and RELATIVEJUMP_OPCODE instead of the
> > constants (0xE8, 0xE9), just as you do later in the patch.
> 
> Those are private to kprobes..
> 
> but I can do something like so:
> 
> --- a/arch/x86/include/asm/text-patching.h
> +++ b/arch/x86/include/asm/text-patching.h
> @@ -48,8 +48,14 @@ static inline void int3_emulate_jmp(stru
>  	regs->ip = ip;
>  }
>  
> -#define INT3_INSN_SIZE 1
> -#define CALL_INSN_SIZE 5
> +#define INT3_INSN_SIZE		1
> +#define INT3_INSN_OPCODE	0xCC
> +
> +#define CALL_INSN_SIZE		5
> +#define CALL_INSN_OPCODE	0xE8
> +
> +#define JMP_INSN_SIZE		5
> +#define JMP_INSN_OPCODE		0xE9
>  
>  static inline void int3_emulate_push(struct pt_regs *regs, unsigned long val)
>  {
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -952,11 +952,11 @@ int poke_int3_handler(struct pt_regs *re
>  	opcode = *(struct opcode *)bp_int3_opcode;
>  
>  	switch (opcode.insn) {
> -	case 0xE8: /* CALL */
> +	case CALL_INSN_OPCODE:
>  		int3_emulate_call(regs, ip + opcode.rel);
>  		break;
>  
> -	case 0xE9: /* JMP */
> +	case JMP_INSN_OPCODE:
>  		int3_emulate_jmp(regs, ip + opcode.rel);
>  		break;
>  

This looks good. I don't want to make those opcode as private.
I would like to share it.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>