Re: [PATCH 1/3] x86/alternatives: Teach text_poke_bp() to emulate instructions

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Oct 03, 2019 at 10:27:51AM +0200, Peter Zijlstra wrote:
> On Thu, Oct 03, 2019 at 02:00:50PM +0900, Masami Hiramatsu wrote:
> 
> > > This fits almost all text_poke_bp() users, except
> > > arch_unoptimize_kprobe() which restores random text, and for that site
> > > we have to build an explicit emulate instruction.
> > 
> > OK, and in this case, I would like to change RELATIVEJUMP_OPCODE
> > to JMP32_INSN_OPCODE for readability. (or at least
> > making RELATIVEJUMP_OPCODE as an alias of JMP32_INSN_OPCODE)
> 
> > > @@ -448,12 +447,18 @@ void arch_optimize_kprobes(struct list_h
> > >  void arch_unoptimize_kprobe(struct optimized_kprobe *op)
> > >  {
> > >  	u8 insn_buff[RELATIVEJUMP_SIZE];
> > > +	u8 emulate_buff[RELATIVEJUMP_SIZE];
> > >  
> > >  	/* Set int3 to first byte for kprobes */
> > >  	insn_buff[0] = BREAKPOINT_INSTRUCTION;
> > >  	memcpy(insn_buff + 1, op->optinsn.copied_insn, RELATIVE_ADDR_SIZE);
> > > +
> > > +	emulate_buff[0] = RELATIVEJUMP_OPCODE;
> > > +	*(s32 *)(&emulate_buff[1]) = (s32)((long)op->optinsn.insn -
> > > +			((long)op->kp.addr + RELATIVEJUMP_SIZE));
> 
> I'm halfway through a patch introducing:
> 
>   union text_poke_insn {
> 	u8 code[POKE_MAX_OPCODE_SUZE];
> 	struct {
> 		u8 opcode;
> 		s32 disp;
> 	} __attribute__((packed));
>   };
> 
> to text-patching.h to unify all such custom unions we have all over the
> place. I'll mob up the above in that.
> 
> > > +
> > >  	text_poke_bp(op->kp.addr, insn_buff, RELATIVEJUMP_SIZE,
> > > -		     op->optinsn.insn);
> > > +		     emulate_buff);
> > >  }
> 
> As argued in a previous thread, text_poke_bp() is broken when it changes
> more than a single instruction at a time.
> 
> Now, ISTR optimized kprobes does something like:
> 
> 	poke INT3

Hmm, it does this using text_poke(), but lacks a
on_each_cpu(do_sync_core, NULL, 1), which I suppose is OK-ish IFF you do
that synchronize_rcu_tasks() after it, but less so if you don't.

That is, without either, you can't really tell if the kprobe is in
effect or not.

Also, I think text_poke_bp(INT3) is broken, although I don't think
anybody actually does that. Still, let me fix that.

> 	synchronize_rcu_tasks() /* waits for all tasks to schedule
> 				   guarantees instructions after INT3
> 				   are unused */
> 	install optimized probe /* overwrites multiple instrctions with
> 				   JMP.d32 */
> 
> And the above then undoes that by:
> 
> 	poke INT3 on top of the optimzed probe
> 
> 	poke tail instructions back /* guaranteed safe because the
> 				       above INT3 poke ensures the
> 				       JMP.d32 instruction is unused */
> 
> 	poke head byte back
> 
> Is this correct? If so, we should probably put a comment in there
> explaining how all this is unusual but safe.