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

[Peter Zijlstra]

On Sat, Jun 08, 2019 at 12:47:08AM +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.
> 
> Hm, actually it doesn't restores randome text, since the first byte
> must always be int3. As the function name means, it just unoptimizes
> (jump based optprobe -> int3 based kprobe).
> Anyway, that is not an issue. With this patch, optprobe must still work.

I thought it basically restored 5 bytes of original text (with no
guarantee it is a single instruction, or even a complete instruction),
with the first byte replaced with INT3.

> > @@ -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;
> > +
> > +	default: /* assume NOP */
> 
> Shouldn't we check whether it is actually NOP here?

I was/am lazy and didn't want to deal with:

arch/x86/include/asm/nops.h:#define GENERIC_NOP5_ATOMIC NOP_DS_PREFIX,GENERIC_NOP4
arch/x86/include/asm/nops.h:#define K8_NOP5_ATOMIC 0x66,K8_NOP4
arch/x86/include/asm/nops.h:#define K7_NOP5_ATOMIC NOP_DS_PREFIX,K7_NOP4
arch/x86/include/asm/nops.h:#define P6_NOP5_ATOMIC P6_NOP5

But maybe we should check for all the various NOP5 variants and BUG() on
anything unexpected.

> > +		int3_emulate_jmp(regs, ip);
> > +		break;
> > +	}
> 
> BTW, if we fix the length of patching always 5 bytes and allow user
> to apply it only from/to jump/call/nop, we may be better to remove
> "len" and rename it, something like "text_poke_branch" etc.

I considered it; but was thinking we could still allow patching other
instructions, we'd just have to extend the emulation in
poke_int3_handler().

Then again, if/when we want to do that, we can also restore the @len
argument again.