linux-kernel - Re: [PATCH 08/15] x86/alternatives: Teach text_poke

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Date:   Wed, 12 Jun 2019 19:09:30 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...LAB.COM>,
        Borislav Petkov <bp@...en8.de>,
        Julia Cartwright <julia@...com>, Jessica Yu <jeyu@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Nadav Amit <namit@...are.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Edward Cree <ecree@...arflare.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH 08/15] x86/alternatives: Teach text_poke_bp() to emulate
 instructions

On Fri, Jun 07, 2019 at 07:34:27PM +0200, Peter Zijlstra wrote:
> On Sat, Jun 08, 2019 at 12:47:08AM +0900, Masami Hiramatsu wrote:

> > > @@ -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.

I realized we never actually poke a !ideal nop5_atomic, so I've added
that to the latest versions.