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

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Date:   Tue, 11 Jun 2019 17:52:48 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.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 Tue, Jun 11, 2019 at 11:14:10AM -0400, Steven Rostedt wrote:
> On Wed, 05 Jun 2019 15:08:01 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > -void text_poke_bp(void *addr, const void *opcode, size_t len, void *handler)
> > +void text_poke_bp(void *addr, const void *opcode, size_t len, const void *emulate)
> >  {
> >  	unsigned char int3 = 0xcc;
> >  
> > -	bp_int3_handler = handler;
> > +	bp_int3_opcode = emulate ?: opcode;
> >  	bp_int3_addr = (u8 *)addr + sizeof(int3);
> >  	bp_patching_in_progress = true;
> >  
> >  	lockdep_assert_held(&text_mutex);
> >  
> >  	/*
> > +	 * poke_int3_handler() relies on @opcode being a 5 byte instruction;
> > +	 * notably a JMP, CALL or NOP5_ATOMIC.
> > +	 */
> > +	BUG_ON(len != 5);
> 
> If we have a bug on here, why bother with passing in len at all? Just
> force it to be 5.

Masami said the same.

> We could make it a WARN_ON() and return without doing anything.
> 
> This also prevents us from ever changing two byte jmps.

It doesn't; that is, we'd need to add emulation for the 3 byte jump, but
that'd be pretty trivial.