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

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Date:   Tue, 11 Jun 2019 14:42:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>, 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 Tue, Jun 11, 2019 at 02:34:02PM +0200, Peter Zijlstra wrote:

> Bugger, this isn't right. It'll jump to the beginning of the trampoline,
> even if it is multiple instructions in, which would lead to executing
> instructions twice, which would be BAD.
> 
> _maybe_, depending on what the slot looks like, we could do something
> like:
> 
> 	offset = regs->ip - (unsigned long)bp_int3_addr;
> 	regs->ip = bp_int3_handler + offset;
> 
> That is; jump into the slot at the same offset we hit the INT3, but this
> is quickly getting yuck.

Yeah, that won't work either... it needs something far more complex :/