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Date:   Tue, 11 Jun 2019 14:42:40 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Masami Hiramatsu <mhiramat@kernel.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Andy Lutomirski <luto@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>,
        Julia Cartwright <julia@ni.com>, Jessica Yu <jeyu@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Nadav Amit <namit@vmware.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Edward Cree <ecree@solarflare.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
Subject: Re: [PATCH 08/15] x86/alternatives: Teach text_poke_bp() to emulate
 instructions
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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 :/