linux-kernel - Re: [PATCH] x86/alternatives: Fix optimize

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Date:   Wed, 10 Jan 2018 21:55:44 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        David Woodhouse <dwmw2@...radead.org>,
        Andi Kleen <andi@...stfloor.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Jiri Kosina <jikos@...nel.org>, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] x86/alternatives: Fix optimize_nops() checking

On Wed, Jan 10, 2018 at 12:26:25PM -0800, Linus Torvalds wrote:
> > I think Boris had a patch floating around to add an instruction decoder
> > to alternatives, so you can do a call/jmp anywhere.
> 
> .. and no, we're not doing that. Christ.
> 
> People, we need to try to be *robust* here. That's doubly (triply!)
> true of things like altinstructions where people - very much by design
> - won't even *test* the alternatives very much, because very much by
> design the altinstructions are only used on certain architectures or
> in certain situations.

Ok, so the problem was: how to fixup jumps which are not the first
instruction which is being replaced but a following one in the
instruction bytes with which we replace.

I used the insn decoder to get insn boundaries so that I can know
whether bytes 0xeb or 0xe9 are the actual JMP opcode and not some other
bytes from the stream.

So how do you suggest I do that without the decoder? I still need some
sort of parsing to find out where the boundaries are...

Thx.

-- 
Regards/Gruss,
    Boris.

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