Date: Wed, 10 Jan 2018 21:55:44 +0100
From: Borislav Petkov <bp@alien8.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        David Woodhouse <dwmw2@infradead.org>,
        Andi Kleen <andi@firstfloor.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Paul Turner <pjt@google.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Greg Kroah-Hartman <gregkh@linux-foundation.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Jiri Kosina <jikos@kernel.org>, Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH] x86/alternatives: Fix optimize_nops() checking
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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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