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

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Message-ID: <1515618537.22302.253.camel@infradead.org>
Date:   Wed, 10 Jan 2018 21:08:57 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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, 2018-01-10 at 13:05 -0800, Linus Torvalds wrote:
> On Wed, Jan 10, 2018 at 12:55 PM, Borislav Petkov <bp@...en8.de>
> wrote:
> >
> > 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.
> 
> What jumps do you have that need to be fixed up?
> 
> I really think we should avoid having things like that.
> 
> Any jumps *within* the alternatives should have been handled by the
> assembler already.
> 
> And jumps between the alternatives and other places? Why do they
> exist?

There are a few of the form 'call *somefunc'.

The existing code handles them not by virtue of the relocs, as I said,
but by a simple delta of the old and new location of the instruction.

But it only does so for the *first* instruction of the altinstr, if it
happens to be a (4-byte?) branch.

Right now for retpoline I am just studiously avoiding doing anything
that the alternatives mechanism isn't going to get right, or might
change in future. I think ;)


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