Date: Wed, 10 Jan 2018 14:15:32 -0600
From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Borislav Petkov <bp@alien8.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
Message-ID: <20180110201532.5jnji6ypfl6slzvb@treble>
References: <20180110003139.10531-1-andi@firstfloor.org>
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On Wed, Jan 10, 2018 at 08:55:40PM +0100, Thomas Gleixner wrote:
> On Wed, 10 Jan 2018, Linus Torvalds wrote:
> 
> > On Wed, Jan 10, 2018 at 3:28 AM, Borislav Petkov <bp@alien8.de> wrote:
> > >
> > > Make sure we scan all bytes before we decide to optimize the NOPs in
> > > there.
> > 
> > Can we also add compile-time checking (presumably in objtool, but who
> > knows) that there are no relocations in the alternative section?
> 
> Cc'ing the overlor^Haded objtool wizard
> 
> > Because that was the other "oops, this really doesn't work with
> > altinstructions" issue, wasn't it?

I think .altinstruction relocations *do* work if they're for the first
instruction, and it's a jump or a call.  There's some alternatives code
which adjusts the jump/call offset in that case, and there are some
users of alternatives who rely on that.

I think Boris had a patch floating around to add an instruction decoder
to alternatives, so you can do a call/jmp anywhere.

-- 
Josh