Re: [PATCH] x86/alternatives: Fix optimize_nops() checking

[Josh Poimboeuf]

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@xxxxxxxxx> 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