On Fri, 2018-01-05 at 17:45 +0100, Borislav Petkov wrote:
> On Fri, Jan 05, 2018 at 04:41:46PM +0000, Woodhouse, David wrote:
> > Nope, alternatives are broken. Only a jmp as the *first* opcode of
> > altinstr gets handled by recompute_jump(), while any subsequent insn is
> > just copied untouched.
> 
> Not broken - simply no one needed it until now. I'm looking into it.
> Looks like the insn decoder might come in handy finally.#

I typed 'jmp __x86.indirect_thunk' and it actually jumped to an address
which I believe is (__x86.indirect_thunk + &altinstr - &oldinstr).
Which made me sad, and took a while to debug.

Let's call it "not working for this use case", and I'll stick with
X86_FEATURE_NO_RETPOLINE for now, so the jump can go in the oldinstr
case not in altinstr.