Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation

Sami Tolvanen
2022-07-19T18:06:55Z

On Tue, Jul 19, 2022 at 10:27:00AM -0700, Linus Torvalds wrote:
> On Tue, Jul 19, 2022 at 10:23 AM Sami Tolvanen <samitolvanen@google.com> wrote:
> >
> > The preamble hash is encoded into an instruction just to avoid special
> > casing objtool, which would otherwise get confused about the random
> > bytes. On arm64, we just emit a bare constant before the function.
>
> Ahh.
>
> I think objtool would want to understand about kCFI anyway, so I think
> in the long run that hack isn't a goog idea.
>
> But I get why you'd do it as a "do this as just a compiler thing and
> hide it from objtool" as a development strategy.

I believe it was actually Peter's idea to use an instruction. :) In
earlier revisions of KCFI, I did teach objtool about the preambles, but
that was just so it can ignore them.

Sami