linux-kernel - Re: [PATCH v4 04/15] cfi: Add DEFINE_CFI_IMMEDIATE_RETURN

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Date:   Tue, 5 Oct 2021 14:53:25 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     X86 ML <x86@...nel.org>, Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, llvm@...ts.linux.dev
Subject: Re: [PATCH v4 04/15] cfi: Add DEFINE_CFI_IMMEDIATE_RETURN_STUB

On Tue, Oct 5, 2021 at 1:58 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Oct 05, 2021 at 01:29:02PM -0700, Sami Tolvanen wrote:
> > On Mon, Oct 4, 2021 at 11:59 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> > > For x86_64 it should indeed never get called, however if you plan on
> > > supporting i386 then you need the annotation. Also, it might get called
> > > on arm64 which is about to grow basic HAVE_STATIC_CALL support.
> >
> > Good point. I read through the latest arm64 static call proposal and
> > while it can fall back to an indirect call, it doesn't look like that
> > would cause issues with CFI.
>
> Because that call is outside of compiler control?

Correct.

> Same will be true for
> any HAVE_STATIC_CALL implementation I suppose. The trampoline will be
> outside of compiler control.

True, so it shouldn't be a problem.

Sami