linux-kernel - Re: [RFC][PATCH 1/5] static_call: Make NULL static calls consistent

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Message-ID: <CABCJKuezD9sXTUTCecARUzfig+B=VirGBiVNM5bADvKLG3+YYw@mail.gmail.com>
Date:   Mon, 13 Mar 2023 10:48:58 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        Jason Baron <jbaron@...mai.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [RFC][PATCH 1/5] static_call: Make NULL static calls consistent

On Sun, Mar 12, 2023 at 8:17 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, Mar 10, 2023 at 05:20:04PM -0800, Josh Poimboeuf wrote:
> >   2) Create yet another "tier" of static call implementations, for
> >      arches which can have the unfortunate combo of CFI_CLANG +
> >      !HAVE_STATIC_CALL.  CONFIG_ALMOST_DONT_HAVE_STATIC_CALL?
> >
> >      The arch can define ARCH_DEFINE_STATIC_CALL_NOP() which uses inline
> >      asm to create a CFI-compliant NOP/BUG/whatever version of the
> >      function (insert lots of hand-waving).  Is the kcfi hash available
> >      to inline asm at build time?
>
> Yes, clang creates magic symbol for everything it sees a declaration
> for. This symbols can be referenced from asm, linking will make it all
> work.
>
> And yes, C sucks, you can't actually create a function definition from a
> type :/ Otherwise this could be trivially fixable.

Wouldn't creating a separate inline assembly nop function that
references the CFI hash of another function with the correct type
potentially solve this issue like Josh suggested?

Sami