linux-kernel - Re: [PATCH v4 09/17] perf/core: Use static_call to optimize perf_guest_info

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Message-ID: <YwZVj+8H4rGjEHyH@google.com>
Date:   Wed, 24 Aug 2022 16:45:03 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Will McVicker <willmcvicker@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        kvmarm <kvmarm@...ts.cs.columbia.edu>, kvm@...r.kernel.org
Subject: Re: [PATCH v4 09/17] perf/core: Use static_call to optimize
 perf_guest_info_callbacks

On Fri, Feb 18, 2022, Will McVicker wrote:
> On Sun, Feb 6, 2022 at 6:56 PM Kees Cook <keescook@...omium.org> wrote:
> >
> > On Sun, Feb 06, 2022 at 09:28:52PM +0100, Peter Zijlstra wrote:
> > > On Sun, Feb 06, 2022 at 10:45:15AM -0800, Kees Cook wrote:
> > >
> > > > I'm digging through the macros to sort this out, but IIUC, an example of
> > > > the problem is:
> > > >
> > >
> > > > so the caller is expecting "unsigned int (*)(void)" but the prototype
> > > > of __static_call_return0 is "long (*)(void)":
> > > >
> > > > long __static_call_return0(void);
> > > >
> > > > Could we simply declare a type-matched ret0 trampoline too?
> > >
> > > That'll work for this case, but the next case the function will have
> > > arguments we'll need even more nonsense...
> >
> > Shouldn't the typeof() work there too, though? I.e. as long as the
> > return value can hold a "0", it'd work.
> >
> > > And as stated in that other email, there's tb_stub_func() having the
> > > exact same problem as well.
> >
> > Yeah, I'd need to go look at that again.
> >
> > > The x86_64 CFI patches had a work-around for this, that could trivially
> > > be lifted I suppose.
> >
> > Yeah, I think it'd be similar. I haven't had a chance to go look at that
> > again...

Peter and/or Kees, can you provide a pointer to the patches that could potentially
be used as a basis for fixing ARM CFI?  Or even better, send a patch to actually
fix this?  :-)