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    SubjectRe: [PATCH] static_call: fix function type mismatch
    On Wed, Mar 24, 2021 at 3:53 PM Rasmus Villemoes
    <linux@rasmusvillemoes.dk> wrote:
    >
    > On 24/03/2021 23.34, Sami Tolvanen wrote:
    > > On Wed, Mar 24, 2021 at 2:51 PM Rasmus Villemoes
    > > <linux@rasmusvillemoes.dk> wrote:
    > >>
    > >> On 24/03/2021 18.33, Peter Zijlstra wrote:
    > >>> On Wed, Mar 24, 2021 at 05:45:52PM +0100, Rasmus Villemoes wrote:
    > >>>> Sorry, I think I misread the code. The static calls are indeed
    > >>>> initialized with a function with the right prototype. Try adding
    > >>>> "preempt=full" on the command line so that we exercise these lines
    > >>>>
    > >>>> static_call_update(cond_resched,
    > >>>> (typeof(&__cond_resched)) __static_call_return0);
    > >>>> static_call_update(might_resched,
    > >>>> (typeof(&__cond_resched)) __static_call_return0);
    > >>>>
    > >>>> I would expect that to blow up, since we end up calling a long (*)(void)
    > >>>> function using a function pointer of type int (*)(void).
    > >>>
    > >>> Note that on x86 there won't actually be any calling of function
    > >>> pointers. See what arch/x86/kernel/static_call.c does :-)
    > >>
    > >> I know, but so far x86 is the only one with HAVE_STATIC_CALL, so for
    > >> arm64 which is where CFI seems to be targeted initially, static_calls
    > >> are function pointers. And unless CFI ignores the return type, I'd
    > >> really expect the above to fail.
    > >
    > > I think you're correct, this would trip CFI without HAVE_STATIC_CALL.
    > > However, arm64 also doesn't support PREEMPT_DYNAMIC at the moment, so
    > > this isn't currently a problem there.
    >
    > Well, there's PREEMPT_DYNAMIC and HAVE_PREEMPT_DYNAMIC. The former
    > doesn't depend on the latter (and the latter does depend on
    > HAVE_STATIC_CALL, so effectively not for anything but x86). You should
    > be able to select both PREEMPT_DYNAMIC and CFI_CLANG, and test if
    > booting with preempt=full does give the fireworks one expects.

    Actually, it looks like I can't select PREEMPT_DYNAMIC, and tweaking
    Kconfig to force enable it on arm64 results in a build error
    ("implicit declaration of function 'static_call_mod'").

    Sami

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