Re: [PATCH] static_call: fix function type mismatch

[Sami Tolvanen]

On Wed, Mar 24, 2021 at 2:51 PM Rasmus Villemoes
<linux@xxxxxxxxxxxxxxxxxx> 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.

Sami