Re: [PATCH] static_call: fix function type mismatch

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

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.

Rasmus