Re: [PATCH] static_call: fix function type mismatch

[Rasmus Villemoes]

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.

> But I think some of this code might need some __va_function() love when
> combined with CFI.

Well, that was also my first thought when reading through the CFI
patches, I hoped that might salvage my
reduce-boilerplate-and-get-better-type-safety proposal for the
devm_*_action APIs [1]. But I don't think that would help at all;
storing __va_function(__static_call_return0) instead of
&__static_call_return0  (i.e., __static_call_return0 instead of
__static_call_return0.cfi_jt) doesn't help the call sites of that
static_call at all, neither address belongs to the range of jump table
entries corresponding to the prototype "int (*)(void)". So I think it
would be the static_call macro that would somehow need to grow a way to
suppress the cfi checking.

Rasmus

[1]
https://lore.kernel.org/lkml/20210309235917.2134565-1-linux@xxxxxxxxxxxxxxxxxx/