Re: [PATCH] sched: Work around undefined behavior in sched class checking

[Florian Weimer]

* Peter Zijlstra:

> On Tue, May 04, 2021 at 08:39:45PM -0700, Andi Kleen wrote:
>> From: Andi Kleen <andi@xxxxxxxxxxxxxx>
>> 
>> The scheduler initialization code checks that the scheduling
>> classes are consecutive in memory by comparing the end
>> addresses with the next address.
>> 
>> Technically in ISO C comparing symbol addresseses outside different objects
>> is undefined. With LTO gcc 10 tries to exploits this and creates an
>> unconditional BUG_ON in the scheduler initialization, resulting
>> in a boot hang.
>> 
>> Use RELOC_HIDE to make this work. This hides the symbols from gcc,
>> so the optimizer won't make these assumption. I also split
>> the BUG_ONs in multiple.
>
> Urgh, that insanity again :/ Can't we pretty please get a GCC flag to
> disable that?

Context:

  <https://lore.kernel.org/lkml/20210505033945.1282851-1-ak@xxxxxxxxxxxxxxx/>

Obviously, GCC doesn't do this in general.  Would you please provide a
minimal test case?

Thanks,
Florian