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

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 05, 2021 at 10:47:07AM +0200, Florian Weimer wrote:
> * Peter Zijlstra:
> 
> > On Tue, May 04, 2021 at 08:39:45PM -0700, Andi Kleen wrote:
> >> From: Andi Kleen <andi@firstfloor.org>
> >> 
> >> 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@linux.intel.com/>
> 
> Obviously, GCC doesn't do this in general.  Would you please provide a
> minimal test case?

Andi has this GCC-LTO patch-set that triggers this, but the thing I'd
like fixed is the UB mentioned above. Not this particular instance.

And, we've had the problem before, see all the RELOC_HIDE crud. Having
this pointer arith outside object be UB is just really annoying. And in
the spirit of UB bad, can we please get a flag to remove the UB and have
it do the obvious, just do the arithmetic and don't do daft things.

Pretty please.