Subject: Re: [PATCH] powerpc: Avoid panic during boot due to divide by zero in init_cache_info()

From: Segher Boessenkool <[email-masked]>
Date: Sun, 5 Mar 2017 11:24:56 -0600

On Sun, Mar 05, 2017 at 05:58:37PM +0100, Gabriel Paubert wrote:
> > > Erk sorry. One of the static checkers spotted it, but I hadn't got
> > > around to fixing it because it seemed to not actually blow up, guess
> > > not.
> > 
> > The PowerPC divw etc. instructions do not trap by themselves, but recent
> > GCC inserts trap instructions on code paths that are always undefined
> > behaviour (like, dividing by zero).
> 
> Is it systematic or does it depend from, e.g., optimization levels?

In this case it needs -fisolate-erroneous-paths-dereference which is
default at -O2 and higher.

> Is there anything in the standards about this feature?

The compiler can do whatever it likes with code that has undefined
behaviour.  With this optimisation it a) can compile the conforming
code to something better; and b) undefined behaviour will trap instead
of doing something random (which often is exploitable).

Segher