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

From: Segher Boessenkool <segher@kernel.crashing.org>
To: Benjamin Herrenschmidt <benh@au1.ibm.com>
Cc: Gabriel Paubert <paubert@iram.es>,
	paulus@samba.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH] powerpc: Avoid panic during boot due to divide by zero in init_cache_info()
Date: Sun, 5 Mar 2017 18:10:47 -0600	[thread overview]
Message-ID: <20170306001047.GO31469@gate.crashing.org> (raw)
In-Reply-To: <1488755341.2870.117.camel@au1.ibm.com>

On Mon, Mar 06, 2017 at 10:09:01AM +1100, Benjamin Herrenschmidt wrote:
> > 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).
> 
> I actually like that feature,

Yeah, me too -- it also (currently) makes *smaller* code than it would
without it.  Win-win-win.

> except it did bite me once or twice in the past
> adding traps to intentional NULL dereferences ;-) Ah the joys of writing
> a firmware where you poke at stuff at fixed addresses in low memory :-)

You cannot really have something at address 0, the way NULL pointers
are represented in GCC.  0 in firmware, so *fun*, especially before the
CFAR was invented.  "Something jumped to 0, CTR is 0 so it's probably
a BCTR, but which one of the 6000?"

What do you have at 0?  Not anything you need often I hope?

Segher