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

On Sun, 2017-03-05 at 11:24 -0600, Segher Boessenkool wrote:

On Sun, Mar 05, 2017 at 05:58:37PM +0100, Gabriel Paubert wrote:

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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.

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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).

I actually like that feature, 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 :-)

Cheers,
Ben.