From: Gabriel Paubert <paubert@iram•es>
To: Segher Boessenkool <segher@kernel•crashing.org>
Cc: Michael Ellerman <mpe@ellerman•id.au>,
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: Mon, 6 Mar 2017 13:03:19 +0100 [thread overview]
Message-ID: <20170306120319.GA17443@visitor2.iram.es> (raw)
In-Reply-To: <20170305172456.GN31469@gate.crashing.org>
On Sun, Mar 05, 2017 at 11:24:56AM -0600, Segher Boessenkool wrote:
> 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.
Great, another optimization-dependent behaviour. :-(
But this is not the most serious issue: on PPC, when you #include
<limits>, the numeric_limits<any_integer_type>::traps is false on PPC,
and on no other architecture that I know of (in practice this trap
reflects the hardware behaviour on division by zero).
By generating a trap in this case, I believe that the compiler violates
a contract given by <limits>, and the standard.
I'd certainly prefer a compile time warning, easily convertible to an
error.
>
> > 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).
It may be undefined, but I believe that the numeric_limits<>::traps
value clearly prohibits generating a trap in this case.
Gabriel
next prev parent reply other threads:[~2017-03-06 12:04 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-04 23:54 [PATCH] powerpc: Avoid panic during boot due to divide by zero in init_cache_info() Anton Blanchard
2017-03-05 0:25 ` Benjamin Herrenschmidt
2017-03-05 0:27 ` Benjamin Herrenschmidt
2017-03-05 10:26 ` Michael Ellerman
2017-03-05 12:37 ` Segher Boessenkool
2017-03-05 16:58 ` Gabriel Paubert
2017-03-05 17:24 ` Segher Boessenkool
2017-03-05 23:09 ` Benjamin Herrenschmidt
2017-03-06 0:10 ` Segher Boessenkool
2017-03-06 0:20 ` Benjamin Herrenschmidt
2017-03-06 12:03 ` Gabriel Paubert [this message]
2017-03-06 14:17 ` Segher Boessenkool
2017-03-06 15:18 ` David Laight
2017-03-05 10:36 ` Michael Ellerman
2017-03-08 7:25 ` Michael Ellerman
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