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From: David Laight <David.Laight@ACULAB.COM>
To: 'Segher Boessenkool' <segher@kernel.crashing.org>,
	Gabriel Paubert <paubert@iram.es>
Cc: "paulus@samba.org" <paulus@samba.org>,
	"linuxppc-dev@lists.ozlabs.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 15:18:37 +0000	[thread overview]
Message-ID: <063D6719AE5E284EB5DD2968C1650D6DCFE78D11@AcuExch.aculab.com> (raw)
In-Reply-To: <20170306141735.GR31469@gate.crashing.org>

From: Segher Boessenkool
> Sent: 06 March 2017 14:18
> On Mon, Mar 06, 2017 at 01:03:19PM +0100, Gabriel Paubert wrote:
> > > > > The PowerPC divw etc. instructions do not trap by themselves, but=
 recent
> > > > > GCC inserts trap instructions on code paths that are always undef=
ined
> > > > > 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. :-(
>=20
> It makes the "behaviour" for undefined behaviour *less* surprising.
> It does not change anything else: malformed programs stay malformed,
> correct programs do exactly what they did before, too.

Yep, 'undefined behaviour' is exactly that.
It doesn't mean 'undefined result', or 'maybe a signal'.
Wiping the disk and targeting the user with an ICBM are both valid.

	David

  reply	other threads:[~2017-03-06 15:18 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
2017-03-06 14:17           ` Segher Boessenkool
2017-03-06 15:18             ` David Laight [this message]
2017-03-05 10:36 ` Michael Ellerman
2017-03-08  7:25 ` Michael Ellerman

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