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From:   "Kaneda, Erik" <erik.kaneda@intel.com>
To:     Kees Cook <keescook@chromium.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>
CC:     "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        "Moore, Robert" <robert.moore@intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@intel.com>,
        Len Brown <lenb@kernel.org>,
        ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
        "open list:ACPI COMPONENT ARCHITECTURE (ACPICA)" <devel@acpica.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Subject: RE: [PATCH] ACPICA: Replace one-element array and use struct_size()
 helper
Thread-Topic: [PATCH] ACPICA: Replace one-element array and use struct_size()
 helper
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Date:   Thu, 21 May 2020 17:30:11 +0000
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> -----Original Message-----
> From: linux-acpi-owner@vger.kernel.org <linux-acpi-
> owner@vger.kernel.org> On Behalf Of Kees Cook
> Sent: Wednesday, May 20, 2020 8:41 AM
> To: Rafael J. Wysocki <rafael@kernel.org>
> Cc: Gustavo A. R. Silva <gustavoars@kernel.org>; Moore, Robert
> <robert.moore@intel.com>; Kaneda, Erik <erik.kaneda@intel.com>; Wysocki,
> Rafael J <rafael.j.wysocki@intel.com>; Len Brown <lenb@kernel.org>; ACPI
> Devel Maling List <linux-acpi@vger.kernel.org>; open list:ACPI COMPONENT
> ARCHITECTURE (ACPICA) <devel@acpica.org>; Linux Kernel Mailing List
> <linux-kernel@vger.kernel.org>; Gustavo A. R. Silva
> <gustavo@embeddedor.com>
> Subject: Re: [PATCH] ACPICA: Replace one-element array and use
> struct_size() helper
>=20
> On Wed, May 20, 2020 at 11:15:18AM +0200, Rafael J. Wysocki wrote:
> > On Wed, May 20, 2020 at 12:46 AM Gustavo A. R. Silva
> > <gustavoars@kernel.org> wrote:
> > >
> > > On Tue, May 19, 2020 at 12:25:13PM +0200, Rafael J. Wysocki wrote:
> > > > On Tue, May 19, 2020 at 12:22 AM Gustavo A. R. Silva
> > > > <gustavoars@kernel.org> wrote:
> > > > >
> > > > > The current codebase makes use of one-element arrays in the
> > > > > following
> > > > > form:
> > > > >
> > > > > struct something {
> > > > >     int length;
> > > > >     u8 data[1];
> > > > > };
> > > > >
> > > > > struct something *instance;
> > > > >
> > > > > instance =3D kmalloc(sizeof(*instance) + size, GFP_KERNEL);
> > > > > instance->length =3D size;
> > > > > memcpy(instance->data, source, size);
> > > > >
> > > > > but the preferred mechanism to declare variable-length types
> > > > > such as these ones is a flexible array member[1][2], introduced i=
n C99:
> > > > >
> > > > > struct foo {
> > > > >         int stuff;
> > > > >         struct boo array[];
> > > > > };
> > > > >
> > > > > By making use of the mechanism above, we will get a compiler
> > > > > warning in case the flexible array does not occur last in the
> > > > > structure, which will help us prevent some kind of undefined
> > > > > behavior bugs from being inadvertently introduced[3] to the
> codebase from now on.
> > > >
> > > > However, the ACPICA code in the kernel comes from an external
> > > > project and changes of this type are generally not applicable to
> > > > it unless accepted upstream.
> > >
> > > Hi Rafael,
> > >
> > > By _accepted upstream_, in this case, you mean the adoption of the
> > > flexible-arrays in the whole codebase, first?
> >
> > I meant whether or not the patch is accepted by the ACPICA upstream.
>=20
> Is that here? https://github.com/acpica/acpica/commits/master
>=20
> >
> > >  If this is the case
> > > notice that there are hundreds of these flexible-array conversions
> > > in mainline, already:
> > >
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/l
> > > og/?qt=3Dgrep&q=3Dflexible-array
> > >
> > > Is this what you mean?
> >
> > I'm not actually sure what you mean here.
>=20
> I think this was just a misunderstanding about what "upstream" meant. :)
>=20
> I hope ACPICA will take these changes -- it seems like we keep running in=
to
> these issues with the kernel's language feature clean-ups and ACPICA
> upstream, though each have been resolved so far! :) Flexible array member=
s
> are a C99 feature, so it's hardly a new way to express things.
> In fact, it looks like ACPICA already builds with -c99 by default:
> https://github.com/acpica/acpica/blob/master/generate/unix/Makefile.conf
> ig#L202
> https://github.com/acpica/acpica/blob/master/generate/efi/Makefile.config
> #L93
>=20
> MSVC has supported them (called "unsized arrays") since 7.1 in 2003.
>=20
> Gustavo, can you build a merge request for the ACPICA project directly?

The flexible array members will be fine.

The struct_size helper uses the typeof operator which is a GCC extension.

ACPICA codebase is intended to be compiled by many different compilers (not
just GCC and MSVC) so we cannot support struct_size.

Erik
>=20
> --
> Kees Cook