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Date:   Wed, 20 May 2020 08:40:49 -0700
From:   Kees Cook <keescook@chromium.org>
To:     "Rafael J. Wysocki" <rafael@kernel.org>
Cc:     "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Robert Moore <robert.moore@intel.com>,
        Erik Kaneda <erik.kaneda@intel.com>,
        "Rafael J. Wysocki" <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
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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 = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
> > > > instance->length = 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 in 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.

Is that here? https://github.com/acpica/acpica/commits/master

> 
> >  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/log/?qt=grep&q=flexible-array
> >
> > Is this what you mean?
> 
> I'm not actually sure what you mean here.

I think this was just a misunderstanding about what "upstream" meant. :)

I hope ACPICA will take these changes -- it seems like we keep running
into these issues with the kernel's language feature clean-ups and ACPICA
upstream, though each have been resolved so far! :) Flexible array
members 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.config#L202
https://github.com/acpica/acpica/blob/master/generate/efi/Makefile.config#L93

MSVC has supported them (called "unsized arrays") since 7.1 in 2003.

Gustavo, can you build a merge request for the ACPICA project directly?

-- 
Kees Cook