linux-hardening - Re: [PATCH] [next] drm/amdgpu: Replace one-element array with flexible-array member

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Message-ID: <202210280855.9DF8E4D72@keescook>
Date:   Fri, 28 Oct 2022 09:36:19 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Christian König <christian.koenig@....com>
Cc:     Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@...il.com>,
        Alex Deucher <alexander.deucher@....com>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Hans de Goede <hdegoede@...hat.com>,
        Grigory Vasilyev <h0tc0d3@...il.com>,
        Claudio Suarez <cssk@...-c.es>,
        Slark Xiao <slark_xiao@....com>,
        Rongguang Wei <weirongguang@...inos.cn>,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] [next] drm/amdgpu: Replace one-element array with
 flexible-array member

On Fri, Oct 28, 2022 at 09:18:39AM +0200, Christian König wrote:
> Am 28.10.22 um 07:10 schrieb Paulo Miguel Almeida:
> > One-element arrays are deprecated, and we are replacing them with
> > flexible array members instead. So, replace one-element array with
> > flexible-array member in struct _ATOM_FAKE_EDID_PATCH_RECORD and
> > refactor the rest of the code accordingly.
> > 
> > This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
> > routines on memcpy() and help us make progress towards globally
> > enabling -fstrict-flex-arrays=3 [1].
> > 
> > Link: https://github.com/KSPP/linux/issues/79
> > Link: https://github.com/KSPP/linux/issues/238
> > Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836 [1]
> 
> I'm not sure if that's a good idea. We had multiple attempts to refactor
> this now and it always caused a regression.
> 
> Additional to that the header in question came from our BIOS team and isn't
> following Linux styles in general.
> 
> Alex what do you think?

Fake flexible arrays (i.e. 1-element arrays) are deprecated in Linux[1]
(and, frankly, deprecated in C since 1999 and even well before then given
the 0-sized extension that was added in GCC), so we can't continue to
bring them into kernel sources. Their use breaks both compile-time and
run-time bounds checking efforts, etc.

All that said, converting away from them can be tricky, and I think such
conversions need to explicitly show how they were checked for binary
differences[2].

Paulo, can you please check for deltas and report your findings in the
commit log? Note that add struct_size() use in the same patch may result
in binary differences, so for more complex cases, you may want to split
the 1-element conversion from the struct_size() conversions.

-Kees

[1] https://docs.kernel.org/process/deprecated.html#zero-length-and-one-element-arrays
[2] https://outflux.net/blog/archives/2022/06/24/finding-binary-differences/

-- 
Kees Cook