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Date:   Wed, 18 Oct 2023 15:36:00 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Kent Overstreet <kent.overstreet@linux.dev>
Cc:     Brian Foster <bfoster@redhat.com>, linux-bcachefs@vger.kernel.org,
        kernel test robot <lkp@intel.com>,
        linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH] bcachefs: Refactor bkey_i to use a flexible array
Message-ID: <202310181534.54934E68@keescook>
References: <20231010235609.work.594-kees@kernel.org>
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 <202310131637.D0C66BFBA@keescook>
 <20231018220407.b5dvm2ehibcqvhzq@moria.home.lan>
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On Wed, Oct 18, 2023 at 06:04:07PM -0400, Kent Overstreet wrote:
> On Fri, Oct 13, 2023 at 04:44:21PM -0700, Kees Cook wrote:
> > On Fri, Oct 13, 2023 at 07:26:11AM -0400, Brian Foster wrote:
> > > Hi Kees,
> > > 
> > > I'm curious if this is something that could be buried in bch_val given
> > > it's already kind of a fake structure..? If not, my only nitty comment
> > 
> > I was thinking it would be best to keep the flexible array has "high" in
> > the struct as possible, as in the future more refactoring will be needed
> > to avoid having flex arrays overlap with other members in composite
> > structures. So instead of pushing into bch_val, I left it at the highest
> > level possible, bch_i, as that's the struct being used by the memcpy().
> 
> I agree with Brian here - I'd like this buried in bch_val, if possible.
> 
> I also went with unsafe_memcpy() for now - that's now in my for-next
> tree. I'm not seeing any advantage of DECLARE_FLEX_ARRAY over that -
> perhaps later if we could use __counted_by that would make more sense.

This won't help here because of the combination of -fstrict-flex-arrays=3
and -Wstringop-overflow (the latter is in W=1 builds). The builtin memcpy
still complains about the 0-sized destination. I'll send a v3 with this
in bch_val.

-- 
Kees Cook