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Date:   Fri, 18 Sep 2020 21:02:52 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.org>,
        Christoph Lameter <cl@linux.com>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>
Subject: Re: [GIT PULL] percpu fix for v5.9-rc6
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On Fri, Sep 18, 2020 at 12:37:48PM -0700, Linus Torvalds wrote:
> > Applying sizeof() to a structure containing a flex-array member is allowed,
> 
> Yes, and that's wrong and inconsistent, but what else is new about the
> C standard. It's what allows these kinds of bugs to slip through.

Hmm.  We actually do that in our implementation of struct_size()

#define struct_size(p, member, count)                                   \
        __ab_c_size(count,                                              \
                    sizeof(*(p)->member) + __must_be_array((p)->member),\
                    sizeof(*(p)))

I suppose it's not really necessary, we could do offsetof here, right?

#define struct_size(p, member, count)                                   \
        __ab_c_size(count,                                              \
                    sizeof(*(p)->member) + __must_be_array((p)->member),\
                    offsetof(typeof(*(p)), member))