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Date:   Sat, 19 Sep 2020 03:45:56 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        "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
Message-ID: <20200919024556.GJ32101@casper.infradead.org>
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On Fri, Sep 18, 2020 at 06:39:57PM -0400, Arvind Sankar wrote:
> On Fri, Sep 18, 2020 at 02:18:20PM -0700, Linus Torvalds wrote:
> > On Fri, Sep 18, 2020 at 2:00 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
> > >
> > > You could just assert that offsetof(typeof(s),flex) == sizeof(s), no?
> > 
> > No, because the whole point is that I want that "sizeof(s)" to *WARN*.
> 
> Ouch, offsetof() and sizeof() will give different results in the
> presence of alignment padding.
> 
> https://godbolt.org/z/rqnxTK

We really should be using offsetof() then.  It's harmless because we're
currently overallocating, not underallocating.  The test case I did was:

struct s {
        int count;
        char *p[];
};

struct_size(&s, p, 5); (48 bytes)
struct_size2(&s, p, 5); (also 48 bytes)

struct_size2 uses offsetof instead of sizeof.

Your case is different because the chars fit in the padding at the end
of the struct.