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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 18 Sep 2020 13:40:44 -0700
Message-ID: <CAHk-=wh-ryuY7KBNWr1n+kgQ5_CHB3-X+od-djBV4W-1kQFokA@mail.gmail.com>
Subject: Re: [GIT PULL] percpu fix for v5.9-rc6
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Matthew Wilcox <willy@infradead.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>
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On Fri, Sep 18, 2020 at 1:29 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> In general (i.e. outside the implementation of the macro itself), what
> is the preferred way of getting the size of just the header?
>   1) offsetof(typeof(s),flex)
>   2) struct_size(s, flex, 0)

I think those two should end up being equivalent.

>   3) sizeof(s)

This works right now, but exactly *because* it works, we're not seeing
the questionable cases.

Of course, _also_ exactly because it just silently works, I also don't
know if there may be thousands of perfectly fine uses where people
really do want the header, and a "sizeof()" is simpler than
alternatives 1-2.

It's possible that there really are a lot of "I want to know just the
header size" cases. It sounds odd, but I could _imagine_ situations
like that, even though no actual case comes to mind.

>   4) new macro that's easier to read than 1 or 2, but makes it clear
>      what you're doing?

I don't think this would have any real advantage, would it?

Now what might be good is if we can make "struct_size()" also actually
verify that the member that is passed in is that last non-sized
member. I'm not sure how to do that.

I know how to check that it's *not* that last unsized member (just do
"sizeof(s->flex)", and it should error), but I don't see how to assert
the reverse of that).

Because that kind of "yes, we actually pass in the right member" check
would be good to have too.

              Linus