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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 10 Mar 2022 18:01:56 -0800
Message-ID: <CAHk-=wjF4d_v5t=ht_vCOHxNDoPvsBuK-2jiEBus-__oPZuPFQ@mail.gmail.com>
Subject: Re: [RFC PATCH v1 1/2] list: add type-safer list_head wrapper
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Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Xiaomeng Tong <xiam0nd.tong@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Jakob Koschel <jakobkoschel@gmail.com>,
        Arnd Bergmann <arnd@arndb.de>
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On Thu, Mar 10, 2022 at 5:42 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> That one didn't do the automatic offset thing, but see
>
>    https://lore.kernel.org/all/CAADWXX-Pr-D3wSr5wsqTEOBSJzB9k7bSH+7hnCAj0AeL0=U4mg@mail.gmail.com/
>
> on the problems that has.

Note: I think the problems are serious enough that it almost certainly
isn't worth doing - it makes the code uglier for very little upside.

So I tried to explain how it _could_ be done, but that doesn't mean
that it _should_ be done.

Having the member name as part of the list traversal macro isn't
actually generally a real problem.

I added it to the list_traversal_head() macro in that original patch
because I think we can easily use the member head name to _verify_
that the declaration and the use match.

Yes, squirrelling  off the offset and not needing the member head name
at all at when traversing the list is obviously simpler syntax, but
that part has never been the real problem with list traversal. And
verifying that the member name that is passed in is the same as in the
list_traversal_head() would be trivial.

To verify it, we could simply change that type name from:

     type *name##_traversal_type;

to be

     type *name##_traversal_type_##member;

instead, and suddenly the member name in 'list_traverse()' has to
match that thing that list_traversal_head() created.

So yes, you'd have that third argument in list_traverse(), but it
would be trivially checked at compile-time.

And you'd avoid all the ugly complexities (described above) with lists
that are embedded inside data structures that refer to each other)

                  Linus