netdev - Re: [PATCH v4 1/2] kernel.h: Introduce const

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Message-ID: <CA+55aFzO12QMFJHe8osoWfph8jnjgp_jLUQWmgbNmth_KwPxXw@mail.gmail.com>
Date:   Thu, 15 Mar 2018 16:34:13 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Randy Dunlap <rdunlap@...radead.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        David Laight <David.Laight@...lab.com>,
        Ian Abbott <abbotti@....co.uk>,
        linux-input <linux-input@...r.kernel.org>,
        linux-btrfs <linux-btrfs@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v4 1/2] kernel.h: Introduce const_max() for VLA removal

On Thu, Mar 15, 2018 at 3:46 PM, Kees Cook <keescook@...omium.org> wrote:
>
> So, AIUI, I can either get strict type checking, in which case, this
> is rejected (which I assume there is still a desire to have):
>
> int foo[const_max(6, sizeof(whatever))];

Ehh, yes, that looks fairly sane, and erroring out would be annoying.

But maybe we should just make the type explicit, and make it "const_max_t()"?

I think all the existing users are of type "max_t()" anyway due to the
very same issue, no?

At least if there's an explicit type like 'size_t', then passing in
"-1" becoming a large unsigned integer is understandable and clear,
not just some odd silent behavior.

Put another way: I think it's unacceptable that

     const_max(-1,6)

magically becomes a huge positive number like in that patch of yours, but

     const_max_t(size_t, -1, 6)

*obviously* is a huge positive number.

The two things would *do* the same thing, but in the second case the
type is explicit and visible.

> due to __builtin_types_compatible_p() rejecting it, or I can construct
> a "positive arguments only" test, in which the above is accepted, but
> this is rejected:

That sounds acceptable too, although the "const_max_t()" thing is
presumably going to be simpler?

                 Linus