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

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Message-ID: <CA+55aFwDJ906oQ-98L2DNrjfKtb6cd5ykwMxpG942qxCFmAoEQ@mail.gmail.com>
Date:   Thu, 15 Mar 2018 15:23:16 -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 3:16 PM, Kees Cook <keescook@...omium.org> wrote:
>
> size_t __error_not_const_arg(void) \
> __compiletime_error("const_max() used with non-compile-time constant arg");
> #define const_max(x, y)                                         \
>         __builtin_choose_expr(__builtin_constant_p(x) &&        \
>                               __builtin_constant_p(y),          \
>                               (typeof(x))(x) > (typeof(y))(y) ? \
>                                         (x) : (y),              \
>                               __error_not_const_arg())
>
> Is typeof() forcing enums to int? Regardless, I'll put this through
> larger testing. How does that look?

Ok, that alleviates my worry about one class of insane behavior, but
it does raise a few other questions:

 - what drugs is gcc on where (typeof(x)(x)) makes a difference? Funky.

 - this does have the usual "what happen if you do

     const_max(-1,sizeof(x))

where the comparison will now be done in 'size_t', and -1 ends up
being a very very big unsigned integer.

Is there no way to get that type checking inserted? Maybe now is a
good point for that __builtin_types_compatible(), and add it to the
constness checking (and change the name of that error case function)?

          Linus