Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in min()/max()

[Miguel Ojeda]

On Sat, Mar 10, 2018 at 5:30 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Sat, Mar 10, 2018 at 7:33 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>
>> Alright, I'm giving up on fixing max(). I'll go back to STACK_MAX() or
>> some other name for the simple macro. Bleh.
>
> Oh, and I'm starting to see the real problem.
>
> It's not that our current "min/max()" are broiken. It's that "-Wvla" is garbage.
>
> Lookie here:
>
>         int array[(1,2)];
>
> results in gcc saying
>
>      warning: ISO C90 forbids variable length array âarrayâ [-Wvla]
>        int array[(1,2)];
>        ^~~
>
> and that error message - and the name of the flag - is obviously pure garbage.
>
> What is *actually* going on is that ISO C90 requires an array size to
> be not a constant value, but a constant *expression*. Those are two
> different things.
>
> A constant expression has little to do with "compile-time constant".
> It's a more restricted form of it, and has actual syntax requirements.
> A comma expression is not a constant expression, for example, which
> was why I tested this.
>
> So "-Wvla" is garbage, with a misleading name, and a misleading
> warning string. It has nothing to do with "variable length" and
> whether the compiler can figure it out at build time, and everything
> to do with a _syntax_ rule.

The warning string is basically the same to the one used for C++, i.e.:

    int size2 = 2;
    constexpr int size3 = 2;

    int array1[(2,2)];
    int array2[(size2, size2)];
    int array3[(size3, size3)];

only warns for array2 with:

    warning: ISO C++ forbids variable length array 'array2' [-Wvla]
     int array2[(size2, size2)];

So the warning is probably implemented to just trigger whenever VLAs
are used but the given standard does not allow them, for all
languages. The problem is why the ISO C90 frontend is not giving an
error for using invalid syntax for array sizes to begin with?

Miguel