[PATCH v5 0/2] Remove false-positive VLAs when using max()

[Kees Cook]

Patch 1 adds const_max_t(), patch 2 uses it in all the places max()
was used for stack arrays. Commit log from patch 1:

---snip---
kernel.h: Introduce const_max_t() for VLA removal

In the effort to remove all VLAs from the kernel[1], it is desirable to
build with -Wvla. However, this warning is overly pessimistic, in that
it is only happy with stack array sizes that are declared as constant
expressions, and not constant values. One case of this is the evaluation
of the max() macro which, due to its construction, ends up converting
constant expression arguments into a constant value result. Attempts
to adjust the behavior of max() ran afoul of version-dependent compiler
behavior[2].

To work around this and still gain -Wvla coverage, this patch introduces
a new macro, const_max_t(), for use in these cases of stack array size
declaration, where the constant expressions are retained. Since this means
losing the double-evaluation protections of the max() macro, this macro is
designed to explicitly fail if used on non-constant arguments.

Older compilers will fail with the unhelpful message:

    error: first argument to â__builtin_choose_exprâ not a constant

Newer compilers will fail with a hopefully more helpful message:

    error: call to â__error_non_const_argâ declared with attribute error: const_max_t() used with non-constant expression

To gain the ability to compare differing types, the desired type must
be explicitly declared, as with the existing max_t() macro. This is
needed when comparing different enum types and to allow things like:

    int foo[const_max_t(size_t, 6, sizeof(something))];

[1] https://lkml.org/lkml/2018/3/7/621
[2] https://lkml.org/lkml/2018/3/10/170
---eol---

Hopefully this reads well as a summary from all the things that got tried.
I've tested this on allmodconfig builds with gcc 4.4.4 and 6.3.0, with and
without -Wvla.

-Kees

v5: explicit type argument
v4: forced size_t type