KVM: selftests: Define and use a custom static assert in lib headers

KVM: selftests: Define and use a custom static assert in lib headers

Define and use kvm_static_assert() in the common KVM selftests headers to
provide deterministic behavior, and to allow creating static asserts
without dummy messages.

The kernel's static_assert() makes the message param optional, and on the
surface, tools/include/linux/build_bug.h appears to follow suit.  However,
glibc may override static_assert() and redefine it as a direct alias of
_Static_assert(), which makes the message parameter mandatory.  This leads
to non-deterministic behavior as KVM selftests code that utilizes
static_assert() without a custom message may or not compile depending on
the order of includes.  E.g. recently added asserts in
x86_64/processor.h fail on some systems with errors like

  In file included from lib/memstress.c:11:0:
  include/x86_64/processor.h: In function ‘this_cpu_has_p’:
  include/x86_64/processor.h:193:34: error: expected ‘,’ before ‘)’ token
    static_assert(low_bit < high_bit);     \
                                    ^
due to _Static_assert() expecting a comma before a message.  The "message
optional" version of static_assert() uses macro magic to strip away the
comma when presented with empty an __VA_ARGS__

  #ifndef static_assert
  #define static_assert(expr, ...) __static_assert(expr, ##__VA_ARGS__, #expr)
  #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
  #endif // static_assert

and effectively generates "_Static_assert(expr, #expr)".

The incompatible version of static_assert() gets defined by this snippet
in /usr/include/assert.h:

  #if defined __USE_ISOC11 && !defined __cplusplus
  # undef static_assert
  # define static_assert _Static_assert
  #endif

which yields "_Static_assert(expr)" and thus fails as above.

KVM selftests don't actually care about using C11, but __USE_ISOC11 gets
defined because of _GNU_SOURCE, which many tests do #define.  _GNU_SOURCE
triggers a massive pile of defines in /usr/include/features.h, including
_ISOC11_SOURCE:

  /* If _GNU_SOURCE was defined by the user, turn on all the other features.  */
  #ifdef _GNU_SOURCE
  # undef  _ISOC95_SOURCE
  # define _ISOC95_SOURCE 1
  # undef  _ISOC99_SOURCE
  # define _ISOC99_SOURCE 1
  # undef  _ISOC11_SOURCE
  # define _ISOC11_SOURCE 1
  # undef  _POSIX_SOURCE
  # define _POSIX_SOURCE  1
  # undef  _POSIX_C_SOURCE
  # define _POSIX_C_SOURCE        200809L
  # undef  _XOPEN_SOURCE
  # define _XOPEN_SOURCE  700
  # undef  _XOPEN_SOURCE_EXTENDED
  # define _XOPEN_SOURCE_EXTENDED 1
  # undef  _LARGEFILE64_SOURCE
  # define _LARGEFILE64_SOURCE    1
  # undef  _DEFAULT_SOURCE
  # define _DEFAULT_SOURCE        1
  # undef  _ATFILE_SOURCE
  # define _ATFILE_SOURCE 1
  #endif

which further down in /usr/include/features.h leads to:

  /* This is to enable the ISO C11 extension.  */
  #if (defined _ISOC11_SOURCE \
       || (defined __STDC_VERSION__ && __STDC_VERSION__ >= 201112L))
  # define __USE_ISOC11   1
  #endif

To make matters worse, /usr/include/assert.h doesn't guard against
multiple inclusion by turning itself into a nop, but instead #undefs a
few macros and continues on.  As a result, it's all but impossible to
ensure the "message optional" version of static_assert() will actually be
used, e.g. explicitly including assert.h and #undef'ing static_assert()
doesn't work as a later inclusion of assert.h will again redefine its
version.

  #ifdef  _ASSERT_H

  # undef _ASSERT_H
  # undef assert
  # undef __ASSERT_VOID_CAST

  # ifdef __USE_GNU
  #  undef assert_perror
  # endif

  #endif /* assert.h      */

  #define _ASSERT_H       1
  #include <features.h>

Fixes: fcba483e8246 ("KVM: selftests: Sanity check input to ioctls() at build time")
Fixes: ee3795536664 ("KVM: selftests: Refactor X86_FEATURE_* framework to prep for X86_PROPERTY_*")
Fixes: 53a7dc0f215e ("KVM: selftests: Add X86_PROPERTY_* framework to retrieve CPUID values")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221122013309.1872347-1-seanjc@google.com