KVM: selftests: Define _GNU_SOURCE for all selftests code

Define _GNU_SOURCE is the base CFLAGS instead of relying on selftests to
manually #define _GNU_SOURCE, which is repetitive and error prone.  E.g.
kselftest_harness.h requires _GNU_SOURCE for asprintf(), but if a selftest
includes kvm_test_harness.h after stdio.h, the include guards result in
the effective version of stdio.h consumed by kvm_test_harness.h not
defining asprintf():

  In file included from x86_64/fix_hypercall_test.c:12:
  In file included from include/kvm_test_harness.h:11:
 ../kselftest_harness.h:1169:2: error: call to undeclared function
  'asprintf'; ISO C99 and later do not support implicit function declarations
  [-Wimplicit-function-declaration]
   1169 |         asprintf(&test_name, "%s%s%s.%s", f->name,
        |         ^

When including the rseq selftest's "library" code, #undef _GNU_SOURCE so
that rseq.c controls whether or not it wants to build with _GNU_SOURCE.

Reported-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Acked-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Acked-by: Oliver Upton <oliver.upton@linux.dev>
Acked-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Link: https://lore.kernel.org/r/20240423190308.2883084-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 6 insertions, 6 deletions

diff --git a/tools/testing/selftests/kvm/include/kvm_util_base.h b/tools/testing/selftests/kvm/include/kvm_util_base.h
index 8acca8237687..af02308e264e 100644
--- a/tools/testing/selftests/kvm/include/kvm_util_base.h
+++ b/tools/testing/selftests/kvm/include/kvm_util_base.h
@@ -27,12 +27,12 @@
 
 /*
  * Provide a version of static_assert() that is guaranteed to have an optional
- * message param.  If _ISOC11_SOURCE is defined, glibc (/usr/include/assert.h)
- * #undefs and #defines static_assert() as a direct alias to _Static_assert(),
- * i.e. effectively makes the message mandatory.  Many KVM selftests #define
- * _GNU_SOURCE for various reasons, and _GNU_SOURCE implies _ISOC11_SOURCE.  As
- * a result, static_assert() behavior is non-deterministic and may or may not
- * require a message depending on #include order.
+ * message param.  _GNU_SOURCE is defined for all KVM selftests, _GNU_SOURCE
+ * implies _ISOC11_SOURCE, and if _ISOC11_SOURCE is defined, glibc #undefs and
+ * #defines static_assert() as a direct alias to _Static_assert() (see
+ * usr/include/assert.h).  Define a custom macro instead of redefining
+ * static_assert() to avoid creating non-deterministic behavior that is
+ * dependent on include order.
  */
 #define __kvm_static_assert(expr, msg, ...) _Static_assert(expr, msg)
 #define kvm_static_assert(expr, ...) __kvm_static_assert(expr, ##__VA_ARGS__, #expr)