KVM: selftests: Add -U_FORTIFY_SOURCE to avoid some unpredictable test failures

commit e396a74222 upstream.

Some distributions (such as Ubuntu) configure GCC so that
_FORTIFY_SOURCE is automatically enabled at -O1 or above.  This results
in some fortified version of definitions of standard library functions
are included.  While linker resolves the symbols, the fortified versions
might override the definitions in lib/string_override.c and reference to
those PLT entries in GLIBC.  This is not a problem for the code in host,
but it is a disaster for the guest code.  E.g., if build and run
x86/nested_emulation_test on Ubuntu 24.04 will encounter a L1 #PF due to
memset() reference to __memset_chk@plt.

The option -fno-builtin-memset is not helpful here, because those
fortified versions are not built-in but some definitions which are
included by header, they are for different intentions.

In order to eliminate the unpredictable behaviors may vary depending on
the linker and platform, add the "-U_FORTIFY_SOURCE" into CFLAGS to
prevent from introducing the fortified definitions.

Signed-off-by: Zhiquan Li <zhiquan_li@163.com>
Link: https://patch.msgid.link/20260122053551.548229-1-zhiquan_li@163.com
Fixes: 6b6f71484b ("KVM: selftests: Implement memcmp(), memcpy(), and memset() for guest use")
Cc: stable@vger.kernel.org
[sean: tag for stable]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

tools/testing/selftests/kvm/Makefile.kvm

@@ -245,6 +245,7 @@ LINUX_TOOL_INCLUDE = $(top_srcdir)/tools/include
 LINUX_TOOL_ARCH_INCLUDE = $(top_srcdir)/tools/arch/$(ARCH)/include
 CFLAGS += -Wall -Wstrict-prototypes -Wuninitialized -O2 -g -std=gnu99 \
 	-Wno-gnu-variable-sized-type-not-at-end -MD -MP -DCONFIG_64BIT \
+	-U_FORTIFY_SOURCE \
 	-fno-builtin-memcmp -fno-builtin-memcpy \
 	-fno-builtin-memset -fno-builtin-strnlen \
 	-fno-stack-protector -fno-PIE -fno-strict-aliasing \