Re: [PATCH 11/14] KVM: selftests: Disable "gnu-variable-sized-type-not-at-end" warning

Nick Desaulniers
2022-12-13T01:37:09Z

On Mon, Dec 12, 2022 at 4:17 PM Sean Christopherson <seanjc@google.com> wrote:
>
> Disable gnu-variable-sized-type-not-at-end so that tests and libraries
> can create overlays of variable sized arrays at the end of structs when
> using a fixed number of entries, e.g. to get/set a single MSR.
>
> It's possible to fudge around the warning, e.g. by defining a custom
> struct that hardcodes the number of entries, but that is a burden for
> both developers and readers of the code.
>
> lib/x86_64/processor.c:664:19: warning: field 'header' with variable sized type 'struct kvm_msrs'
> not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
> struct kvm_msrs header;
> ^
> lib/x86_64/processor.c:772:19: warning: field 'header' with variable sized type 'struct kvm_msrs'
> not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
> struct kvm_msrs header;
> ^
> lib/x86_64/processor.c:787:19: warning: field 'header' with variable sized type 'struct kvm_msrs'
> not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
> struct kvm_msrs header;
> ^
> 3 warnings generated.
>
> x86_64/hyperv_tlb_flush.c:54:18: warning: field 'hv_vp_set' with variable sized type 'struct hv_vpset'
> not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
> struct hv_vpset hv_vp_set;
> ^
> 1 warning generated.
>
> x86_64/xen_shinfo_test.c:137:25: warning: field 'info' with variable sized type 'struct kvm_irq_routing'
> not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
> struct kvm_irq_routing info;
> ^
> 1 warning generated.
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> tools/testing/selftests/kvm/Makefile | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile
> index 2487db21b177..9cff99a1cb2e 100644
> --- a/tools/testing/selftests/kvm/Makefile
> +++ b/tools/testing/selftests/kvm/Makefile
> @@ -196,6 +196,7 @@ else
> LINUX_TOOL_ARCH_INCLUDE = $(top_srcdir)/tools/arch/$(ARCH)/include
> endif
> CFLAGS += -Wall -Wstrict-prototypes -Wuninitialized -O2 -g -std=gnu99 \
> + -Wno-gnu-variable-sized-type-not-at-end \

This is a clang-specific warning. This will need to be wrapped in a
cc-option check.

tools/build/Build.include seems to redefine that make macro, so be
sure to test it first.

> -fno-builtin-memcmp -fno-builtin-memcpy -fno-builtin-memset \
> -fno-stack-protector -fno-PIE -I$(LINUX_TOOL_INCLUDE) \
> -I$(LINUX_TOOL_ARCH_INCLUDE) -I$(LINUX_HDR_PATH) -Iinclude \
> --
> 2.39.0.rc1.256.g54fd8350bd-goog
>
>

--
Thanks,
~Nick Desaulniers