Re: [PATCH][next] hyperv: Avoid -Wflex-array-member-not-at-end warning

[Gustavo A. R. Silva]

On 12/16/25 03:47, Wei Liu wrote:

On Fri, Dec 12, 2025 at 03:44:50PM +0900, Gustavo A. R. Silva wrote:

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Use the new __TRAILING_OVERLAP() helper to fix the following warning:

include/hyperv/hvgdk_mini.h:581:25: warning: structure containing a flexible 
array member is not at the end of another structure 
[-Wflex-array-member-not-at-end]

This helper creates a union between a flexible-array member (FAM) and a
set of MEMBERS that would otherwise follow it.

This overlays the trailing MEMBER u64 gva_list[]; onto the FAM
struct hv_tlb_flush_ex::hv_vp_set.bank_contents[], while keeping
the FAM and the start of MEMBER aligned.

The static_assert() ensures this alignment remains, and it's
intentionally placed inmediately after the related structure --no
blank line in between.

Signed-off-by: Gustavo A. R. Silva <gustavo...@kernel.org>

Gustavo, what is your build command? I would like to incorporate that
into my own build tests.

I'm using the following patch (which is the one I'll actually submit
upstream once all these warnings are fixed).

diff --git a/Makefile b/Makefile
index e404e4767944..0d6a0d8f791b 100644
--- a/Makefile
+++ b/Makefile
@@ -1086,6 +1086,8 @@ KBUILD_CFLAGS += $(call cc-option, -fstrict-flex-arrays=3)
 # Allow including a tagged struct or union anonymously in another struct/union.
 KBUILD_CFLAGS += -fms-extensions

+KBUILD_CFLAGS += $(call cc-option, -Wflex-array-member-not-at-end)
+
 # disable invalid "can't wrap" optimizations for signed / pointers
 KBUILD_CFLAGS  += -fno-strict-overflow


Thanks
-Gustavo

Thanks,
Wei

---
  include/hyperv/hvgdk_mini.h | 7 +++++--
  1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/hyperv/hvgdk_mini.h b/include/hyperv/hvgdk_mini.h
index 04b18d0e37af..30fbbde81c5c 100644
--- a/include/hyperv/hvgdk_mini.h
+++ b/include/hyperv/hvgdk_mini.h
@@ -578,9 +578,12 @@ struct hv_tlb_flush {       /* 
HV_INPUT_FLUSH_VIRTUAL_ADDRESS_LIST */
  struct hv_tlb_flush_ex {
        u64 address_space;
        u64 flags;
-       struct hv_vpset hv_vp_set;
-       u64 gva_list[];
+       __TRAILING_OVERLAP(struct hv_vpset, hv_vp_set, bank_contents, __packed,
+               u64 gva_list[];
+       );
  } __packed;
+static_assert(offsetof(struct hv_tlb_flush_ex, hv_vp_set.bank_contents) ==
+             offsetof(struct hv_tlb_flush_ex, gva_list));

struct ms_hyperv_tsc_page { /* HV_REFERENCE_TSC_PAGE */

        volatile u32 tsc_sequence;
--
2.43.0