Re: [PATCH] Bluetooth: btusb: Use DECLARE_FLEX_ARRAY() for memdump segment data

[Luiz Augusto von Dentz]

Hi Zijun,

On Mon, Nov 9, 2026 at 8:21 AM Zijun Hu <zijun.hu@xxxxxxxxxxxxxxxx> wrote:
>
> On 11/8/2026 12:20 PM, Hex Lindqvist wrote:
> > struct qhci_vse_memdump declares segdata as a zero-length array in a
> > union. Zero-length arrays are deprecated [1], so use DECLARE_FLEX_ARRAY()
> > instead.
> >
> > No functional change: segdata is only used with offsetof(). With debug
> > info stripped, btusb.o is byte-for-byte identical before and after,
> > built with GCC 16 and Clang 21 on x86_64 and GCC 16 on arm64, with
> > UBSAN_BOUNDS and FORTIFY_SOURCE enabled.
> >
> > An AI assistant helped find and check this change and drafted this
> > message.
> >
> > Link: https://docs.kernel.org/process/deprecated.html#zero-length-and-one-element-arrays [1]
> > Signed-off-by: Hex Lindqvist <hex@xxxxxxxxxxxxx>
> > ---
> >  drivers/bluetooth/btusb_qcom.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/bluetooth/btusb_qcom.c b/drivers/bluetooth/btusb_qcom.c
> > index cc4cfcde2afa..97be9078e045 100644
> > --- a/drivers/bluetooth/btusb_qcom.c
> > +++ b/drivers/bluetooth/btusb_qcom.c
> > @@ -533,7 +533,7 @@ struct qhci_vse_memdump {
> >       __le16 seqno;
> >       u8     subsys;
> >       union {
> > -             u8 segdata[0];
> > +             DECLARE_FLEX_ARRAY(u8, segdata);
> >               struct {
> >                       __le32 dump_size;
> >                       u8 first_segdata[];
>
> NAK. Please keep this as-is. since:
>
> 1) segdata[0] is a familiar GNU C idiom and, since it is only used as an offsetof() anchor, is more straightforward than the
>   DECLARE_FLEX_ARRAY() expansion.
>
> 2) I guess this kind of isolated cleanup may not be welcome.As noted in Documentation/process/maintainer-netdev.rst, simple cleanups outside
>    the context of other work are discouraged

While this maybe true if the above code does produce new warnings then
it needs to be fixed anyway, from the commit message it doesn't
mention any build warning but that perhaps is because it is enclosed
into a union which mask it or something, anyway trainling variable
length should be accompany with __counted_by; so the compiler can do
bounds checking.

-- 
Luiz Augusto von Dentz