Subject: Re: [PATCH][next] wifi: mt76: mt7921: Replace fake flex-arrays with flexible-array members

From: Simon Horman <[email-masked]>
Date: Mon, 20 Mar 2023 15:56:54 +0100

On Fri, Mar 17, 2023 at 02:56:39PM -0600, Gustavo A. R. Silva wrote:
> Zero-length arrays as fake flexible arrays are deprecated and we are
> moving towards adopting C99 flexible-array members instead.
> 
> Address the following warnings found with GCC-13 and
> -fstrict-flex-arrays=3 enabled:
> drivers/net/wireless/mediatek/mt76/mt7921/acpi_sar.c:266:25: warning: array subscript 0 is outside array bounds of ‘struct mt7921_asar_dyn_limit_v2[0]’ [-Warray-bounds=]
> drivers/net/wireless/mediatek/mt76/mt7921/acpi_sar.c:263:25: warning: array subscript 0 is outside array bounds of ‘struct mt7921_asar_dyn_limit[0]’ [-Warray-bounds=]
> drivers/net/wireless/mediatek/mt76/mt7921/acpi_sar.c:223:28: warning: array subscript <unknown> is outside array bounds of ‘struct mt7921_asar_geo_limit_v2[0]’ [-Warray-bounds=]
> drivers/net/wireless/mediatek/mt76/mt7921/acpi_sar.c:220:28: warning: array subscript <unknown> is outside array bounds of ‘struct mt7921_asar_geo_limit[0]’ [-Warray-bounds=]
> drivers/net/wireless/mediatek/mt76/mt7921/acpi_sar.c:334:37: warning: array subscript i is outside array bounds of ‘u8[0]’ {aka ‘unsigned char[]’} [-Warray-bounds=]
> 
> Notice that the DECLARE_FLEX_ARRAY() helper allows for flexible-array
> members in unions.
> 
> This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
> routines on memcpy() and help us make progress towards globally
> enabling -fstrict-flex-arrays=3 [1].
> 
> Link: https://github.com/KSPP/linux/issues/21
> Link: https://github.com/KSPP/linux/issues/272
> Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [1]
> Signed-off-by: Gustavo A. R. Silva <[email-masked]>
> ---
>  drivers/net/wireless/mediatek/mt76/mt7921/acpi_sar.h | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/wireless/mediatek/mt76/mt7921/acpi_sar.h b/drivers/net/wireless/mediatek/mt76/mt7921/acpi_sar.h
> index 35268b0890ad..6f2c4a572572 100644
> --- a/drivers/net/wireless/mediatek/mt76/mt7921/acpi_sar.h
> +++ b/drivers/net/wireless/mediatek/mt76/mt7921/acpi_sar.h

...

> @@ -69,7 +69,7 @@ struct mt7921_asar_geo_v2 {
>  	u8 version;
>  	u8 rsvd;
>  	u8 nr_tbl;
> -	struct mt7921_asar_geo_limit_v2 tbl[0];
> +	DECLARE_FLEX_ARRAY(struct mt7921_asar_geo_limit_v2, tbl);
>  } __packed;
>  
>  struct mt7921_asar_cl {
> @@ -85,7 +85,7 @@ struct mt7921_asar_fg {
>  	u8 rsvd;
>  	u8 nr_flag;
>  	u8 rsvd1;
> -	u8 flag[0];
> +	u8 flag[];

I am curious to know why DECLARE_FLEX_ARRAY isn't used here.

>  } __packed;
>  struct mt7921_acpi_sar {
> -- 
> 2.34.1