Re: [PATCH][next] virtio_net: Fix alignment and avoid -Wflex-array-member-not-at-end warning

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Sep 03, 2025 at 09:36:13PM +0200, 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:
> 
> drivers/net/virtio_net.c:429:46: 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 (in this case
> `u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];`). This
> overlays the trailing members (rss_hash_key_data) onto the FAM
> (hash_key_data) while keeping the FAM and the start of MEMBERS aligned.
> The static_assert() ensures this alignment remains, and it's
> intentionally placed inmediately after `struct virtnet_info` (no
> blank line in between).
> 
> Notice that due to tail padding in flexible `struct
> virtio_net_rss_config_trailer`, `rss_trailer.hash_key_data`
> (at offset 83 in struct virtnet_info) and `rss_hash_key_data` (at
> offset 84 in struct virtnet_info) are misaligned by one byte. See
> below:
> 
> struct virtio_net_rss_config_trailer {
>         __le16                     max_tx_vq;            /*     0     2 */
>         __u8                       hash_key_length;      /*     2     1 */
>         __u8                       hash_key_data[];      /*     3     0 */
> 
>         /* size: 4, cachelines: 1, members: 3 */
>         /* padding: 1 */
>         /* last cacheline: 4 bytes */
> };
> 
> struct virtnet_info {
> ...
>         struct virtio_net_rss_config_trailer rss_trailer; /*    80     4 */
> 
>         /* XXX last struct has 1 byte of padding */
> 
>         u8                         rss_hash_key_data[40]; /*    84    40 */
> ...
>         /* size: 832, cachelines: 13, members: 48 */
>         /* sum members: 801, holes: 8, sum holes: 31 */
>         /* paddings: 2, sum paddings: 5 */
> };
> 
> After changes, those members are correctly aligned at offset 795:
> 
> struct virtnet_info {
> ...
>         union {
>                 struct virtio_net_rss_config_trailer rss_trailer; /*   792     4 */
>                 struct {
>                         unsigned char __offset_to_hash_key_data[3]; /*   792     3 */
>                         u8         rss_hash_key_data[40]; /*   795    40 */
>                 };                                       /*   792    43 */
>         };                                               /*   792    44 */
> ...
>         /* size: 840, cachelines: 14, members: 47 */
>         /* sum members: 801, holes: 8, sum holes: 35 */
>         /* padding: 4 */
>         /* paddings: 1, sum paddings: 4 */
>         /* last cacheline: 8 bytes */
> };
> 
> As a last note `struct virtio_net_rss_config_hdr *rss_hdr;` is also
> moved to the end, since it seems those three members should stick
> around together. :)
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
> 
> This should probably include the following tag:
> 
> 	Fixes: ed3100e90d0d ("virtio_net: Use new RSS config structs")
> 
> but I'd like to hear some feedback, first.
> 
> Thanks!
> 


I would add:

as a result, the RSS key passed to the device is shifted by 1
byte: the last byte is cut off, and instead a (possibly uninitialized) byte
is added at the beginning.

>  drivers/net/virtio_net.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 975bdc5dab84..f4964a18a214 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -425,9 +425,6 @@ struct virtnet_info {
>  	u16 rss_indir_table_size;
>  	u32 rss_hash_types_supported;
>  	u32 rss_hash_types_saved;
> -	struct virtio_net_rss_config_hdr *rss_hdr;
> -	struct virtio_net_rss_config_trailer rss_trailer;
> -	u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];
>  
>  	/* Has control virtqueue */
>  	bool has_cvq;
> @@ -493,7 +490,16 @@ struct virtnet_info {
>  	struct failover *failover;
>  
>  	u64 device_stats_cap;
> +
> +	struct virtio_net_rss_config_hdr *rss_hdr;
> +
> +	/* Must be last --ends in a flexible-array member. */
> +	TRAILING_OVERLAP(struct virtio_net_rss_config_trailer, rss_trailer, hash_key_data,
> +		u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];
> +	);
>  };
> +static_assert(offsetof(struct virtnet_info, rss_trailer.hash_key_data) ==
> +	      offsetof(struct virtnet_info, rss_hash_key_data));
>  
>  struct padded_vnet_hdr {
>  	struct virtio_net_hdr_v1_hash hdr;
> -- 
> 2.43.0