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 <gustavo...@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
