Re: [PATCH v2][next] virtio_net: Fix misalignment bug in struct virtnet_info

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

On Tue, Jan 13, 2026 at 04:06:05PM +0100, Paolo Abeni wrote:
> On 1/13/26 3:39 PM, Michael S. Tsirkin wrote:
> > On Tue, Jan 13, 2026 at 03:30:00PM +0100, Paolo Abeni wrote:
> >> On 1/10/26 9:07 AM, Gustavo A. R. Silva wrote:
> >>> Use the new TRAILING_OVERLAP() helper to fix a misalignment bug
> >>> along with 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.
> >>>
> >>> 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 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.
> >>>
> >>> 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. :)
> >>>
> >>> Cc: stable@vger.kernel.org
> >>> Fixes: ed3100e90d0d ("virtio_net: Use new RSS config structs")
> >>> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> >>> ---
> >>> Changes in v2:
> >>>  - Update subject and changelog text (include feedback from Simon and
> >>>    Michael --thanks folks)
> >>>  - Add Fixes tag and CC -stable.
> >>
> >> @Michael, @Jason: This is still apparently targeting 'net-next', but I
> >> think it should land in the 'net' tree, right?
> >>
> >> /P
> > 
> > Probably but I'm yet to properly review it. The thing that puzzles me at
> > a first glance is how are things working right now then?
> 
> Apparently they aren't ?!?
> 
> rss self-tests for virtio_net are failing:
> 
> https://netdev-ctrl.bots.linux.dev/logs/vmksft/drv-hw-dbg/results/471521/15-rss-api-py/stdout
> 
> but the result is into the CI reported as success (no idea why?!?)
> 
> /P

and this fixes it?