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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 13 Jan 2026 09:39:54 -0500

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: [email-masked]
> > Fixes: ed3100e90d0d ("virtio_net: Use new RSS config structs")
> > Signed-off-by: Gustavo A. R. Silva <[email-masked]>
> > ---
> > 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?

-- 
MST