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

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

On Thu, Sep 04, 2025 at 08:53:31PM +0200, Gustavo A. R. Silva wrote:
> 
> 
> On 9/4/25 11:13, Simon Horman wrote:
> > 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.
> > 
> > I tend to agree given that:
> > 
> > On the one hand:
> > 
> > 1) in virtnet_init_default_rss(), netdev_rss_key_fill() is used
> >     to write random data to .rss_hash_key_data
> > 
> > 2) In virtnet_set_rxfh() key data written to .rss_hash_key_data
> > 
> > While
> > 
> > 3) In virtnet_commit_rss_command() virtio_net_rss_config_trailer,
> >     including the contents of .hash_key_data based on the length of
> >     that data provided in .hash_key_length is copied.
> > 
> > It seems to me that step 3 will include 1 byte of uninitialised data
> > at the start of .hash_key_data. And, correspondingly, truncate
> > .rss_hash_key_data by one byte.
> > 
> > It's unclear to me what the effect of this - perhaps they key works
> > regardless. But it doesn't seem intended. And while the result may be
> > neutral, I do  suspect this reduces the quality of the key. And I more
> > strongly suspect it doesn't have any positive outcome.
> > 
> > So I would lean towards playing it safe and considering this as a bug.
> > 
> > Of course, other's may have better insight as to the actual effect of this.
> 
> Yeah, in the meantime I'll prepare v2 with both the 'Fixes' and 'stable'
> tags.
> 
> Thanks for the feedback!
> -Gustavo
> 
> 


I agree. It looks like that commit completely broke RSS
configuration. Akihiko do you mind sharing how that was
tested? Maybe help testing the fix? Thanks!