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

[Simon Horman <horms@kernel.org>]

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.

...