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

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.

...