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

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

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