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 <gustavo...@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
