-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@xxxxxxxxxx>
---
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.
Thanks!
drivers/net/virtio_net.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 975bdc5dab84..f4964a18a214 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -425,9 +425,6 @@ struct virtnet_info {
u16 rss_indir_table_size;
u32 rss_hash_types_supported;
u32 rss_hash_types_saved;
- struct virtio_net_rss_config_hdr *rss_hdr;
- struct virtio_net_rss_config_trailer rss_trailer;
- u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];
/* Has control virtqueue */
bool has_cvq;
@@ -493,7 +490,16 @@ struct virtnet_info {
struct failover *failover;
u64 device_stats_cap;
+
+ struct virtio_net_rss_config_hdr *rss_hdr;
+
+ /* Must be last --ends in a flexible-array member. */
+ TRAILING_OVERLAP(struct virtio_net_rss_config_trailer, rss_trailer, hash_key_data,
+ u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];
+ );
};
+static_assert(offsetof(struct virtnet_info, rss_trailer.hash_key_data) ==
+ offsetof(struct virtnet_info, rss_hash_key_data));
struct padded_vnet_hdr {
struct virtio_net_hdr_v1_hash hdr;
--
2.43.0
