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virtio-devices: net: Gracefully handle MTU query failure

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If querying the fd's MTU fails (because it was from a different network
namespace). Degrade gracefully by not advertising the VIRTIO_NET_F_MTU
feature and instead let the guest kernel use the default 1500 Ethernet
MTU.

Signed-off-by: Rob Bradford <rbradford@meta.com>
This commit is contained in:
Rob Bradford
2026-04-29 18:10:29 +01:00
parent 475c8d3efb
commit 87ffc620e4
1 changed files with 15 additions and 17 deletions
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32
virtio-devices/src/net.rs
View File

@@ -481,7 +481,14 @@ impl Net {
) -> Result<Self> {
assert!(!taps.is_empty());
let mtu = taps[0].mtu().map_err(Error::TapError)? as u16;
// Skip advertising VIRTIO_NET_F_MTU and let the guest fall back to the Ethernet default if querying failed
let mtu = match taps[0].mtu() {
Ok(m) => Some(m as u16),
Err(e) => {
warn!("Failed to query tap MTU; not advertising VIRTIO_NET_F_MTU: {e}");
None
}
};
let (avail_features, acked_features, config, queue_sizes, paused) = if let Some(state) =
state
@@ -495,9 +502,11 @@ impl Net {
true,
)
} else {
let mut avail_features = (1 << VIRTIO_NET_F_MTU)
| (1 << VIRTIO_RING_F_EVENT_IDX)
| (1 << VIRTIO_F_VERSION_1);
let mut avail_features = (1 << VIRTIO_RING_F_EVENT_IDX) | (1 << VIRTIO_F_VERSION_1);
if mtu.is_some() {
avail_features |= 1 << VIRTIO_NET_F_MTU;
}
if access_platform_enabled {
avail_features |= 1u64 << VIRTIO_F_ACCESS_PLATFORM;
@@ -528,20 +537,9 @@ impl Net {
let mut config = VirtioNetConfig::default();
if let Some(mac) = guest_mac {
build_net_config_space(
&mut config,
mac,
num_queues,
Some(mtu),
&mut avail_features,
);
build_net_config_space(&mut config, mac, num_queues, mtu, &mut avail_features);
} else {
build_net_config_space_with_mq(
&mut config,
num_queues,
Some(mtu),
&mut avail_features,
);
build_net_config_space_with_mq(&mut config, num_queues, mtu, &mut avail_features);
}
(