On Fri, 2016-01-29 at 13:06 -0500, Sowmini Varadhan wrote:
> There is an unaligned access at __skb_flow_dissect when it calls
> ip6_flowlabel() with the call stack
>
> __skb_flow_dissect+0x2a8/0x87c
> eth_get_headlen+0x5c/0xaxa4
> ixgbe_clean_rx_irq+0x5cc/0xb20 [ixgbe]
> ixgbe_poll+0x5a4/0x760 [ixgbe]
> net_rx_action+0x13c/0x354
> :
>
> Essentially, ixgbe_pull_tail() is trying to figure out how much
> to pull, in order to have an aligned buffer:
>
> pull_len = eth_get_headlen(va, IXGBE_RX_HDR_SIZE);
>
> /* align pull length to size of long to optimize memcpy performance */
> skb_copy_to_linear_data(skb, va, ALIGN(pull_len, sizeof(long)));
>
> and seems like the unaligned access is unavoidable here (see comments
> in __skb_get_poff, for example).
>
> This (below) is what I came up with, to get rid of the unaligned access
> errors on sparc, Is there a better solution? (Not having access
> to struct ip6_hdr in this file made put_unaligned usage non-obvious)
>
>
> --- a/net/core/flow_dissector.c
> +++ b/net/core/flow_dissector.c
> @@ -102,6 +102,17 @@ __be32 __skb_flow_get_ports(const struct sk_buff *skb,
> int
> }
> EXPORT_SYMBOL(__skb_flow_get_ports);
>
> +static inline __be32 ip6_flowlabel_align(const u8 *hdr)
> +{
> + union {
> + __u8 w[4];
> + __u32 flow;
> + } ip6_flow;
> +
> + memcpy(ip6_flow.w, hdr, 4);
> + return (ip6_flow.flow & IPV6_FLOWLABEL_MASK);
> +}
> +
> /**
> * __skb_flow_dissect - extract the flow_keys struct and return it
> * @skb: sk_buff to extract the flow from, can be NULL if the rest are
> specifie
> @@ -230,7 +241,7 @@ ipv6:
> key_control->addr_type =
> FLOW_DISSECTOR_KEY_IPV6_ADDRS;
> }
>
> - flow_label = ip6_flowlabel(iph);
> + flow_label = ip6_flowlabel_align((const u8 *)iph);
> if (flow_label) {
> if (dissector_uses_key(flow_dissector,
>
> FLOW_DISSECTOR_KEY_FLOW_LABEL)) {
>
>
Why ipv6 stack itself does not trigger the issue ?
Maybe the driver itself does not properly align IP headers on sparc ?
Make sure NET_IP_ALIGN is 2 on your build.
Note that x86 does not care, but a driver should always align Ethernet
header to NET_IP_ALIGN, so that IP headers are aligned to 4 bytes
boundaries.
