Re: [PATCH net] net: fall back to skb_iif for the timestamping pktinfo if_index

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

JR Lanteigne wrote:
> Miroslav Lichvar wrote:
> > That would cause applications to receive a wrong index in some cases,
> > right? If it's possible, it should report the index of the physical
> > interface, but it should not be guessing.
> 
> Right: with the fallback a nonzero if_index is no longer guaranteed
> to be the physical timestamping device. An application that maps the
> index to a PHC without validating it could see the aggregating
> device where it previously saw 0.
> 
> Reporting the physical interface does not look implementable in the
> two failing cases. Without CONFIG_NET_RX_BUSY_POLL there is no napi
> id recorded at all. For cpsw the napi belongs to a DMA channel
> shared by both ports, so there is no per-napi net_device to attach;
> the delivering port is only known per packet, and there skb->dev is
> set correctly, so skb_iif is exactly the physical interface. The
> index only becomes an aggregate with bonding or bridging on top,
> where the napi lookup would have failed anyway.
> 
> So the choice for these packets is between 0 and the skb_iif value,
> which is what IP_PKTINFO already reports. No objection from me if
> this is dropped; I can respin if there is a form of the fallback
> worth keeping.

It may be possible to detect whether an skb_iif points to a physical
or aggregate device (e.g., through lower_dev)? If so, then the value
can still be used in most cases.