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

[JR Lanteigne <root@dnim.dev>]

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.