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

[Miroslav Lichvar <mlichvar@redhat.com>]

On Sun, Aug 23, 2026 at 02:21:00AM -0300, JR Lanteigne wrote:
> put_ts_pktinfo() resolves the interface index of a received packet
> from its napi id. The lookup fails on drivers whose napi instances
> are not attached to the delivering net_device (e.g. ti cpsw, which
> keeps them on an internal dummy device) and on kernels built without
> CONFIG_NET_RX_BUSY_POLL, where skb_napi_id() is always 0. In those
> cases SCM_TIMESTAMPING_PKTINFO carries if_index 0 and applications
> cannot tell which interface produced the hardware timestamp. chronyd,
> for example, then ignores valid hardware timestamps.

That's the intended behavior, from timestamping.rst:
  A valid (non-zero) interface index will be returned only if
  CONFIG_NET_RX_BUSY_POLL is enabled and the driver is using NAPI. 
 
> Fall back to skb->skb_iif, which is set for every received packet.
> On aggregated interfaces this reports the aggregating device instead
> of the physical one, but only in cases where the napi lookup already
> failed and nothing was reported at all.

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.

> A userspace fallback for existing kernels was proposed to chrony
> separately.

That makes sense to me. It's a bug that it doesn't check for
zero value.

-- 
Miroslav Lichvar