Subject: Re: [PATCH net-next] stmmac: align RX buffers

From: Marc Zyngier <[email-masked]>
Date: Fri, 20 Aug 2021 17:26:21 +0100

On Fri, 20 Aug 2021 11:37:03 +0100,
Matteo Croce <[email-masked]> wrote:
> 
> On Thu, Aug 19, 2021 at 6:29 PM Marc Zyngier <[email-masked]> wrote:

[...]

> > diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac.h b/drivers/net/ethernet/stmicro/stmmac/stmmac.h
> > index fcdb1d20389b..244aa6579ef4 100644
> > --- a/drivers/net/ethernet/stmicro/stmmac/stmmac.h
> > +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac.h
> > @@ -341,7 +341,7 @@ static inline unsigned int stmmac_rx_offset(struct stmmac_priv *priv)
> >         if (stmmac_xdp_is_enabled(priv))
> >                 return XDP_PACKET_HEADROOM + NET_IP_ALIGN;
> >
> > -       return NET_SKB_PAD + NET_IP_ALIGN;
> > +       return 8 + NET_IP_ALIGN;
> >  }
> >
> >  void stmmac_disable_rx_queue(struct stmmac_priv *priv, u32 queue);
> >
> > I don't see the system corrupting packets anymore. Is that exactly
> > what you had in mind? This really seems to point to a basic buffer
> > overflow.

[...]

> Sorry, I meant something like:
> 
> -       return NET_SKB_PAD + NET_IP_ALIGN;
> +       return 8;
> 
> I had some hardware which DMA fails if the receive buffer was not word
> aligned, but this seems not the case, as 8 + NET_IP_ALIGN = 10, and
> it's not aligned too.

No error in that case either, as expected. Given that NET_SKB_PAD is
likely to expand to 64, it is likely a DMA buffer overflow which
probably only triggers for large-ish packets.

Now, we're almost at -rc7, and we don't have a solution in sight.

Can we please revert this until we have an understanding of what is
happening? I'll hopefully have more cycles to work on the issue once
5.14 is out, and hopefully the maintainers of this driver can chime in
(they have been pretty quiet so far).

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.