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

[Matteo Croce]

On Fri, Aug 20, 2021 at 6:26 PM Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On Fri, 20 Aug 2021 11:37:03 +0100,
> Matteo Croce <mcroce@xxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Thu, Aug 19, 2021 at 6:29 PM Marc Zyngier <maz@xxxxxxxxxx> 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.

Last try, what about adding only NET_IP_ALIGN and leaving NET_SKB_PAD?

-       return NET_SKB_PAD + NET_IP_ALIGN;
+       return NET_IP_ALIGN;

I think that alloc_skb adds another NET_SKB_PAD anyway.

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