[PATCH net v5] net: stmmac: Prevent NULL deref when RX memory exhausted

[Sam Edwards]

On Wed, Apr 15, 2026 at 9:28 AM Russell King (Oracle)
<linux at armlinux.org.uk> wrote:
>
> On Wed, Apr 15, 2026 at 01:56:32PM +0100, Russell King (Oracle) wrote:
> > Locally, while debugging my issues, I used this to prevent cur_rx
> > catching up with dirty_rx:
> >
> >                 status = stmmac_rx_status(priv, &priv->xstats, p);
> >                 /* check if managed by the DMA otherwise go ahead */
> >                 if (unlikely(status & dma_own))
> >                         break;
> >
> >                 next_entry = STMMAC_NEXT_ENTRY(rx_q->cur_rx,
> >                                                priv->dma_conf.dma_rx_size);
> >                 if (unlikely(next_entry == rx_q->dirty_rx))
> >                         break;
> >
> >                 rx_q->cur_rx = next_entry;
> >
> > If we care about the cost of reloading rx_q->dirty_rx on every
> > iteration, then I'd suggest that the cost we already incur reading and
> > writing rx_q->cur_rx is something that should be addressed, and
> > eliminating that would counter the cost of reading rx_q->dirty_rx. I
> > suspect, however, that the cost is minimal, as cur_tx and dirty_rx are
> > likely in the same cache line.

No, no, I like your approach better. :) It also removes the need for
the `limit` clamp at the top of the function, so later code can assume
limit==budget.

> > It looks like any fix to stmmac_rx() will also need a corresponding
> > fix for stmmac_rx_zc().

I agree that stmmac_rx_zc() is likely also broken (in a similar way,
but not similar enough to permit a "corresponding" fix), but I don't
agree that there's a dependency relationship here. This patch is
addressing #221010, which affects the generic/non-ZC codepath; I'm
afraid the ZC codepath warrants its own investigation.

> I have some further information, but a new curveball has just been
> chucked... and I've no idea what this will mean at this stage. Just
> take it that I won't be responding for a while.

I think I follow your meaning. Good luck getting it straightened out!