Re: [PATCH net v2] net: stmmac: restore NET_IP_ALIGN in the RX DMA offset

[David Laight <david.laight.linux@gmail.com>]

On Mon, 24 Aug 2026 14:50:14 +0200 (CEST)
Pascal Kneuper <PKneuper@dspace.de> wrote:

> Since the RX path was converted to zero-copy, the page pool page is handed
> to the stack directly as the skb head, and the offset the DMA engine writes
> at is what determines the alignment of the packet headers.
> 
> Before the conversion the payload was copied into an skb obtained from
> napi_alloc_skb(), which reserves NET_SKB_PAD + NET_IP_ALIGN. The
> conversion moved the headroom into stmmac_rx_offset() but did not carry
> over NET_IP_ALIGN, so on architectures where NET_IP_ALIGN is 2 the IP
> header now lands misaligned:
> 
>   64 (NET_SKB_PAD) + 14 (ethernet) + 20 (IP) = 98
> 
> Same for the XDP branch:
> 
>   256 (XDP_PACKET_HEADROOM) + 14 (ethernet) + 20 (IP) = 290
> 
> On ARM32 this is fatal, because ldm and ldrd trap on unaligned addresses
> even when CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS is set.

I suspect an alternative is mark the structure(s) as __packed
when CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS is set.
The compiler will then only use 'normal' memory instructions which
won't fault.
OTOH aligning the buffers is likely to be better.

David

> 
> Any received echo request panics the machine, e.g:
> 
>   Unhandled fault: alignment exception (0x001) at 0x81873062
>   Internal error: : 1 [#1] SMP ARM
>   Hardware name: Altera SOCFPGA Arria10
>   PC is at icmp_echo+0x38/0xa8
>   LR is at icmp_rcv+0x22c/0x370
>   Call trace:
>    icmp_echo from icmp_rcv+0x22c/0x370
>    icmp_rcv from ip_protocol_deliver_rcu+0x2c/0x224
>    ip_protocol_deliver_rcu from ip_local_deliver+0xc8/0x1a0
>    ip_local_deliver from ip_sublist_rcv_finish+0x3c/0x50
>    ip_sublist_rcv_finish from ip_list_rcv_finish+0x110/0x118
>    ip_list_rcv_finish from ip_list_rcv+0xc8/0xdc
>    ip_list_rcv from __netif_receive_skb_list_core+0x170/0x1c0
>    ...
>    napi_complete_done from stmmac_napi_poll_rx+0xcb0/0x1030
>   Code: e24dd068 e59020a0 e28dc010 e0822001 (e8920003)
>   Kernel panic - not syncing: Fatal exception in interrupt
> 
> The faulting instruction is the ldm of *icmp_hdr(skb) in icmp_echo().
> 
> Fix by adding NET_IP_ALIGN back to the RX offset, which restores the
> alignment the stack used to get.
> 
> Note that commit a955318fe67e ("stmmac: align RX buffers") made a similar
> change in 2021 and was reverted by commit 12d125b4574b ("stmmac: Revert
> "stmmac: align RX buffers"") because it caused packet corruption. That
> patch raised the offset from 0 without adjusting the buffer size
> accounting, so the DMA engine could arguably write past the end of the RX
> buffers, though this was never root caused.
> Commit df542f669307 ("net: stmmac: Switch to zero-copy in non-XDP RX
> path") since derives the page pool allocation from stmmac_rx_offset(), so
> the extra bytes are accounted for.
> 
> Fixes: df542f669307 ("net: stmmac: Switch to zero-copy in non-XDP RX path")
> Cc: Daniel Baldin <DBaldin@dspace.de>
> Assisted-by: GitHub-Copilot-CLI:claude-opus-5
> Signed-off-by: Pascal Kneuper <PKneuper@dspace.de>
> ---
> v2:
>  - also add NET_IP_ALIGN to the XDP branch, reproduced the same panic
>    with an XDP_PASS program attached (Jakub Kicinski)
>  - retitle accordingly, v1 was non-XDP only
> 
>  drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> index a71f0df263785..4d4b155d0d931 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> @@ -1527,9 +1527,9 @@ static void stmmac_display_rings(struct stmmac_priv *priv,
>  static unsigned int stmmac_rx_offset(struct stmmac_priv *priv)
>  {
>  	if (stmmac_xdp_is_enabled(priv))
> -		return XDP_PACKET_HEADROOM;
> +		return XDP_PACKET_HEADROOM + NET_IP_ALIGN;
>  
> -	return NET_SKB_PAD;
> +	return NET_SKB_PAD + NET_IP_ALIGN;
>  }
>  
>  static int stmmac_set_bfsize(int mtu)