From 2ea42936416b29cca42b2c7bc7d3d3768f77c199 Mon Sep 17 00:00:00 2001 From: Pascal Kneuper Date: Mon, 24 Aug 2026 14:50:14 +0200 Subject: [PATCH] net: stmmac: restore NET_IP_ALIGN in the RX DMA offset [ Upstream commit 23680bf5f8c69c923546b84a8e6c401bef8b88fe ] 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. 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 Signed-off-by: Pascal Kneuper Link: https://patch.msgid.link/20260824125014.47862-1-PKneuper@dspace.de Signed-off-by: Jakub Kicinski Signed-off-by: Sasha Levin --- 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 f6cf44d7bb642..a3eeb68c8d3f4 100644 --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c @@ -1326,9 +1326,9 @@ 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, int bufsize) -- 2.53.0