From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.tipi-net.de (mail.tipi-net.de [194.13.80.246]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id BD95D4E0B91; Mon, 19 Oct 2026 11:08:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=194.13.80.246 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792408109; cv=none; b=DHxbOt3F+pNvXnfXnwVpkaUZ/yBzCfT1gha8AL2lnOtnQMEO26BxlsFSyiZynjUldFyNH3ZRnA32806V6p88aetk+9+gz0cfnChYYUZAutOeJ68EMEUMaRmn2adB0sQxLOU/50FAwskOPQkG0WVznGSwZZYSzBKk1u/6d4Maw/M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792408109; c=relaxed/simple; bh=gIMM3VGfooYX/bh/LO8HXS+L2AyHpHz2uFiMjt1vRJI=; h=MIME-Version:Date:From:To:Cc:Subject:In-Reply-To:References: Message-ID:Content-Type; b=gO16xWMojNw3ILfb3/r/oWF27FLNr6rjP/Y7KLpA8NvNgMNInfw81lC8TsddS7G8oipRm7CGHlNn7xmiWdY3ueOLonmJWUOYcwi5nVVy3QKC8U7yLDGX5rKbIXgWkoUMMCxt615F5rM6cLD0fvkpNuuksXNkcZlYQ/YFPR4xk4E= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=tipi-net.de; spf=pass smtp.mailfrom=tipi-net.de; dkim=pass (2048-bit key) header.d=tipi-net.de header.i=@tipi-net.de header.b=xBjblPJE; arc=none smtp.client-ip=194.13.80.246 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=tipi-net.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=tipi-net.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=tipi-net.de header.i=@tipi-net.de header.b="xBjblPJE" Received: from [127.0.0.1] (localhost [127.0.0.1]) by localhost (Mailerdaemon) with ESMTPSA id 7DBADA012B; Mon, 19 Oct 2026 13:08:15 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=tipi-net.de; s=dkim; t=1792408102; h=from:subject:date:message-id:to:cc:mime-version:content-type: content-transfer-encoding:in-reply-to:references; bh=5bO4U+trkWr254I1Yj+4SsCvrERs9vmKIAeF7UeSbL8=; b=xBjblPJEXvZWghFd34CrF/W7QjSUcxZ4nOjuxKyN907/1xutdrSPOeIlyX9AA+6Cqfs5DJ vs/yOGugxIpjsQdEpotJjp8ACT2OqiyrVYNz20pzhl0e22V13ivz+s2s6if1BYg3rAg1B5 8uhSYAIYgvLN63wa3DnkCDgyOWrK4aFr5Y1xJ4ovzDa51ruKv2rfFTzRAZfIhK3Sv38E7z oIyQGhN1FWCN9d1FQb8gw9SSheXEHVKWBA8+zdtmaidoyHOwRa2jCP25a/+ElVLYUixkpq j56hWYodLEySRQlDZzKY7wImTfpfCxi2X0avE1dA9S9jaPAr8RJ2qUAbt7jgdg== Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Date: Mon, 19 Oct 2026 13:08:15 +0200 From: Nicolai Buchwitz To: Lorenzo Bianconi Cc: Maxime Chevallier , Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Maxime Coquelin , Alexandre Torgue , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , Stanislav Fomichev , Jose Abreu , Ong Boon Leong , netdev@vger.kernel.org, linux-arm-kernel@lists.infradead.org, bpf@vger.kernel.org Subject: Re: [PATCH net-next v8 3/5] net: stmmac: drop multi-buffer frames in XDP mode In-Reply-To: <20261018-stmmac-rx-mb-v8-3-5f58c0b25849@oss.qualcomm.com> References: <20261018-stmmac-rx-mb-v8-0-5f58c0b25849@oss.qualcomm.com> <20261018-stmmac-rx-mb-v8-3-5f58c0b25849@oss.qualcomm.com> Message-ID: <8af4012ae73827a721533e2734b80ea9@tipi-net.de> X-Sender: nb@tipi-net.de Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit X-Last-TLS-Session-Version: TLSv1.3 On 18.10.2026 19:07, Lorenzo Bianconi wrote: > stmmac_rx() runs the XDP program once per descriptor buffer and uses > skb == NULL as the only marker for the start of a frame. A frame larger > than one buffer is split by the hardware across several descriptors; > GMAC4 enables jumbo frames unconditionally (GMAC_CONFIG_JE in > GMAC_CORE_INIT), so an on-link peer can deliver such a frame even when > the interface MTU is left at 1500. > > When an XDP verdict ends a non-last descriptor, skb is cleared and the > next descriptor of the same hardware frame is parsed as a new frame. > For XDP_CONSUMED this happens via read_again with len preserved, so > stmmac_rx_buf1_len() then returns min(dma_buf_sz, plen - len) for the > last descriptor, which can be as small as 0-3 bytes. buf1_len -= > ETH_FCS_LEN underflows and xdp_prepare_buff() puts data_end far past > data; on XDP_PASS this produces an skb whose length is close to 4G that > reaches the network stack (skb->tail and skb->len are 32-bit and wrap > below skb->end, so skb_over_panic() does not fire). A 4+ byte tail is > instead offered to the XDP program and delivered as a standalone > mid-payload Ethernet frame on XDP_PASS. XDP_TX/XDP_REDIRECT instead > falls through to the next iteration, resetting the running length, so > the tail is re-parsed as a full-size new frame. > > The same fault exists when a single descriptor uses its second buffer: > GMAC4/XGMAC always provide a second data buffer per descriptor, so a > frame up to 2 * dma_buf_sz can land on a single descriptor while the > XDP > program still only sees the first buffer. XDP_TX then transmits a > truncated frame, and XDP_PASS appends the second buffer only after the > program has already run over the first one. > > XDP does not support scatter-gather frames until the multi-buffer > rework, so drop the whole frame instead: mark it as errored as soon as > a non-last descriptor, or a descriptor using its second buffer, is seen > while an XDP program is attached, drain and release every descriptor - > including the last. Removing the re-parse lets both XDP verdict > branches > simply complete the packet via next. > > Note: the subsequent "net: stmmac: rework stmmac_rx to support XDP rx > multi-buff" accumulates the whole frame into a single xdp_buff and > supersedes this patch; the fix is kept so the net portion of the series > stands on its own. > > Fixes: 5fabb01207a2 ("net: stmmac: Add initial XDP support") > Signed-off-by: Lorenzo Bianconi > --- > [...] Reviewed-by: Nicolai Buchwitz Thanks, Nicolai