From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 BADF24FDE56; Tue, 13 Oct 2026 12:11:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791893463; cv=none; b=kVshj3ChE3dJ9hR7ICjTtBF0d2hufk4qCmySoOXa32D36S4TGIY6Cw4NNINTjSTzv33euZdM6nmkwi8Haye2ySu3aLJruacjkIgvmELgiriA3BIbo2BPV257lAFBYST32nk9tbnX+SstPeLOXgTcPm3dEF5okG6VJ6/jnQXj5gI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791893463; c=relaxed/simple; bh=gp9YKR5Ffank944Qbl/YQ1f3bO8lrZdVlbqyKGACJRw=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=Vai3LP1cdEqawsRTCyxslJNfpY966pUd181UYENcZXydafOxakOmWMXbkJTLweL7+doCetaF8N5wDrNMdLI2F9wB2WRhZPKRXwYrg7U+/ldPKdMusVTJ6SJYxQVzLxWjpu1FLwzz7bef4WETV4wePQhVh0JqZtSBmCBNFak8pwI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=D3pSxhYC; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="D3pSxhYC" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0FD141F000FF; Tue, 13 Oct 2026 12:11:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1791893462; bh=FOl4Rce9aa2KEBvehoiLW7hbIIJXjMfmVszbEwYifok=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=D3pSxhYCsVtjZMSagR43SfQY4fY4S/3sqLIr4q9nopBryLMyCl5HpAUmrjQqv16O2 QzPOAgASoltNPlTnCE+UVHnSL7Zcuq8EKUB9UlYl3StSuSrYGxile5YS7O7w4nR5rU O67IEbJmkko7Cv/F/wE9MLWp8kz6xj3b9oDEMnvw= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Stephen Hemminger , Jamal Hadi Salim , Paolo Abeni , Sasha Levin Subject: [PATCH 6.18 233/621] net: Introduce skb tc depth field to track packet loops Date: Tue, 13 Oct 2026 13:33:48 +0200 Message-ID: <20261013112926.389101041@linuxfoundation.org> X-Mailer: git-send-email 2.56.0 In-Reply-To: <20261013112920.855194808@linuxfoundation.org> References: <20261013112920.855194808@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Jamal Hadi Salim [ Upstream commit 98b34f3e8c3492cfc89ff943c9d92b4d52863d1d ] Add a 2-bit per-skb tc depth field to track packet loops across the stack. The previous per-CPU loop counters like MIRRED_NEST_LIMIT assume a single call stack and lose state in two cases: 1) When a packet is queued and reprocessed later (e.g., egress->ingress via backlog), the per-cpu state is gone by the time it is dequeued. 2) With XPS/RPS a packet may arrive on one CPU and be processed on another. A per-skb field solves both by travelling with the packet itself. The field fits in existing padding, using 2 bits that were previously a hole: pahole before(-) and after (+) diff looks like: __u8 slow_gro:1; /* 132: 3 1 */ __u8 csum_not_inet:1; /* 132: 4 1 */ __u8 unreadable:1; /* 132: 5 1 */ + __u8 tc_depth:2; /* 132: 6 1 */ - /* XXX 2 bits hole, try to pack */ /* XXX 1 byte hole, try to pack */ __u16 tc_index; /* 134 2 */ There used to be a ttl field which was removed as part of tc_verd in commit aec745e2c520 ("net-tc: remove unused tc_verd fields"). It was already unused by that time, due to remove earlier in commit c19ae86a510c ("tc: remove unused redirect ttl"). The first user of this field is netem, which increments tc_depth on duplicated packets before re-enqueueing them at the root qdisc. On re-entry, netem skips duplication for any skb with tc_depth already set, bounding recursion to a single level regardless of tree topology. The other user is mirred which increments it on each pass and limits to depth to MIRRED_DEFER_LIMIT (3). The new field was called ttl in earlier versions of this patch but renamed to tc_depth to avoid confusion with IP ttl. Note (looking at you Sashiko! Dont ignore me and continue bringing this up): 1. Since both mirred and netem utilize the same 2-bit tc_depth field it is possible when netem and mirred are used together that netem qdisc to skip the duplication step. This is a known trade-off, as a 2-bit field cannot independently track both features' recursion depths and it is not considered sane to have a setup that addresses both features on at the same time. 2. skb_scrub_packet does not clear tc_depth. This means a packet's loop history is preserved even across namespaces. While this might be restrictive for some topologies, it is also design intent to provide robustness against loops across namespaces. Reviewed-by: Stephen Hemminger Signed-off-by: Jamal Hadi Salim Link: https://patch.msgid.link/20260525122556.973584-2-jhs@mojatatu.com Signed-off-by: Paolo Abeni Stable-dep-of: 9552b11e3eda ("net/sched: fix packet loop on netem when duplicate is on") Signed-off-by: Sasha Levin --- include/linux/skbuff.h | 2 ++ 1 file changed, 2 insertions(+) diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h index 237e8b0cb08ab..5af1127dd870c 100644 --- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h @@ -821,6 +821,7 @@ enum skb_tstamp_type { * @_sk_redir: socket redirection information for skmsg * @_nfct: Associated connection, if any (with nfctinfo bits) * @skb_iif: ifindex of device we arrived on + * @tc_depth: counter for packet duplication * @tc_index: Traffic control index * @hash: the packet hash * @queue_mapping: Queue mapping for multiqueue devices @@ -1030,6 +1031,7 @@ struct sk_buff { __u8 csum_not_inet:1; #endif __u8 unreadable:1; + __u8 tc_depth:2; #if defined(CONFIG_NET_SCHED) || defined(CONFIG_NET_XGRESS) __u16 tc_index; /* traffic control index */ #endif -- 2.53.0