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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH net-next v4 1/2] l2tp: account for IP version in SKB headroom Date: Sun, 9 Aug 2026 00:43:12 +0200 Message-ID: <20260808224315.1092263-1-mail@david-bauer.net> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Bar: - X-Rspamd-Report: MID_CONTAINS_FROM(1) BAYES_HAM(-3) MIME_GOOD(-0.1) R_MISSING_CHARSET(0.5) X-Rspamd-Score: -1.6 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=david-bauer.net; s=uberspace; h=from:to:cc:subject:date; bh=7x84D7GlaxbGBWZuSkoAsGjqXhoQRBpfOh2j4lOnzio=; b=dedJAOQW7oiAjjy8NaUzNWtDeUeJ0/Xe63SliISvu65JGNt4RXAnQesvwyvtTNFAZKjcfr4eNd OFVeU+5r+1x8Nj5/N+nSEqAmz/LSqEa0vMuyS+C3UFXi83d3VXIMuZr4xEiqTHJiIjt7B0iZ6q7N MAET5jKaBegNC/KcV3bDO3fR9RDp5SDlKsR4AIrl4UevbGun0EAI2MYUiDl9XgbMk05XlwMb40S7 jn35VUnPFGMgtQ1xuEy27GSyooS7Vr5QWIgqQACtMgjd6A/BGp7saNc0hAoFFnXfgsXi8jNUa50a 12EMK4YRPK8/3ljTAw4Oy2aSnGusWEdq8kGHjpxqwmSoHRh8h2qccSmgBByTkc65y5sQxqm4XpEO LR2iQFWWj810AbWlLPEjZLdUaD/2PgWJYQ8aWNgAnnEra/9V7PUWuMFbj8Rtfb6suIKHTTwtCFal ymSSEAkcc9gtVj2EMkGzuaVecYzlvp65mzXyO1kZrDscJN9DvvixEfzxaV9hflVeKiML8a3ovXjM j0gqn6ntMjYJpHtX4I7tuuAn06PKDoqhbGoVcnuplLu2gT2cADy468xUFAcrDf/fasB2TY1Xuoug SuT113S9ToCCrSVGIxyq+7YgRU1dwvch1lQHHHQBgiPoRyy5rJdvJ3aItKLtANv3+c6J3v+rR0od 4= Account for the IP version of the tunnel when accounting skb headroom on xmit. This avoids having to potentially copy the skb a second time down the stack due to allocating not enough space for IPv6 headers in case the tunnel uses IPv6. Signed-off-by: David Bauer --- v4: - Update comment to reflect new code behavior net/l2tp/l2tp_core.c | 3 ++- net/l2tp/l2tp_core.h | 1 + net/l2tp/l2tp_eth.c | 14 ++++---------- 3 files changed, 7 insertions(+), 11 deletions(-) diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c index 4712cc41881a3..d6fa0f7436629 100644 --- a/net/l2tp/l2tp_core.c +++ b/net/l2tp/l2tp_core.c @@ -1237,7 +1237,7 @@ static int l2tp_xmit_core(struct l2tp_session *session, struct sk_buff *skb, uns * make room. Adjust truesize. */ uhlen = (tunnel->encap == L2TP_ENCAPTYPE_UDP) ? sizeof(*uh) : 0; - headroom = NET_SKB_PAD + sizeof(struct iphdr) + uhlen + session->hdr_len; + headroom = NET_SKB_PAD + tunnel->l3_overhead + uhlen + session->hdr_len; if (skb_cow_head(skb, headroom)) { kfree_skb(skb); return NET_XMIT_DROP; @@ -1688,6 +1688,7 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net, } sk->sk_allocation = GFP_ATOMIC; + tunnel->l3_overhead = kernel_sock_ip_overhead(sk); release_sock(sk); sock_hold(sk); diff --git a/net/l2tp/l2tp_core.h b/net/l2tp/l2tp_core.h index ffd8ced3a51ff..aab574376d95f 100644 --- a/net/l2tp/l2tp_core.h +++ b/net/l2tp/l2tp_core.h @@ -167,6 +167,7 @@ struct l2tp_tunnel { u32 tunnel_id; u32 peer_tunnel_id; int version; /* 2=>L2TPv2, 3=>L2TPv3 */ + int l3_overhead; /* IP header overhead */ char name[L2TP_TUNNEL_NAME_MAX]; /* for logging */ enum l2tp_encap_type encap; diff --git a/net/l2tp/l2tp_eth.c b/net/l2tp/l2tp_eth.c index a4956ef9574cc..66d1fc3ad409a 100644 --- a/net/l2tp/l2tp_eth.c +++ b/net/l2tp/l2tp_eth.c @@ -188,7 +188,6 @@ static void l2tp_eth_adjust_mtu(struct l2tp_tunnel *tunnel, struct net_device *dev) { unsigned int overhead = 0; - u32 l3_overhead = 0; u32 mtu; /* if the encap is UDP, account for UDP header size */ @@ -197,22 +196,17 @@ static void l2tp_eth_adjust_mtu(struct l2tp_tunnel *tunnel, dev->needed_headroom += sizeof(struct udphdr); } - lock_sock(tunnel->sock); - l3_overhead = kernel_sock_ip_overhead(tunnel->sock); - release_sock(tunnel->sock); - - if (l3_overhead == 0) { + if (tunnel->l3_overhead == 0) { /* L3 Overhead couldn't be identified, this could be - * because tunnel->sock was NULL or the socket's - * address family was not IPv4 or IPv6, - * dev mtu stays at 1500. + * because the socket's address family was not IPv4 + * or IPv6 - dev mtu stays at 1500. */ return; } /* Adjust MTU, factor overhead - underlay L3, overlay L2 hdr * UDP overhead, if any, was already factored in above. */ - overhead += session->hdr_len + ETH_HLEN + l3_overhead; + overhead += session->hdr_len + ETH_HLEN + tunnel->l3_overhead; mtu = l2tp_tunnel_dst_mtu(tunnel) - overhead; if (mtu < dev->min_mtu || mtu > dev->max_mtu) -- 2.53.0