Re: [PATCH net v2] net: Reset MAC header for direct packet transmission
From: Eric Dumazet <edumazet@google.com>
Date: 2021-03-29 08:54:40
Subsystem:
hsr network protocol, networking [general], the rest · Maintainers:
"David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds
On Mon, Mar 29, 2021 at 9:17 AM Kurt Kanzenbach [off-list ref] wrote:
quoted hunk ↗ jump to hunk
Reset MAC header in case of using dev_direct_xmit(), e.g. by specifying PACKET_QDISC_BYPASS. This is needed, because other code such as the HSR layer expects the MAC header to be correctly set. This has been observed using the following setup: |$ ip link add name hsr0 type hsr slave1 lan0 slave2 lan1 supervision 45 version 1 |$ ifconfig hsr0 up |$ ./test hsr0 The test binary is using mmap'ed sockets and is specifying the PACKET_QDISC_BYPASS socket option. This patch resolves the following warning on a non-patched kernel: |[ 112.725394] ------------[ cut here ]------------ |[ 112.731418] WARNING: CPU: 1 PID: 257 at net/hsr/hsr_forward.c:560 hsr_forward_skb+0x484/0x568 |[ 112.739962] net/hsr/hsr_forward.c:560: Malformed frame (port_src hsr0) The MAC header is also reset unconditionally in case of PACKET_QDISC_BYPASS is not used at the top of __dev_queue_xmit(). Fixes: d346a3fae3ff ("packet: introduce PACKET_QDISC_BYPASS socket option") Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de> --- Changes since v1: * Move skb_reset_mac_header() to __dev_direct_xmit() * Add Fixes tag * Target net tree Previous versions: * https://lkml.kernel.org/netdev/20210326154835.21296-1-kurt@linutronix.de/ net/core/dev.c | 2 ++ 1 file changed, 2 insertions(+)diff --git a/net/core/dev.c b/net/core/dev.c index b4c67a5be606..b5088223dc57 100644 --- a/net/core/dev.c +++ b/net/core/dev.c@@ -4297,6 +4297,8 @@ int __dev_direct_xmit(struct sk_buff *skb, u16 queue_id) !netif_carrier_ok(dev))) goto drop; + skb_reset_mac_header(skb); + skb = validate_xmit_skb_list(skb, dev, &again); if (skb != orig_skb) goto drop; --2.20.1
Note that last year, I addressed the issue differently in commit
96cc4b69581db68efc9749ef32e9cf8e0160c509
("macvlan: do not assume mac_header is set in macvlan_broadcast()")
(amended with commit 1712b2fff8c682d145c7889d2290696647d82dab
"macvlan: use skb_reset_mac_header() in macvlan_queue_xmit()")
My reasoning was that in TX path, when ndo_start_xmit() is called, MAC
header is essentially skb->data,
so I was hoping to _remove_ skb_reset_mac_header(skb) eventually from
the fast path (aka __dev_queue_xmit),
because most drivers do not care about MAC header, they just use skb->data.
I understand it is more difficult to review drivers instead of just
adding more code in __dev_direct_xmit()
In hsr case, I do not really see why the existing check can not be
simply reworked ?
mac_header really makes sense in input path, when some layer wants to
get it after it has been pulled.
diff --git a/net/hsr/hsr_forward.c b/net/hsr/hsr_forward.c
index ed82a470b6e154be28d7e53be57019bccd4a964d..cda495cb1471e23e6666c1f2e9d27a01694f997f100644
--- a/net/hsr/hsr_forward.c
+++ b/net/hsr/hsr_forward.c@@ -555,11 +555,7 @@ void hsr_forward_skb(struct sk_buff *skb, structhsr_port *port)
{
struct hsr_frame_info frame;
- if (skb_mac_header(skb) != skb->data) {
- WARN_ONCE(1, "%s:%d: Malformed frame (port_src %s)\n",
- __FILE__, __LINE__, port->dev->name);
- goto out_drop;
- }
+ skb_reset_mac_header(skb);
if (fill_frame_info(&frame, skb, port) < 0)
goto out_drop;