Re: [PATCH net v2] net: Reset MAC header for direct packet transmission

On Mon, Mar 29, 2021 at 12:30 PM Kurt Kanzenbach [off-list ref] wrote:

On Mon Mar 29 2021, Eric Dumazet wrote:

quoted

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 ?

It can be reworked, no problem. I just thought it might be better to add
it to the generic code just in case there are more drivers suffering
from the issue.

Note that I have a similar issue pending in ipvlan.

Still, I think I prefer the non easy way to not add more stuff in fast path.

quoted

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..cda495cb1471e23e6666c1f2e9d27a01694f997f

100644

--- a/net/hsr/hsr_forward.c
+++ b/net/hsr/hsr_forward.c

@@ -555,11 +555,7 @@ void hsr_forward_skb(struct sk_buff *skb, struct

hsr_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);

hsr_forward_skb() has four call sites. Three of them make sure that the
header is properly set. The Tx path does not. So, maybe something like
below?

Yes, this should be fine.