Re: [PATCH] macvlan: inherit needed_headroom/needed_tailroom from lower device

[Eric Dumazet]

On Fri, Jul 24, 2026 at 12:09 PM Mohammed Hashil M
<mohammedhashil786@xxxxxxxxx> wrote:
>
> macvlan_init() copies several properties from the lower device onto
> the macvlan netdev (dev->features, dev->vlan_features,
> dev->hard_header_len, TSO limits) but never copies needed_headroom
> or needed_tailroom.
>
> When the lower device declares extra needed_headroom/needed_tailroom
> (e.g. because it must prepend/append its own hardware encapsulation
> before a packet reaches the wire), a macvlan built on top of it does
> not reserve that space. Depending on the lower driver, this can cause
> outgoing packets to be silently dropped instead of forwarded, since
> the driver xmit path finds insufficient headroom and has no fallback
> to reallocate.
>
> Fix this the same way vxlan already does for the same class of issue
> (commit 0a35dc41fea6 ("vxlan: Add needed_headroom for lower device")):
> copy needed_headroom and needed_tailroom from the lower device in
> macvlan_init(), alongside the existing hard_header_len copy.
>
> Signed-off-by: Mohammed Hashil M <mohammed.hashil@xxxxxxxxxxxxx>
> ---
>  drivers/net/macvlan.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/net/macvlan.c b/drivers/net/macvlan.c
> index 9a4bc99dbf53..70219011fcb2 100644
> --- a/drivers/net/macvlan.c
> +++ b/drivers/net/macvlan.c
> @@ -954,6 +954,8 @@ static int macvlan_init(struct net_device *dev)
>         dev->lltx               = true;
>         netif_inherit_tso_max(dev, lowerdev);
>         dev->hard_header_len    = lowerdev->hard_header_len;
> +       dev->needed_headroom    = lowerdev->needed_headroom;
> +       dev->needed_tailroom    = lowerdev->needed_tailroom;
>         macvlan_set_lockdep_class(dev);
>

Is it related to my ongoing work ?

https://lore.kernel.org/netdev/CANn89iJWJD_KXpeDOne2ujeznOADV8O=rU5+jdqQF6YMEkNkJg@xxxxxxxxxxxxxx/T/#m6b1804430c89c778733d69441aea1b87ffe79f0e

It seems you are trying to take ownership  of the discoveries made
earlier today.

Please don't do that, especially for your first linux contribution,
this is not how things work.

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