[PATCH net-next] net/packet: Improve the comment about LL header visibility criteria

[PATCH net-next] net/packet: Improve the comment about LL header visibility criteria

[Xie He]

The "dev_has_header" function, recently added in
commit d549699048b4 ("net/packet: fix packet receive on L3 devices
without visible hard header"),
is more accurate as criteria for determining whether a device exposes
the LL header to upper layers, because in addition to dev->header_ops,
it also checks for dev->header_ops->create.

When transmitting an skb on a device, dev_hard_header can be called to
generate an LL header. dev_hard_header will only generate a header if
dev->header_ops->create is present.

Signed-off-by: Xie He <xie.he.0141@gmail.com>
---
 net/packet/af_packet.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 6bbc7a448593..e24b2841c643 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -132,17 +132,17 @@ Resume
     because it is invisible to us.
 
 
 On transmit:
 ------------
 
-dev->header_ops != NULL
+dev_has_header(dev) == true
    mac_header -> ll header
    data       -> ll header
 
-dev->header_ops == NULL (ll header is invisible to us)
+dev_has_header(dev) == false (ll header is invisible to us)
    mac_header -> data
    data       -> data
 
    We should set network_header on output to the correct position,
    packet classifier depends on it.
  */
-- 
2.27.0

Re: [PATCH net-next] net/packet: Improve the comment about LL header visibility criteria

[Willem de Bruijn]

On Fri, Feb 5, 2021 at 5:42 PM Xie He <xie.he.0141@gmail.com> wrote:
>
> The "dev_has_header" function, recently added in
> commit d549699048b4 ("net/packet: fix packet receive on L3 devices
> without visible hard header"),
> is more accurate as criteria for determining whether a device exposes
> the LL header to upper layers, because in addition to dev->header_ops,
> it also checks for dev->header_ops->create.
>
> When transmitting an skb on a device, dev_hard_header can be called to
> generate an LL header. dev_hard_header will only generate a header if
> dev->header_ops->create is present.
>
> Signed-off-by: Xie He <xie.he.0141@gmail.com>

Acked-by: Willem de Bruijn <willemb@google.com>

Indeed, existence of dev->header_ops->create is the deciding factor. Thanks Xie.

Re: [PATCH net-next] net/packet: Improve the comment about LL header visibility criteria

[Eyal Birger]

Hi,

On Sat, Feb 6, 2021 at 4:52 AM Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>
> On Fri, Feb 5, 2021 at 5:42 PM Xie He <xie.he.0141@gmail.com> wrote:
> >
> > The "dev_has_header" function, recently added in
> > commit d549699048b4 ("net/packet: fix packet receive on L3 devices
> > without visible hard header"),
> > is more accurate as criteria for determining whether a device exposes
> > the LL header to upper layers, because in addition to dev->header_ops,
> > it also checks for dev->header_ops->create.
> >
> > When transmitting an skb on a device, dev_hard_header can be called to
> > generate an LL header. dev_hard_header will only generate a header if
> > dev->header_ops->create is present.
> >
> > Signed-off-by: Xie He <xie.he.0141@gmail.com>
>
> Acked-by: Willem de Bruijn <willemb@google.com>
>
> Indeed, existence of dev->header_ops->create is the deciding factor. Thanks Xie.

As such, may I suggest making this explicit by making
dev_hard_header() use dev_has_header()?

Eyal.

Re: [PATCH net-next] net/packet: Improve the comment about LL header visibility criteria

[Xie He]

On Sat, Feb 6, 2021 at 7:43 AM Eyal Birger <eyal.birger@gmail.com> wrote:
>
> As such, may I suggest making this explicit by making
> dev_hard_header() use dev_has_header()?

That is a good idea. We may submit another patch for that.

Re: [PATCH net-next] net/packet: Improve the comment about LL header visibility criteria

[Jakub Kicinski]

On Fri, 5 Feb 2021 21:51:36 -0500 Willem de Bruijn wrote:
> On Fri, Feb 5, 2021 at 5:42 PM Xie He <xie.he.0141@gmail.com> wrote:
> >
> > The "dev_has_header" function, recently added in
> > commit d549699048b4 ("net/packet: fix packet receive on L3 devices
> > without visible hard header"),
> > is more accurate as criteria for determining whether a device exposes
> > the LL header to upper layers, because in addition to dev->header_ops,
> > it also checks for dev->header_ops->create.
> >
> > When transmitting an skb on a device, dev_hard_header can be called to
> > generate an LL header. dev_hard_header will only generate a header if
> > dev->header_ops->create is present.
> >
> > Signed-off-by: Xie He <xie.he.0141@gmail.com>  
> 
> Acked-by: Willem de Bruijn <willemb@google.com>
> 
> Indeed, existence of dev->header_ops->create is the deciding factor. Thanks Xie.

Applied, thanks!