netdev - Re: [PATCH v3 net-next] net: drop_monitor: support drop reason

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Date:   Thu, 27 Jan 2022 18:02:56 +0200
From:   Ido Schimmel <idosch@...sch.org>
To:     David Ahern <dsahern@...il.com>
Cc:     menglong8.dong@...il.com, kuba@...nel.org, nhorman@...driver.com,
        davem@...emloft.net, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, dsahern@...nel.org,
        rostedt@...dmis.org, Menglong Dong <imagedong@...cent.com>
Subject: Re: [PATCH v3 net-next] net: drop_monitor: support drop reason

On Thu, Jan 27, 2022 at 08:53:04AM -0700, David Ahern wrote:
> On 1/26/22 8:33 PM, menglong8.dong@...il.com wrote:
> > From: Menglong Dong <imagedong@...cent.com>
> > 
> > In the commit c504e5c2f964 ("net: skb: introduce kfree_skb_reason()")
> > drop reason is introduced to the tracepoint of kfree_skb. Therefore,
> > drop_monitor is able to report the drop reason to users by netlink.
> > 
> > For now, the number of drop reason is passed to users ( seems it's
> > a little troublesome to pass the drop reason as string ). Therefore,
> > users can do some customized description of the reason.
> > 
> > Signed-off-by: Menglong Dong <imagedong@...cent.com>
> > ---
> > v3:
> > - referring to cb->reason and cb->pc directly in
> >   net_dm_packet_report_fill()
> > 
> > v2:
> > - get a pointer to struct net_dm_skb_cb instead of local var for
> >   each field
> > ---
> >  include/uapi/linux/net_dropmon.h |  1 +
> >  net/core/drop_monitor.c          | 16 ++++++++++++----
> >  2 files changed, 13 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/uapi/linux/net_dropmon.h b/include/uapi/linux/net_dropmon.h
> > index 66048cc5d7b3..b2815166dbc2 100644
> > --- a/include/uapi/linux/net_dropmon.h
> > +++ b/include/uapi/linux/net_dropmon.h
> > @@ -93,6 +93,7 @@ enum net_dm_attr {
> >  	NET_DM_ATTR_SW_DROPS,			/* flag */
> >  	NET_DM_ATTR_HW_DROPS,			/* flag */
> >  	NET_DM_ATTR_FLOW_ACTION_COOKIE,		/* binary */
> > +	NET_DM_ATTR_REASON,			/* u32 */
> >  
> 
> For userspace to properly convert reason from id to string, enum
> skb_drop_reason needs to be moved from skbuff.h to a uapi file.
> include/uapi/linux/net_dropmon.h seems like the best candidate to me.
> Maybe others have a better idea.

I think the best option would be to convert it to a string in the kernel
(or report both). Then you don't need to update user space tools such as
the Wireshark dissector [1] and DropWatch every time a new reason is
added.

[1] https://www.wireshark.org/docs/dfref/n/net_dm.html