linux-kernel - Re: [PATCH v3 net-next] net: drop

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Date:   Fri, 28 Jan 2022 10:56:38 +0800
From:   Menglong Dong <menglong8.dong@...il.com>
To:     Ido Schimmel <idosch@...sch.org>
Cc:     David Ahern <dsahern@...il.com>, Jakub Kicinski <kuba@...nel.org>,
        Neil Horman <nhorman@...driver.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        David Ahern <dsahern@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Menglong Dong <imagedong@...cent.com>
Subject: Re: [PATCH v3 net-next] net: drop_monitor: support drop reason

On Fri, Jan 28, 2022 at 12:03 AM Ido Schimmel <idosch@...sch.org> wrote:
>
> 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.

I think reporting it as a string would be a good choice. Is it ok if we do like
this (not tested yet)?

--- a/net/core/drop_monitor.c
+++ b/net/core/drop_monitor.c
@@ -48,6 +48,16 @@
 static int trace_state = TRACE_OFF;
 static bool monitor_hw;

+#undef EM
+#undef EMe
+
+#define EM(a, b) [a] = #b,
+#define EMe(a, b) [a] = #b
+
+static const char *drop_reasons[SKB_DROP_REASON_MAX + 1] = {
+       TRACE_SKB_DROP_REASON
+};
+
 /* net_dm_mutex
  *
  * An overall lock guarding every operation coming from userspace.
@@ -628,7 +638,8 @@ static int net_dm_packet_report_fill(struct
sk_buff *msg, struct sk_buff *skb,
                              NET_DM_ATTR_PAD))
                goto nla_put_failure;

-       if (nla_put_u32(msg, NET_DM_ATTR_REASON, cb->reason))
+       if (nla_put_string(msg, NET_DM_ATTR_REASON,
+                          drop_reasons[cb->reason]))
                goto nla_put_failure;

        snprintf(buf, sizeof(buf), "%pS", cb->pc);

Besides, I still think moving these reasons to uapi is necessary.
@David Ahern Is it ok to create a new file (such as net_skbuff.h)
for these reasons? Maybe other users need these enum in the
feature, and this job is done the sooner the better.

Thanks!
Menglong Dong

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