netdev - Re: [PATCH net-next v2] net: skb: export skb drop reaons to user by TRACE_DEFINE

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Date:   Fri, 2 Sep 2022 08:43:07 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Menglong Dong <menglong8.dong@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Menglong Dong <imagedong@...cent.com>,
        David Ahern <dsahern@...nel.org>,
        Hao Peng <flyingpeng@...cent.com>,
        Dongli Zhang <dongli.zhang@...cle.com>, robh@...nel.org,
        Pavel Begunkov <asml.silence@...il.com>,
        Luiz Augusto von Dentz <luiz.von.dentz@...el.com>,
        Vasily Averin <vasily.averin@...ux.dev>,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v2] net: skb: export skb drop reaons to user by TRACE_DEFINE_ENUM

On Fri, Sep 2, 2022 at 7:18 AM <menglong8.dong@...il.com> wrote:
>
> From: Menglong Dong <imagedong@...cent.com>
>
> As Eric reported, the 'reason' field is not presented when trace the
> kfree_skb event by perf:
>
> $ perf record -e skb:kfree_skb -a sleep 10
> $ perf script
>   ip_defrag 14605 [021]   221.614303:   skb:kfree_skb:
>   skbaddr=0xffff9d2851242700 protocol=34525 location=0xffffffffa39346b1
>   reason:
>
> The cause seems to be passing kernel address directly to TP_printk(),
> which is not right. As the enum 'skb_drop_reason' is not exported to
> user space through TRACE_DEFINE_ENUM(), perf can't get the drop reason
> string from the 'reason' field, which is a number.
>
> Therefore, we introduce the macro DEFINE_DROP_REASON(), which is used
> to define the trace enum by TRACE_DEFINE_ENUM(). With the help of
> DEFINE_DROP_REASON(), now we can remove the auto-generate that we
> introduced in the commit ec43908dd556
> ("net: skb: use auto-generation to convert skb drop reason to string"),
> and define the string array 'drop_reasons'.
>
> Hmmmm...now we come back to the situation that have to maintain drop
> reasons in both enum skb_drop_reason and DEFINE_DROP_REASON. But they
> are both in dropreason.h, which makes it easier.
>
> After this commit, now the format of kfree_skb is like this:
>
> $ cat /tracing/events/skb/kfree_skb/format
> name: kfree_skb
> ID: 1524
> format:
>         field:unsigned short common_type;       offset:0;       size:2; signed:0;
>         field:unsigned char common_flags;       offset:2;       size:1; signed:0;
>         field:unsigned char common_preempt_count;       offset:3;       size:1; signed:0;
>         field:int common_pid;   offset:4;       size:4; signed:1;
>
>         field:void * skbaddr;   offset:8;       size:8; signed:0;
>         field:void * location;  offset:16;      size:8; signed:0;
>         field:unsigned short protocol;  offset:24;      size:2; signed:0;
>         field:enum skb_drop_reason reason;      offset:28;      size:4; signed:0;
>
> print fmt: "skbaddr=%p protocol=%u location=%p reason: %s", REC->skbaddr, REC->protocol, REC->location, __print_symbolic(REC->reason, { 1, "NOT_SPECIFIED" }, { 2, "NO_SOCKET" } ......
>
> Reported-by: Eric Dumazet <edumazet@...gle.com>

Note that I also provided the sha1 of the faulty patch.

You should add a corresponding Fixes: tag, to help both humans and bots.

This would also hint that this patch should target net tree, not net-next ?

> Signed-off-by: Menglong Dong <imagedong@...cent.com>
> ---
> v2:
> - undef FN/FNe after use it (Jakub Kicinski)

I would love some feedback from Steven :)