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

[Menglong Dong]

On Fri, Sep 2, 2022 at 11:43 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> On Fri, Sep 2, 2022 at 7:18 AM <menglong8.dong@xxxxxxxxx> wrote:
> >
> > From: Menglong Dong <imagedong@xxxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
>
> Note that I also provided the sha1 of the faulty patch.
>
> You should add a corresponding Fixes: tag, to help both humans and bots.
>

Okay, I'll add the Fixes tag......and  the Link tag.

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

It seems that the net tree is more suitable. I'll change it.

Thanks!

> > Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx>
> > ---
> > v2:
> > - undef FN/FNe after use it (Jakub Kicinski)
>
> I would love some feedback from Steven :)

Hi, Steven! Do you have any feedback? Yes, I add the
TRACE_DEFINE_ENUM() back! (Hmm...I deleted it before,
without knowing its function :/ )

Thanks!
Menglong Dong