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Date: Mon, 10 Jun 2024 12:54:22 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Yan Zhai <yan@cloudflare.com>
Cc: netdev@vger.kernel.org, "David S. Miller" <davem@davemloft.net>, Eric
 Dumazet <edumazet@google.com>, Jakub Kicinski <kuba@kernel.org>, Paolo
 Abeni <pabeni@redhat.com>, Simon Horman <horms@kernel.org>, David Ahern
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 Masami Hiramatsu <mhiramat@kernel.org>, Mathieu Desnoyers
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 linux-trace-kernel@vger.kernel.org, Dan Carpenter
 <dan.carpenter@linaro.org>
Subject: Re: [RFC v3 net-next 1/7] net: add rx_sk to trace_kfree_skb
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On Thu, 6 Jun 2024 10:37:46 -0500
Yan Zhai <yan@cloudflare.com> wrote:

> > name: kfree_skb
> > ID: 1799
> > 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;
> >
> > Notice that "protocol" is 2 bytes in size at offset 24, but "reason" starts
> > at offset 28. This means at offset 26, there's a 2 byte hole.
> >  
> The reason I added the pointer as the last argument is trying to
> minimize the surprise to existing TP users, because for common ABIs
> it's fine to omit later arguments when defining a function, but it
> needs change and recompilation if the order of arguments changed.

Nothing should be hard coding the offsets of the fields. This is
exported to user space so that tools can see where the fields are.
That's the purpose of libtraceevent. The fields should be movable and
not affect anything. There should be no need to recompile.

> 
> Looking at the actual format after the change, it does not add a new
> hole since protocol and reason are already packed into the same 8-byte
> block, so rx_skaddr starts at 8-byte aligned offset:
> 
> # cat /sys/kernel/debug/tracing/events/skb/kfree_skb/format
> name: kfree_skb
> ID: 2260
> 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;
>         field:void * rx_skaddr; offset:32;      size:8; signed:0;
> 
> Do you think we still need to change the order?

Up to you, just wanted to point it out.

-- Steve