linux-kernel - Re: [PATCH] xdp: Remove unused events xdp_redirect_map and xdp_redirect_map

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Message-ID: <20415ab5-5003-4725-bf1b-560f197465c4@kernel.org>
Date: Mon, 16 Jun 2025 14:05:58 +0200
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: Toke Høiland-Jørgensen <toke@...nel.org>,
 Steven Rostedt <rostedt@...dmis.org>, LKML <linux-kernel@...r.kernel.org>,
 Linux trace kernel <linux-trace-kernel@...r.kernel.org>, bpf@...r.kernel.org
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
 John Fastabend <john.fastabend@...il.com>,
 Andrii Nakryiko <andrii@...nel.org>
Subject: Re: [PATCH] xdp: Remove unused events xdp_redirect_map and
 xdp_redirect_map_err

On 12/06/2025 12.54, Jesper Dangaard Brouer wrote:
> 
> 
> On 12/06/2025 12.30, Toke Høiland-Jørgensen wrote:
>> Steven Rostedt <rostedt@...dmis.org> writes:
>>
>>> From: Steven Rostedt <rostedt@...dmis.org>
>>>
>>> Each TRACE_EVENT() defined can take up around 5K of text and meta data
>>> regardless if they are used or not. New code is being developed that 
>>> will
>>> warn when a tracepoint is defined but not used.
>>>
>>> The trace events xdp_redirect_map and xdp_redirect_map_err are 
>>> defined but
>>> not used, but there's also a comment that states these are kept 
>>> around for
>>> backward compatibility. Which is interesting because since they are not
>>> used, any old BPF program that expects them to exist will get incorrect
>>> data (no data) when they use them. It's worse than not working, it's
>>> silently failing.
>>>
>>> Remove them as they will soon cause warnings, or if they really need to
>>> stick around, then code needs to be added to use them.
>>>
>>> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
>>
>> I guess that makes sense; I have no objections to getting rid of them.
>>
>> Reviewed-by: Toke Høiland-Jørgensen <toke@...nel.org>
> 
> Make sense.
> 
> 
> Toke we have to check how XDP-tools handle when these tracepoints 
> disappears.

To Toke, notice that userspace tools expect this tracepoint to be
available will fail as below (for kernel release v6.16):

  $ sudo ./xdp-bench redirect mlx5p1 veth41
   libbpf: prog 'tp_xdp_redirect_map_err': failed to find kernel BTF 
type ID of 'xdp_redirect_map_err': -3
   libbpf: prog 'tp_xdp_redirect_map_err': failed to prepare load 
attributes: -3
   libbpf: prog 'tp_xdp_redirect_map_err': failed to load: -3
   libbpf: failed to load object 'xdp_redirect_basic'
  Failed to attach XDP program: No such process

IMHO this is a userspace problem, that needs to be more flexible and
adapt to this change.

This was changed in kernel v5.6 (Jan 2020) commit 1d233886dd90 ("xdp:
Use bulking for non-map XDP_REDIRECT and consolidate code paths").
So, I'm thinking that xdp-tools could just remove monitoring for these
tracepoints?

--Jesper