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Date:   Tue, 12 Jan 2021 12:19:49 -0800
From:   Yonghong Song <yhs@...com>
To:     Qais Yousef <qais.yousef@....com>, <netdev@...r.kernel.org>,
        <bpf@...r.kernel.org>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/2] trace: bpf: Allow bpf to attach to bare
 tracepoints



On 1/11/21 10:20 AM, Qais Yousef wrote:
> Some subsystems only have bare tracepoints (a tracepoint with no
> associated trace event) to avoid the problem of trace events being an
> ABI that can't be changed.
> 
>  From bpf presepective, bare tracepoints are what it calls
> RAW_TRACEPOINT().
> 
> Since bpf assumed there's 1:1 mapping, it relied on hooking to
> DEFINE_EVENT() macro to create bpf mapping of the tracepoints. Since
> bare tracepoints use DECLARE_TRACE() to create the tracepoint, bpf had
> no knowledge about their existence.
> 
> By teaching bpf_probe.h to parse DECLARE_TRACE() in a similar fashion to
> DEFINE_EVENT(), bpf can find and attach to the new raw tracepoints.
> 
> Enabling that comes with the contract that changes to raw tracepoints
> don't constitute a regression if they break existing bpf programs.
> We need the ability to continue to morph and modify these raw
> tracepoints without worrying about any ABI.
> 
> Update Documentation/bpf/bpf_design_QA.rst to document this contract.
> 
> Signed-off-by: Qais Yousef <qais.yousef@....com>
> ---
>   Documentation/bpf/bpf_design_QA.rst |  6 ++++++
>   include/trace/bpf_probe.h           | 12 ++++++++++--
>   2 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/bpf/bpf_design_QA.rst b/Documentation/bpf/bpf_design_QA.rst
> index 2df7b067ab93..0e15f9b05c9d 100644
> --- a/Documentation/bpf/bpf_design_QA.rst
> +++ b/Documentation/bpf/bpf_design_QA.rst
> @@ -208,6 +208,12 @@ data structures and compile with kernel internal headers. Both of these
>   kernel internals are subject to change and can break with newer kernels
>   such that the program needs to be adapted accordingly.
>   
> +Q: Are tracepoints part of the stable ABI?
> +------------------------------------------
> +A: NO. Tracepoints are tied to internal implementation details hence they are
> +subject to change and can break with newer kernels. BPF programs need to change
> +accordingly when this happens.
> +
>   Q: How much stack space a BPF program uses?
>   -------------------------------------------
>   A: Currently all program types are limited to 512 bytes of stack
> diff --git a/include/trace/bpf_probe.h b/include/trace/bpf_probe.h
> index cd74bffed5c6..cf1496b162b1 100644
> --- a/include/trace/bpf_probe.h
> +++ b/include/trace/bpf_probe.h
> @@ -55,8 +55,7 @@
>   /* tracepoints with more than 12 arguments will hit build error */
>   #define CAST_TO_U64(...) CONCATENATE(__CAST, COUNT_ARGS(__VA_ARGS__))(__VA_ARGS__)
>   
> -#undef DECLARE_EVENT_CLASS
> -#define DECLARE_EVENT_CLASS(call, proto, args, tstruct, assign, print)	\
> +#define __BPF_DECLARE_TRACE(call, proto, args)				\
>   static notrace void							\
>   __bpf_trace_##call(void *__data, proto)					\
>   {									\
> @@ -64,6 +63,10 @@ __bpf_trace_##call(void *__data, proto)					\
>   	CONCATENATE(bpf_trace_run, COUNT_ARGS(args))(prog, CAST_TO_U64(args));	\
>   }
>   
> +#undef DECLARE_EVENT_CLASS
> +#define DECLARE_EVENT_CLASS(call, proto, args, tstruct, assign, print)	\
> +	__BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args))
> +
>   /*
>    * This part is compiled out, it is only here as a build time check
>    * to make sure that if the tracepoint handling changes, the
> @@ -111,6 +114,11 @@ __DEFINE_EVENT(template, call, PARAMS(proto), PARAMS(args), size)
>   #define DEFINE_EVENT_PRINT(template, name, proto, args, print)	\
>   	DEFINE_EVENT(template, name, PARAMS(proto), PARAMS(args))
>   
> +#undef DECLARE_TRACE
> +#define DECLARE_TRACE(call, proto, args)				\
> +	(__BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args))		\
> +	 __DEFINE_EVENT(call, call, PARAMS(proto), PARAMS(args), 0))

I applied the patch to my local bpf-next repo, and got the following
compilation error:

In file included from 
/data/users/yhs/work/net-next/include/trace/define_trace.h:104, 

                  from 
/data/users/yhs/work/net-next/include/trace/events/sched.h:740, 

                  from 
/data/users/yhs/work/net-next/kernel/sched/core.c:10: 

/data/users/yhs/work/net-next/include/trace/bpf_probe.h:59:1: error: 
expected identifier or ‘(’ before ‘static’
  static notrace void       \ 

  ^~~~~~ 

/data/users/yhs/work/net-next/include/trace/bpf_probe.h:119:3: note: in 
expansion of macro ‘__BPF_DECLARE_TRACE’
   (__BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args))  \ 

    ^~~~~~~~~~~~~~~~~~~ 

/data/users/yhs/work/net-next/include/trace/events/sched.h:693:1: note: 
in expansion of macro ‘DECLARE_TRACE’
  DECLARE_TRACE(pelt_cfs_tp, 

  ^~~~~~~~~~~~~ 

/data/users/yhs/work/net-next/include/trace/bpf_probe.h:59:1: error: 
expected identifier or ‘(’ before ‘static’
  static notrace void       \ 

  ^~~~~~ 

/data/users/yhs/work/net-next/include/trace/bpf_probe.h:119:3: note: in 
expansion of macro ‘__BPF_DECLARE_TRACE’
   (__BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args))  \ 

    ^~~~~~~~~~~~~~~~~~~ 

/data/users/yhs/work/net-next/include/trace/events/sched.h:697:1: note: 
in expansion of macro ‘DECLARE_TRACE’
  DECLARE_TRACE(pelt_rt_tp, 

  ^~~~~~~~~~~~~
/data/users/yhs/work/net-next/include/trace/bpf_probe.h:59:1: error: 
expected identifier or ‘(’ before ‘static’
  static notrace void       \

I dumped preprecessor result but after macro expansion, the code
becomes really complex and I have not figured out why it failed.
Do you know what is the possible reason?

> +
>   #include TRACE_INCLUDE(TRACE_INCLUDE_FILE)
>   
>   #undef DEFINE_EVENT_WRITABLE
> 

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