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Date:   Tue, 17 Nov 2020 14:21:45 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        Matt Mullins <mmullins@mmlx.us>,
        Ingo Molnar <mingo@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Dmitry Vyukov <dvyukov@google.com>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andriin@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        netdev <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH] tracepoint: Do not fail unregistering a probe due to
 memory allocation
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On Tue, 17 Nov 2020 14:15:10 -0500 (EST)
Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:


> diff --git a/include/linux/tracepoint-defs.h b/include/linux/tracepoint-defs.h
> index e7c2276be33e..e0351bb0b140 100644
> --- a/include/linux/tracepoint-defs.h
> +++ b/include/linux/tracepoint-defs.h
> @@ -38,6 +38,7 @@ struct tracepoint {
>         int (*regfunc)(void);
>         void (*unregfunc)(void);
>         struct tracepoint_func __rcu *funcs;
> +       void *stub_func;
>  };
>  
>  #ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> index 0f21617f1a66..b0b805de3779 100644
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -287,6 +287,9 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>  #define DEFINE_TRACE_FN(_name, _reg, _unreg, proto, args)              \
>         static const char __tpstrtab_##_name[]                          \
>         __section("__tracepoints_strings") = #_name;                    \
> +       static void __cold __tracepoint_stub_func_##_name(void *__data, proto) \
> +       {                                                               \
> +       }                                                               \

The thing is, tracepoints are already bloated. I do not want to add
something like this that will unnecessarily add more text.

Since all tracepoints callbacks have at least one parameter (__data), we
could declare tp_stub_func as:

static void tp_stub_func(void *data, ...)
{
	return;
}

And now C knows that tp_stub_func() can be called with one or more
parameters, and had better be able to deal with it!

-- Steve


>         extern struct static_call_key STATIC_CALL_KEY(tp_func_##_name); \
>         int __traceiter_##_name(void *__data, proto);                   \
>         struct tracepoint __tracepoint_##_name  __used                  \
> @@ -298,7 +301,8 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>                 .iterator = &__traceiter_##_name,                       \
>                 .regfunc = _reg,                                        \
>                 .unregfunc = _unreg,                                    \
> -               .funcs = NULL };                                        \
> +               .funcs = NULL,                                          \
> +               .stub_func = __tracepoint_stub_func_##_name, };         \
>         __TRACEPOINT_ENTRY(_name);                                      \
>         int __traceiter_##_name(void *__data, proto)                    \
>         {                                                               \
> 
> Thanks,
> 
> Mathieu
>