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Date:   Tue, 12 Apr 2022 09:49:23 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>
Cc:     Jiri Olsa <jolsa@kernel.org>, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>
Subject: Re: [RFC bpf-next 4/4] selftests/bpf: Add attach bench test
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On Mon, 11 Apr 2022 15:15:40 -0700
Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:

> > +#define DEBUGFS "/sys/kernel/debug/tracing/"
> > +
> > +static int get_syms(char ***symsp, size_t *cntp)
> > +{
> > +       size_t cap = 0, cnt = 0, i;
> > +       char *name, **syms = NULL;
> > +       struct hashmap *map;
> > +       char buf[256];
> > +       FILE *f;
> > +       int err;
> > +
> > +       /*
> > +        * The available_filter_functions contains many duplicates,
> > +        * but other than that all symbols are usable in kprobe multi
> > +        * interface.
> > +        * Filtering out duplicates by using hashmap__add, which won't
> > +        * add existing entry.
> > +        */
> > +       f = fopen(DEBUGFS "available_filter_functions", "r");
> 
> I'm really curious how did you manage to attach to everything in
> available_filter_functions because when I'm trying to do that I fail.
> available_filter_functions has a bunch of functions that should not be
> attachable (e.g., notrace functions). Look just at __bpf_tramp_exit:
> 
>   void notrace __bpf_tramp_exit(struct bpf_tramp_image *tr);

Hmm, this sounds like a bug in ftrace side. IIUC, the
"available_filter_functions" only shows the functions which is NOT
instrumented by mcount, we should not see any notrace functions on it.

Technically, this is done by __no_instrument_function__ attribute.

#if defined(CC_USING_HOTPATCH)
#define notrace                 __attribute__((hotpatch(0, 0)))
#elif defined(CC_USING_PATCHABLE_FUNCTION_ENTRY)
#define notrace                 __attribute__((patchable_function_entry(0, 0)))
#else
#define notrace                 __attribute__((__no_instrument_function__))
#endif

> 
> So first, curious what I am doing wrong or rather why it succeeds in
> your case ;)
> 
> But second, just wanted to plea to "fix" available_filter_functions to
> not list stuff that should not be attachable. Can you please take a
> look and checks what's going on there and why do we have notrace
> functions (and what else should *NOT* be there)?

Can you share how did you reproduce the issue? I'll check it.

Thank you,


-- 
Masami Hiramatsu <mhiramat@kernel.org>