linux-kernel - Re: [PATCHv2] ftrace: Show all functions with addresses in available_filter_functions

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Date:   Mon, 12 Jun 2023 07:49:53 -0700
From:   Yonghong Song <yhs@...a.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Jiri Olsa <jolsa@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Andrii Nakryiko <andrii@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Jackie Liu <liu.yun@...ux.dev>
Subject: Re: [PATCHv2] ftrace: Show all functions with addresses in
 available_filter_functions_addrs



On 6/11/23 7:57 PM, Steven Rostedt wrote:
> On Sun, 11 Jun 2023 22:54:07 -0400
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
>>> Also, if there are multiple same static functions with
>>> different addresses, user space might need to check dwarf or
>>> proposed BTF_KIND_KFUNC (which encode kallsyms addresses)
>>> to find whether entry in available_filter_functions_addrs
>>> to be used. But addresses may not match. How this issue could
>>> be resolved?
>>
>> Easy, you use the address between two other addresses in kallsyms. The
>> address is still in the function. The addresses in kallsyms is the starting
>> address, but there's cases that the patch location is not at the start.
> 
> Not to mention, you can still use the kallsyms address. If you did the work
> to find it, then use it (it may not be as efficient as I mentioned before).
> That's basically what is done today (so I am told), and this patch was to
> create a file where you don't need to look up kallsyms when you know which
> function to use. The functions are sorted by address, so if you know of a
> unique function near the duplicate, you just find the duplicate that's near
> the unique function name.

Thanks for explanation. IIUC, typically with endbr enabled, the patch
address typically is the entry_addr + 4. So it is indeed easy to
correlate the entry in available_filter_functions_addrs and in
kallsyms.

I am actually interested in how available_filter_functions_addrs
will be used. For example, bpf_program__attach_kprobe_multi_opts()
can already take addresses from kallsyms. How to use
available_filter_functions_addrs to facilitate kprobe_multi?
Do we need to change kernel APIs? It would be great at least we
got a RFC patch to answer these questions.

> 
> -- Steve