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

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Message-ID: <4c87727b-0b3f-ffc1-d55b-90e75dcae52b@meta.com>
Date:   Mon, 12 Jun 2023 22:04:28 -0700
From:   Yonghong Song <yhs@...a.com>
To:     Jiri Olsa <olsajiri@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     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 6/12/23 1:25 PM, Jiri Olsa wrote:
> On Mon, Jun 12, 2023 at 11:02:22AM -0400, Steven Rostedt wrote:
>> On Mon, 12 Jun 2023 07:49:53 -0700
>> Yonghong Song <yhs@...a.com> wrote:
>>
>>> 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?
> 
> the problem is that we need to do 2 passes:
> 
>   - through available_filter_functions and find out if the function is traceable
>   - through /proc/kallsyms to get the address for traceable function
> 
> having available_filter_functions symbols together with addresses allow
> us to skip the kallsyms step
> 
> and we are ok with the address in available_filter_functions_addr not being the
> function entry, because kprobe_multi uses fprobe and that handles both entry and
> patch-site address properly

Thanks for explanation! It would be great if we can put more details in
this email into the commit message!

> 
>>> Do we need to change kernel APIs? It would be great at least we
>>> got a RFC patch to answer these questions.
>>
>> I agree, having that information would also be useful to me.
>>
>> Jiri? Andrii?
> 
> so we have 2 interfaces how to create kprobe_multi link:
> 
>    a) passing symbols to kernel
> 
>       1) user gathers symbols and need to ensure that they are
>          trace-able -> pass through available_filter_functions file
> 
>       2) kernel takes those symbols and translates them to addresses
>          through kallsyms api
> 
>       3) addresses are passed to fprobe/ftrace through:
> 
>           register_fprobe_ips
>           -> ftrace_set_filter_ips
> 
>    b) passing addresses to kernel
> 
>       1) user gathers symbols and needs to ensure that they are
>          trace-able -> pass through available_filter_functions file
> 
>       2) user takes those symbols and translates them to addresses
>         through /proc/kallsyms
> 
>       3) addresses are passed to the kernel and kernel calls:
> 
>           register_fprobe_ips
>           -> ftrace_set_filter_ips
> 
> 
> The new available_filter_functions_addrs file helps us with option b),
> because we can make 'b 1' and 'b 2' in one step - while filtering traceable
> functions, we get the address directly.
> 
> I tested the new available_filter_functions_addrs changes with some hacked
> selftest changes, you can check it in here [1].
> 
> I assume Jackie Liu will send new version of her patchset [2] based on this
> new available_filter_functions_addrs file.
> 
> I think we should have these changes coming together and add some perf
> measurements from before and after to make the benefit apparent.
> 
> jirka
> 
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/jolsa/perf.git/commit/?h=bpf/avail_addrs&id=fecaeeaf40bae034715ab2e9a46ca1dc16371e8e
> [2] https://lore.kernel.org/bpf/20230526155026.1419390-1-liu.yun@linux.dev/#r