Re: [PATCH v2 3/3] tracing/kprobes: Use APIs that matches symbols without .XXX suffix
From: Song Liu
Date: Tue Aug 06 2024 - 20:19:34 EST
> On Aug 6, 2024, at 5:01 PM, Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>
> On Tue, 6 Aug 2024 20:12:55 +0000
> Song Liu <songliubraving@xxxxxxxx> wrote:
>
>>
>>
>>> On Aug 6, 2024, at 1:01 PM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>>>
>>> On Tue, 6 Aug 2024 16:00:49 -0400
>>> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>>>
>>>>>>> + if (IS_ENABLED(CONFIG_LTO_CLANG) && !addr)
>>>>>>> + addr = kallsyms_lookup_name_without_suffix(trace_kprobe_symbol(tk));
>>>>>>> +
>>>>>>
>>>>>> So you do the lookup twice if this is enabled?
>>>>>>
>>>>>> Why not just use "kallsyms_lookup_name_without_suffix()" the entire time,
>>>>>> and it should work just the same as "kallsyms_lookup_name()" if it's not
>>>>>> needed?
>>>>>
>>>>> We still want to give priority to full match. For example, we have:
>>>>>
>>>>> [root@~]# grep c_next /proc/kallsyms
>>>>> ffffffff81419dc0 t c_next.llvm.7567888411731313343
>>>>> ffffffff81680600 t c_next
>>>>> ffffffff81854380 t c_next.llvm.14337844803752139461
>>>>>
>>>>> If the goal is to explicitly trace c_next.llvm.7567888411731313343, the
>>>>> user can provide the full name. If we always match _without_suffix, all
>>>>> of the 3 will match to the first one.
>>>>>
>>>>> Does this make sense?
>>>>
>>>> Yes. Sorry, I missed the "&& !addr)" after the "IS_ENABLED()", which looked
>>>> like you did the command twice.
>>>
>>> But that said, does this only have to be for llvm? Or should we do this for
>>> even gcc? As I believe gcc can give strange symbols too.
>>
>> I think most of the issue comes with LTO, as LTO promotes local static
>> functions to global functions. IIUC, we don't have GCC built, LTO enabled
>> kernel yet.
>>
>> In my GCC built, we have suffixes like ".constprop.0", ".part.0", ".isra.0",
>> and ".isra.0.cold". We didn't do anything about these before this set. So I
>> think we are OK not handling them now. We sure can enable it for GCC built
>> kernel in the future.
>
> Hmm, I think it should be handled as it is. This means it should do as
> livepatch does. Since I expected user will check kallsyms if gets error,
> we should keep this as it is. (if a symbol has suffix, it should accept
> symbol with suffix, or user will get confused because they can not find
> which symbol is kprobed.)
>
> Sorry about the conclusion (so I NAK this), but this is a good discussion.
Do you mean we do not want patch 3/3, but would like to keep 1/3 and part
of 2/3 (remove the _without_suffix APIs)? If this is the case, we are
undoing the change by Sami in [1], and thus may break some tracing tools.
Sami, could you please share your thoughts on this?
If this works, I will send next version with 1/3 and part of 2/3.
Thanks,
Song
[1] https://lore.kernel.org/all/20210408182843.1754385-8-samitolvanen@xxxxxxxxxx/