Re: [PATCH v2 3/3] tracing/kprobes: Use APIs that matches symbols without .XXX suffix

[Song Liu]

> On Aug 8, 2024, at 2:59 AM, Petr Mladek <pmladek@xxxxxxxx> wrote:
> 
> On Wed 2024-08-07 20:48:48, Song Liu wrote:
>> 
>> 
>>> On Aug 7, 2024, at 8:33 AM, Sami Tolvanen <samitolvanen@xxxxxxxxxx> wrote:
>>> 
>>> Hi,
>>> 
>>> On Wed, Aug 7, 2024 at 3:08 AM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>>>> 
>>>> On Wed, 7 Aug 2024 00:19:20 +0000
>>>> Song Liu <songliubraving@xxxxxxxx> wrote:
>>>> 
>>>>> 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.
>>>> 
>>>> What tracing tools may be broke and why?
>>> 
>>> This was a few years ago when we were first adding LTO support, but
>>> the unexpected suffixes in tracing output broke systrace in Android,
>>> presumably because the tools expected to find specific function names
>>> without suffixes. I'm not sure if systrace would still be a problem
>>> today, but other tools might still make assumptions about the function
>>> name format. At the time, we decided to filter out the suffixes in all
>>> user space visible output to avoid these issues.
>>> 
>>>> For this suffix problem, I would like to add another patch to allow probing on
>>>> suffixed symbols. (It seems suffixed symbols are not available at this point)
>>>> 
>>>> The problem is that the suffixed symbols maybe a "part" of the original function,
>>>> thus user has to carefully use it.
>>>> 
>>>>> 
>>>>> Sami, could you please share your thoughts on this?
>>>> 
>>>> Sami, I would like to know what problem you have on kprobes.
>>> 
>>> The reports we received back then were about registering kprobes for
>>> static functions, which obviously failed if the compiler added a
>>> suffix to the function name. This was more of a problem with ThinLTO
>>> and Clang CFI at the time because the compiler used to rename _all_
>>> static functions, but one can obviously run into the same issue with
>>> just LTO.
>> 
>> I think newer LLVM/clang no longer add suffixes to all static functions
>> with LTO and CFI. So this may not be a real issue any more?
>> 
>> If we still need to allow tracing without suffix, I think the approach
>> in this patch set is correct (sort syms based on full name,
> 
> Yes, we should allow to find the symbols via the full name, definitely.
> 
>> remove suffixes in special APIs during lookup).
> 
> Just an idea. Alternative solution would be to make make an alias
> without the suffix when there is only one symbol with the same
> name.
> 
> It would be complementary with the patch adding aliases for symbols
> with the same name, see
> https://lore.kernel.org/r/20231204214635.2916691-1-alessandro.carminati@xxxxxxxxx

I guess v3 plus this work may work well together.  

> I would allow to find the symbols with and without the suffix using
> a single API.

Could you please describe how this API would work? I tried some 
idea in v1, but it turned out to be quite confusing. So I decided 
to leave this logic to the users of kallsyms APIs in v2. 

Thanks,
Song