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

[Song Liu]

> On Aug 9, 2024, at 8:40 AM, Petr Mladek <pmladek@xxxxxxxx> wrote:
> 
> On Thu 2024-08-08 15:20:26, Song Liu wrote:
>> 
>> 
>>> 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.
> 
> If we create an alias without the suffix but only when there is only
> one symbol with such a name then we have, for example:
> 
>  klp_complete_transition.lwn.123456
>  klp_complete_transition [alias]
> 
>  init_once.lwn.2131221
>  init_once.lwn.3443243
>  init_once.lwn.4324322
>  init_once.lwn.5214121
>  init_once.lwn.2153121
>  init_once.lwn.4342343
> 
> This way, it will be possible to find the static symbol
> "klp_complete_transition" without the suffix via the alias.
> It will have the alias because it has an unique name.
> 
> While "init_once" symbol must always be searched with the suffix
> because it is not unique.
> 
> It looks like >99% of static symbols have unique name.

Got it. The idea is to generate the alias at boot time. I think
this will indeed work. 

IIUC, v3 of this set with Alessandro's work (maybe with some 
variations) should do this. 


Thanks, 
Song