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

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Message-ID: <2622EED3-19EF-4F3B-8681-B4EB19370436@fb.com>
Date: Wed, 7 Aug 2024 19:41:11 +0000
From: Song Liu <songliubraving@...a.com>
To: Masami Hiramatsu <mhiramat@...nel.org>
CC: Song Liu <songliubraving@...a.com>, Steven Rostedt <rostedt@...dmis.org>,
        Song Liu <song@...nel.org>,
        "live-patching@...r.kernel.org"
	<live-patching@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-trace-kernel@...r.kernel.org" <linux-trace-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>, Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>, Petr Mladek <pmladek@...e.com>,
        Joe Lawrence
	<joe.lawrence@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        "morbo@...gle.com" <morbo@...gle.com>,
        Justin Stitt <justinstitt@...gle.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Leizhen <thunder.leizhen@...wei.com>,
        "kees@...nel.org" <kees@...nel.org>,
        Kernel Team <kernel-team@...a.com>,
        Matthew Maurer <mmaurer@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: Re: [PATCH v2 3/3] tracing/kprobes: Use APIs that matches symbols
 without .XXX suffix



> On Aug 7, 2024, at 3:08 AM, Masami Hiramatsu <mhiramat@...nel.org> wrote:
> 
> On Wed, 7 Aug 2024 00:19:20 +0000
> Song Liu <songliubraving@...a.com> wrote:
> 
>> 
>> 
>>> On Aug 6, 2024, at 5:01 PM, Masami Hiramatsu <mhiramat@...nel.org> wrote:
>>> 
>>> On Tue, 6 Aug 2024 20:12:55 +0000
>>> Song Liu <songliubraving@...a.com> wrote:
>>> 
>>>> 
>>>> 
>>>>> On Aug 6, 2024, at 1:01 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
>>>>> 
>>>>> On Tue, 6 Aug 2024 16:00:49 -0400
>>>>> Steven Rostedt <rostedt@...dmis.org> 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.
> 
> What tracing tools may be broke and why?
> 
> 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.

It appears there are multiple APIs that may need change. For example, on gcc
built kernel, /sys/kernel/tracing/available_filter_functions does not show 
the suffix: 

  [root@(none)]# grep cmos_irq_enable /proc/kallsyms
  ffffffff81db5470 t __pfx_cmos_irq_enable.constprop.0
  ffffffff81db5480 t cmos_irq_enable.constprop.0
  ffffffff822dec6e t cmos_irq_enable.constprop.0.cold

  [root@(none)]# grep cmos_irq_enable /sys/kernel/tracing/available_filter_functions
  cmos_irq_enable

perf-probe uses _text+<offset> for such cases:

  [root@(none)]# cat /sys/kernel/tracing/kprobe_events
  p:probe/cmos_irq_enable _text+14374016 

I am not sure which APIs do we need to change here. 

Thanks,
Song