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

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Message-ID: <BEEE3F89-717B-44A4-8571-68DA69408DA4@fb.com>
Date: Wed, 7 Aug 2024 00:19:20 +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 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. 

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@google.com/