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

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Message-ID: <09ED7762-A464-45FF-9062-9560C59F304E@fb.com>
Date: Wed, 7 Aug 2024 20:48:48 +0000
From: Song Liu <songliubraving@...a.com>
To: Sami Tolvanen <samitolvanen@...gle.com>
CC: Masami Hiramatsu <mhiramat@...nel.org>,
        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>
Subject: Re: [PATCH v2 3/3] tracing/kprobes: Use APIs that matches symbols
 without .XXX suffix



> On Aug 7, 2024, at 8:33 AM, Sami Tolvanen <samitolvanen@...gle.com> wrote:
> 
> Hi,
> 
> On Wed, 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:
>> 
>>> 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, remove
suffixes in special APIs during lookup). 

Thanks,
Song