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

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Message-ID: <CABCJKucdMS1hkWjHWty8JyACjZy2R9juusABcbsMYzNej=pB2Q@mail.gmail.com>
Date: Wed, 7 Aug 2024 08:33:07 -0700
From: Sami Tolvanen <samitolvanen@...gle.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>
Subject: Re: [PATCH v2 3/3] tracing/kprobes: Use APIs that matches symbols
 without .XXX suffix

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.

Sami