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

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Message-Id: <20240808062603.f4a180fa2153881874eb0dc0@kernel.org>
Date: Thu, 8 Aug 2024 06:26:03 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Sami Tolvanen <samitolvanen@...gle.com>
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

On Wed, 7 Aug 2024 08:33:07 -0700
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.

Thanks for explanation.
IMHO, those tools might change their assumptions and decide the policy
that it drops suffixes (as you did) or keep the suffixes as it is.

> > 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.

Yeah, without 1/3 of this series, user can not specify llvm suffixed
symbols on kprobe events. However, as perf-probe does, users can use
the relative offset from a unique symbol too. (kprobe does not care
the function boundary.)

Thank you,

> 
> Sami


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>