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

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Message-Id: <20240807090146.88b38c2fbd1cd8db683be22c@kernel.org>
Date: Wed, 7 Aug 2024 09:01:46 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Song Liu <songliubraving@...a.com>
Cc: 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>, Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH v2 3/3] tracing/kprobes: Use APIs that matches symbols
 without .XXX suffix

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. 

Thanks,

> 
> Thanks,
> Song
> 
> 
> 


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