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

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Wed, 7 Aug 2024 00:19:20 +0000
Song Liu <songliubraving@meta.com> wrote:

> 
> 
> > On Aug 6, 2024, at 5:01 PM, Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > 
> > On Tue, 6 Aug 2024 20:12:55 +0000
> > Song Liu <songliubraving@meta.com> wrote:
> > 
> >> 
> >> 
> >>> On Aug 6, 2024, at 1:01 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
> >>> 
> >>> On Tue, 6 Aug 2024 16:00:49 -0400
> >>> Steven Rostedt <rostedt@goodmis.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. 

What tracing tools may be broke and why?

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.

Thank you,

> 
> 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/
> 


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