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

[Petr Mladek]

On Thu 2024-08-08 15:20:26, Song Liu wrote:
> 
> 
> > On Aug 8, 2024, at 2:59 AM, Petr Mladek <pmladek@xxxxxxxx> wrote:
> > 
> > On Wed 2024-08-07 20:48:48, Song Liu wrote:
> >> 
> >> 
> >>> On Aug 7, 2024, at 8:33 AM, Sami Tolvanen <samitolvanen@xxxxxxxxxx> wrote:
> >>> 
> >>> Hi,
> >>> 
> >>> On Wed, Aug 7, 2024 at 3:08 AM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> >>>> 
> >>>> On Wed, 7 Aug 2024 00:19:20 +0000
> >>>> Song Liu <songliubraving@xxxxxxxx> 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,
> > 
> > Yes, we should allow to find the symbols via the full name, definitely.
> > 
> >> remove suffixes in special APIs during lookup).
> > 
> > Just an idea. Alternative solution would be to make make an alias
> > without the suffix when there is only one symbol with the same
> > name.
> > 
> > It would be complementary with the patch adding aliases for symbols
> > with the same name, see
> > https://lore.kernel.org/r/20231204214635.2916691-1-alessandro.carminati@xxxxxxxxx
> 
> I guess v3 plus this work may work well together.  
> 
> > I would allow to find the symbols with and without the suffix using
> > a single API.
> 
> Could you please describe how this API would work? I tried some 
> idea in v1, but it turned out to be quite confusing. So I decided 
> to leave this logic to the users of kallsyms APIs in v2. 

If we create an alias without the suffix but only when there is only
one symbol with such a name then we have, for example:

  klp_complete_transition.lwn.123456
  klp_complete_transition		[alias]

  init_once.lwn.2131221
  init_once.lwn.3443243
  init_once.lwn.4324322
  init_once.lwn.5214121
  init_once.lwn.2153121
  init_once.lwn.4342343

This way, it will be possible to find the static symbol
"klp_complete_transition" without the suffix via the alias.
It will have the alias because it has an unique name.

While "init_once" symbol must always be searched with the suffix
because it is not unique.

It looks like >99% of static symbols have unique name.

Best Regards,
Petr