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

On Thu 2024-08-08 15:20:26, Song Liu wrote:

quoted

On Aug 8, 2024, at 2:59 AM, Petr Mladek [off-list ref] wrote:

On Wed 2024-08-07 20:48:48, Song Liu wrote:

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On Aug 7, 2024, at 8:33 AM, Sami Tolvanen [off-list ref] wrote:

Hi,

On Wed, Aug 7, 2024 at 3:08 AM Masami Hiramatsu [off-list ref] wrote:

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On Wed, 7 Aug 2024 00:19:20 +0000
Song Liu [off-list ref] wrote:

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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.

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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.

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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.

quoted

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@gmail.com (local)

I guess v3 plus this work may work well together.  

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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