Hi Alessandro,

On Aug 8, 2024, at 11:20 PM, Alessandro Carminati [off-list ref] wrote:

Hello,
sorry to join late at the party.

Il giorno gio 8 ago 2024 alle ore 11:59 Petr Mladek [off-list ref]
ha scritto:

quoted

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

quoted

quoted

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:

quoted

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

quoted

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.

quoted

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.

quoted

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 would allow to find the symbols with and without the suffix using
a single API.

kas_alias isn't handling LTO as effectively as it should.
This is something I plan to address in the next patch version.
Introducing aliases is the best approach I found to preserve current
tools behavior while adding this new feature.
While I believe it will deliver the promised benefits, there is a trade-off,
particularly affecting features like live patching, which rely on handling
duplicate symbols.
For instance, kallsyms_lookup_names typically returns the last occurrence
of a symbol when the end argument is not NULL, but introducing aliases
disrupts this behavior.

Do you think with v3 of this set [1], live patching should be fine? The
idea is to let kallsyms_lookup_names() do full name match, then live
patching can find the right symbol with symbol name + old_sympos. 
Did I miss some cases?

I'm working on a solution to manage duplicate symbols, ensuring compatibility
with both LTO and kallsyms_lookup_names.

Thanks,
Song

[1] https://lore.kernel.org/live-patching/20240807220513.3100483-1-song@kernel.org/T/#u (local)