Hello, sorry to join late at the party. Il giorno gio 8 ago 2024 alle ore 11:59 Petr Mladek <pmladek@xxxxxxxx> ha scritto: > > 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 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. I'm working on a solution to manage duplicate symbols, ensuring compatibility with both LTO and kallsyms_lookup_names. thanks, Alessandro > > Best Regards, > Petr
