Hi Alessandro, > On Aug 8, 2024, at 11:20 PM, Alessandro Carminati <alessandro.carminati@xxxxxxxxx> wrote: > > 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. 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@xxxxxxxxxx/T/#u
