linux-kernel - Re: [PATCH v3 0/2] Fix kallsyms with CONFIG_LTO

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Message-Id: <20240813132907.e6f392ec38bcc08e6796a014@kernel.org>
Date: Tue, 13 Aug 2024 13:29:07 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Song Liu <song@...nel.org>
Cc: live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, jpoimboe@...nel.org, jikos@...nel.org,
 mbenes@...e.cz, pmladek@...e.com, joe.lawrence@...hat.com,
 nathan@...nel.org, morbo@...gle.com, justinstitt@...gle.com,
 mcgrof@...nel.org, thunder.leizhen@...wei.com, kees@...nel.org,
 kernel-team@...a.com, mmaurer@...gle.com, samitolvanen@...gle.com,
 mhiramat@...nel.org, rostedt@...dmis.org
Subject: Re: [PATCH v3 0/2] Fix kallsyms with CONFIG_LTO_CLANG

On Wed,  7 Aug 2024 15:05:11 -0700
Song Liu <song@...nel.org> wrote:

> With CONFIG_LTO_CLANG, the compiler/linker adds .llvm.<hash> suffix to
> local symbols to avoid duplications. Existing scripts/kallsyms sorts
> symbols without .llvm.<hash> suffix. However, this causes quite some
> issues later on. Some users of kallsyms, such as livepatch, have to match
> symbols exactly.
> 
> Address this by sorting full symbols at build time, and let kallsyms
> lookup APIs to match the symbols exactly.
> 

I've tested this series and confirmed it makes kprobes work with llvm suffixed
symbols.

/sys/kernel/tracing # echo "p c_start.llvm.8011538628216713357" >> kprobe_events
 /sys/kernel/tracing # cat kprobe_events 
p:kprobes/p_c_start_llvm_8011538628216713357_0 c_start.llvm.8011538628216713357
/sys/kernel/tracing # echo "p c_start" >> kprobe_events 
/sys/kernel/tracing # cat kprobe_events 
p:kprobes/p_c_start_llvm_8011538628216713357_0 c_start.llvm.8011538628216713357
p:kprobes/p_c_start_0 c_start

And ftrace too.

/sys/kernel/tracing # grep ^c_start available_filter_functions
c_start.llvm.8011538628216713357
c_start
c_start.llvm.17132674095431275852

Tested-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

for this series.

> Changes v2 => v3:
> 1. Remove the _without_suffix APIs, as kprobe will not use them.
>    (Masami Hiramatsu)
> 
> v2: https://lore.kernel.org/live-patching/20240802210836.2210140-1-song@kernel.org/T/#u
> 
> Changes v1 => v2:
> 1. Update the APIs to remove all .XXX suffixes (v1 only removes .llvm.*).
> 2. Rename the APIs as *_without_suffix. (Masami Hiramatsu)
> 3. Fix another user from kprobe. (Masami Hiramatsu)
> 4. Add tests for the new APIs in kallsyms_selftests.
> 
> v1: https://lore.kernel.org/live-patching/20240730005433.3559731-1-song@kernel.org/T/#u
> 
> Song Liu (2):
>   kallsyms: Do not cleanup .llvm.<hash> suffix before sorting symbols
>   kallsyms: Match symbols exactly with CONFIG_LTO_CLANG
> 
>  kernel/kallsyms.c          | 55 +++++---------------------------------
>  kernel/kallsyms_selftest.c | 22 +--------------
>  scripts/kallsyms.c         | 31 ++-------------------
>  scripts/link-vmlinux.sh    |  4 ---
>  4 files changed, 9 insertions(+), 103 deletions(-)
> 
> --
> 2.43.5


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>