Re: [PATCH v2] kallsyms: strip LTO-only suffixes from promoted global functions

[Nick Desaulniers <ndesaulniers@google.com>]

On Wed, Jun 28, 2023 at 11:19 AM Yonghong Song <yhs@fb.com> wrote:
>
> Commit 6eb4bd92c1ce ("kallsyms: strip LTO suffixes from static functions")
> stripped all function/variable suffixes started with '.' regardless
> of whether those suffixes are generated at LTO mode or not. In fact,
> as far as I know, in LTO mode, when a static function/variable is
> promoted to the global scope, '.llvm.<...>' suffix is added.
>
> The existing mechanism breaks live patch for a LTO kernel even if
> no <symbol>.llvm.<...> symbols are involved. For example, for the following
> kernel symbols:
>   $ grep bpf_verifier_vlog /proc/kallsyms
>   ffffffff81549f60 t bpf_verifier_vlog
>   ffffffff8268b430 d bpf_verifier_vlog._entry
>   ffffffff8282a958 d bpf_verifier_vlog._entry_ptr
>   ffffffff82e12a1f d bpf_verifier_vlog.__already_done
> 'bpf_verifier_vlog' is a static function. '_entry', '_entry_ptr' and
> '__already_done' are static variables used inside 'bpf_verifier_vlog',
> so llvm promotes them to file-level static with prefix 'bpf_verifier_vlog.'.
> Note that the func-level to file-level static function promotion also
> happens without LTO.
>
> Given a symbol name 'bpf_verifier_vlog', with LTO kernel, current mechanism will
> return 4 symbols to live patch subsystem which current live patching
> subsystem cannot handle it. With non-LTO kernel, only one symbol
> is returned.
>
> In [1], we have a lengthy discussion, the suggestion is to separate two
> cases:
>   (1). new symbols with suffix which are generated regardless of whether
>        LTO is enabled or not, and
>   (2). new symbols with suffix generated only when LTO is enabled.
>
> The cleanup_symbol_name() should only remove suffixes for case (2).
> Case (1) should not be changed so it can work uniformly with or without LTO.
>
> This patch removed LTO-only suffix '.llvm.<...>' so live patching and
> tracing should work the same way for non-LTO kernel.
> The cleanup_symbol_name() in scripts/kallsyms.c is also changed to have the same
> filtering pattern so both kernel and kallsyms tool have the same
> expectation on the order of symbols.
>
>  [1] https://lore.kernel.org/live-patching/20230615170048.2382735-1-song@kernel.org/T/#u
>
> Fixes: 6eb4bd92c1ce ("kallsyms: strip LTO suffixes from static functions")
> Reported-by: Song Liu <song@kernel.org>
> Signed-off-by: Yonghong Song <yhs@fb.com>

Thanks for the patch and improving live patch with LTO.  Looking back
at the internal report that resulted in
commit 6eb4bd92c1ce ("kallsyms: strip LTO suffixes from static functions")
your version was what I originally had.  I did not leave a comment as
to why I changed it when I sent it 2 years ago, and no longer recall
the reason.


Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
>  kernel/kallsyms.c  | 5 ++---
>  scripts/kallsyms.c | 6 +++---
>  2 files changed, 5 insertions(+), 6 deletions(-)
>
> Changelogs:
>   v1 -> v2:
>     . add 'Reported-by: Song Liu <song@kernel.org>'
>     . also fix in scripts/kallsyms.c.
>
> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> index 77747391f49b..4874508bb950 100644
> --- a/kernel/kallsyms.c
> +++ b/kernel/kallsyms.c
> @@ -174,11 +174,10 @@ static bool cleanup_symbol_name(char *s)
>          * LLVM appends various suffixes for local functions and variables that
>          * must be promoted to global scope as part of LTO.  This can break
>          * hooking of static functions with kprobes. '.' is not a valid
> -        * character in an identifier in C. Suffixes observed:
> +        * character in an identifier in C. Suffixes only in LLVM LTO observed:
>          * - foo.llvm.[0-9a-f]+
> -        * - foo.[0-9a-f]+
>          */
> -       res = strchr(s, '.');
> +       res = strstr(s, ".llvm.");
>         if (res) {
>                 *res = '\0';
>                 return true;
> diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
> index 0d2db41177b2..13af6d0ff845 100644
> --- a/scripts/kallsyms.c
> +++ b/scripts/kallsyms.c
> @@ -346,10 +346,10 @@ static void cleanup_symbol_name(char *s)
>          * ASCII[_]   = 5f
>          * ASCII[a-z] = 61,7a
>          *
> -        * As above, replacing '.' with '\0' does not affect the main sorting,
> -        * but it helps us with subsorting.
> +        * As above, replacing the first '.' in ".llvm." with '\0' does not
> +        * affect the main sorting, but it helps us with subsorting.
>          */
> -       p = strchr(s, '.');
> +       p = strstr(s, ".llvm.");
>         if (p)
>                 *p = '\0';
>  }
> --
> 2.34.1
>


-- 
Thanks,
~Nick Desaulniers