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

[Yonghong Song]

On 6/28/23 4:34 AM, Leizhen (ThunderTown) wrote:
> On 2023/6/28 14:44, Yonghong Song 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.
> >
> >   [1] https://lore.kernel.org/live-patching/20230615170048.2382735-1-song@xxxxxxxxxx/T/#u
>
> Missed the addition of:
> Reported-by: Song Liu <song@xxxxxxxxxx>

Will add in the next revision.

> > Fixes: 6eb4bd92c1ce ("kallsyms: strip LTO suffixes from static functions")
> > Signed-off-by: Yonghong Song <yhs@xxxxxx>
> > ---
> >   kernel/kallsyms.c | 5 ++---
> >   1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > 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.");
>
> We'd better modify function cleanup_symbol_name() in scripts/kallsyms.c accordingly.

Thanks, will make corresponding change in scripts/kallsyms.c as well.

> >   	if (res) {
> >   		*res = '\0';
> >   		return true;