Messages in this thread | | | From | Nick Desaulniers <> | Date | Fri, 7 Jul 2023 15:27:23 -0700 | Subject | Re: [PATCH v2] kallsyms: strip LTO-only suffixes from promoted global functions |
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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
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