Messages in this thread | | | Date | Fri, 7 Jul 2023 17:23:04 -0700 | Subject | Re: [PATCH v2] kallsyms: strip LTO-only suffixes from promoted global functions | From | Yonghong Song <> |
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On 7/7/23 3:27 PM, Nick Desaulniers wrote: > 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>
Nick, thanks for the review. I am not sure how this patch could be merged into the mainline. I checked your patch 6eb4bd92c1ce ("kallsyms: strip LTO suffixes from static functions") and it looks like the patch was merged by Kees Cook.
I added Kees in the 'To' list. Kees, could you help merge this patch if you are okay with the change?
Thanks!
Yonghong
> >> --- >> 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 >> > >
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