Commit 8cc32a9b authored Jun 28, 2023 by Yonghong Song Committed by Kees Cook Jul 12, 2023

kallsyms: strip LTO-only suffixes from promoted global functions

Commit 6eb4bd92 ("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: 6eb4bd92 ("kallsyms: strip LTO suffixes from static functions")
Reported-by: Song Liu <song@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Reviewed-by: Zhen Lei <thunder.leizhen@huawei.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230628181926.4102448-1-yhs@fb.com


Signed-off-by: Kees Cook <keescook@chromium.org>

parent 5fc52248

kernel/kallsyms.c

+2 −3

Original line number	Diff line number	Diff line
		@@ -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;

scripts/kallsyms.c

+3 −3

Original line number	Diff line number	Diff line
		@@ -349,10 +349,10 @@ 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';
		}