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

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Date:   Tue, 27 Jun 2023 23:44:33 -0700
From:   Yonghong Song <yhs@...com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Petr Mladek <pmladek@...e.com>, Song Liu <song@...nel.org>
CC:     Fangrui Song <maskray@...gle.com>, <kernel-team@...com>,
        Leizhen <thunder.leizhen@...wei.com>,
        <linux-kernel@...r.kernel.org>, <llvm@...ts.linux.dev>
Subject: [PATCH] kallsyms: strip LTO-only suffixes from promoted global functions

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@kernel.org/T/#u

Fixes: 6eb4bd92c1ce ("kallsyms: strip LTO suffixes from static functions")
Signed-off-by: Yonghong Song <yhs@...com>
---
 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.");
 	if (res) {
 		*res = '\0';
 		return true;
-- 
2.34.1