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Message-ID: <20230628181926.4102448-1-yhs@fb.com>
Date: Wed, 28 Jun 2023 11:19:26 -0700
From: Yonghong Song <yhs@...com>
To: Nick Desaulniers <ndesaulniers@...gle.com>,
Petr Mladek <pmladek@...e.com>, Song Liu <song@...nel.org>,
Steven Rostedt <rostedt@...dmis.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 v2] 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.
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@...nel.org>
Signed-off-by: Yonghong Song <yhs@...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@...nel.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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