[PATCH v5] kallsyms: strip CLANG CFI postfix ".cfi_jt"

From: Padmanabha Srinivasaiah
Date: Sat Aug 14 2021 - 08:43:03 EST


Clang CFI adds a postfix ".cfi_jt" to a symbols of extern functions.
For e.g. this breaks syscall tracer that doesn't expect such postfix,
so strip out the postfix from the expanded symbol.

Signed-off-by: Padmanabha Srinivasaiah <treasure4paddy@xxxxxxxxx>
---
Change in v5:
- Also remove explcit config check for postfix ".llvm." as LLVM
renames even in full LTO case

Change in v4:
- Remove redundant check; irrespective of LTO type (THIN/FULL),
LTO_CLANG will be always enabled. Hence will be used as entry flag
to check various postfix patterns.
- And prior to stripping postfix ".cfi_jt", added a comment to
justify why we are doing so.

Change in v3:
- Modified commit message to indicate fix is for Clang CFI postfix
- Rebased on recent patch from ndesaulniers@xxxxxxxxxx.
https://lore.kernel.org/lkml/
20210707181814.365496-1-ndesaulniers@xxxxxxxxxx/#t
- Fix is enabled even for CONFIG_LTO_CLANG

Change in v2:
- Use existing routine in kallsyms to strip postfix ".cfi_jt" from
extern function name.
- Modified the commit message accordingly

kernel/kallsyms.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 5cabe4dd3ff4..c8ef618e2a71 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -174,7 +174,7 @@ static bool cleanup_symbol_name(char *s)
* foo.llvm.974640843467629774. This can break hooking of static
* functions with kprobes.
*/
- if (!IS_ENABLED(CONFIG_LTO_CLANG_THIN))
+ if (!IS_ENABLED(CONFIG_LTO_CLANG))
return false;

res = strstr(s, ".llvm.");
@@ -194,6 +194,17 @@ static bool cleanup_symbol_name(char *s)
return false;

res = strrchr(s, '$');
+ if (!res) {
+ /*
+ * In case of non static function symbol <funcsym>,
+ * the local jump table will have entry as <funcsym>.cfi_jt.
+ *
+ * Such expansion breaks some built-in components,
+ * e.g. syscall tracer. Hence remove postfix ".cfi_jt".
+ */
+ res = strstr(s, ".cfi_jt");
+ }
+
if (res) {
*res = '\0';
return true;
--
2.17.1