linux-kernel - [PATCH v4] kallsyms: strip CLANG CFI postfix ".cfi

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Message-Id: <20210805232741.9501-1-treasure4paddy@gmail.com>
Date:   Fri,  6 Aug 2021 01:27:38 +0200
From:   Padmanabha Srinivasaiah <treasure4paddy@...il.com>
To:     Jessica Yu <jeyu@...nel.org>, Kees Cook <keescook@...omium.org>,
        nathan@...nel.org, ndesaulniers@...gle.com, samitolvanen@...gle.com
Cc:     treasure4paddy@...il.com, Miroslav Benes <mbenes@...e.cz>,
        Stephen Boyd <swboyd@...omium.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Joe Perches <joe@...ches.com>, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: [PATCH v4] kallsyms: strip CLANG CFI postfix ".cfi_jt"

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@...il.com>
---
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@...gle.com.
    https://lore.kernel.org/lkml/
	20210707181814.365496-1-ndesaulniers@...gle.com/#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 | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 5cabe4dd3ff4..1b40bcf20fe6 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -174,13 +174,15 @@ 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.");
-	if (res) {
-		*res = '\0';
-		return true;
+	if (IS_ENABLED(CONFIG_LTO_CLANG_THIN)) {
+		res = strstr(s, ".llvm.");
+		if (res) {
+			*res = '\0';
+			return true;
+		}
 	}
 
 	/*
@@ -194,6 +196,17 @@ 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