linux-kernel - [PATCH v3] kallsyms: strip CLANG CFI postfix ".cfi

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Message-Id: <20210729205317.25685-1-treasure4paddy@gmail.com>
Date:   Thu, 29 Jul 2021 22:53:12 +0200
From:   Padmanabha Srinivasaiah <treasure4paddy@...il.com>
To:     jeyu@...nel.org, keescook@...omium.org, nathan@...nel.org,
        ndesaulniers@...gle.com
Cc:     samitolvanen@...gle.com, 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 v3] kallsyms: strip CLANG CFI postfix ".cfi_jt"

Clang CFI adds a postfix ".cfi_jt" to a symbols of extern functions.
For example 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 v3:
  - Modified commit message to indicate fix is for Clang CFI postfix
  - Rebased on recent patch from ndesaulniers@...gle.com.
  - 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 | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 5cabe4dd3ff4..67d015854cbd 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -174,7 +174,8 @@ 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) ||
+	      IS_ENABLED(CONFIG_LTO_CLANG_THIN)))
 		return false;
 
 	res = strstr(s, ".llvm.");
@@ -184,16 +185,24 @@ static bool cleanup_symbol_name(char *s)
 	}
 
 	/*
-	 * LLVM appends a hash to static function names when ThinLTO and CFI
-	 * are both enabled, i.e. foo() becomes
-	 * foo$707af9a22804d33c81801f27dcfe489b. This causes confusion and
-	 * potentially breaks user space tools, so we strip the suffix from
-	 * expanded symbol names.
+	 * LLVM appends a hash to static function names when both
+	 * (Thin/FULL) LTO and CFI are enabled, i.e. foo() becomes
+	 * foo$707af9a22804d33c81801f27dcfe489b.
+	 *
+	 * In case of non static function symbol <funcsym>,
+	 * the local jump table will have entry as <funcsym>.cfi_jt.
+	 *
+	 * This causes confusion and potentially breaks
+	 * user space tools and some built-in components.
+	 * So we strip the suffix from expanded symbol names.
 	 */
 	if (!IS_ENABLED(CONFIG_CFI_CLANG))
 		return false;
 
 	res = strrchr(s, '$');
+	if (!res)
+		res = strstr(s, ".cfi_jt");
+
 	if (res) {
 		*res = '\0';
 		return true;
-- 
2.17.1