linux-kernel - [PATCH v2] kallsyms: strip LTO suffixes from static functions

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Message-Id: <20210628190509.2486992-1-ndesaulniers@google.com>
Date:   Mon, 28 Jun 2021 12:05:06 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Fangrui Song <maskray@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "KE . LI" <like1@...o.com>, Nathan Chancellor <nathan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Miroslav Benes <mbenes@...e.cz>, Jessica Yu <jeyu@...nel.org>,
        Joe Perches <joe@...ches.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Stephen Boyd <swboyd@...omium.org>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: [PATCH v2] kallsyms: strip LTO suffixes from static functions

Similar to:
commit 8b8e6b5d3b01 ("kallsyms: strip ThinLTO hashes from static
functions")

It's very common for compilers to modify the symbol name for static
functions as part of optimizing transformations. That makes hooking
static functions (that weren't inlined or DCE'd) with kprobes difficult.

LLVM has yet another name mangling scheme used by thin LTO.  Strip off
these suffixes so that we can continue to hook such static functions.

Reported-by: KE.LI(Lieke) <like1@...o.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
Changes v1 -> v2:
* Both mangling schemes can occur for thinLTO + CFI, this new scheme can
  also occur for thinLTO without CFI. Split cleanup_symbol_name() into
  two function calls.
* Drop KE.LI's tested by tag.
* Do not carry Fangrui's Reviewed by tag.
* Drop the inline keyword; it is meaningless.

 kernel/kallsyms.c | 33 +++++++++++++++++++++++++++++----
 1 file changed, 29 insertions(+), 4 deletions(-)

diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 4067564ec59f..fbce4a1ec700 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -171,14 +171,30 @@ static unsigned long kallsyms_sym_address(int idx)
 	return kallsyms_relative_base - 1 - kallsyms_offsets[idx];
 }
 
-#if defined(CONFIG_CFI_CLANG) && defined(CONFIG_LTO_CLANG_THIN)
+#ifdef CONFIG_LTO_CLANG_THIN
+/*
+ * LLVM appends a suffix for local variables that must be promoted to global
+ * scope as part of thin LTO. foo() becomes foo.llvm.974640843467629774. This
+ * can break hooking of static functions with kprobes.
+ */
+static bool cleanup_symbol_name_thinlto(char *s)
+{
+	char *res;
+
+	res = strstr(s, ".llvm.");
+	if (res)
+		*res = '\0';
+
+	return res != NULL;
+}
+#ifdef CONFIG_CFI_CLANG
 /*
  * 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.
  */
-static inline bool cleanup_symbol_name(char *s)
+static bool cleanup_symbol_name_thinlto_cfi(char *s)
 {
 	char *res;
 
@@ -189,8 +205,17 @@ static inline bool cleanup_symbol_name(char *s)
 	return res != NULL;
 }
 #else
-static inline bool cleanup_symbol_name(char *s) { return false; }
-#endif
+static bool cleanup_symbol_name_thinlto_cfi(char *s) { return false; }
+#endif /* CONFIG_CFI_CLANG */
+#else
+static bool cleanup_symbol_name_thinlto(char *s) { return false; }
+#endif /* CONFIG_LTO_CLANG_THIN */
+
+static bool cleanup_symbol_name(char *s)
+{
+	return cleanup_symbol_name_thinlto(s) &&
+		cleanup_symbol_name_thinlto_cfi(s);
+}
 
 /* Lookup the address for this symbol. Returns 0 if not found. */
 unsigned long kallsyms_lookup_name(const char *name)
-- 
2.32.0.93.g670b81a890-goog