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Date: Mon, 28 Jun 2021 14:19:52 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Nick Desaulniers <ndesaulniers@...gle.com>
Cc: Kees Cook <keescook@...omium.org>,
Fangrui Song <maskray@...gle.com>, "KE . LI" <like1@...o.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Stephen Rothwell <sfr@...b.auug.org.au>,
Miguel Ojeda <ojeda@...nel.org>, Joe Perches <joe@...ches.com>,
"Gustavo A. R. Silva" <gustavoars@...nel.org>,
Randy Dunlap <rdunlap@...radead.org>,
Sami Tolvanen <samitolvanen@...gle.com>,
Stephen Boyd <swboyd@...omium.org>,
linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH v3] kallsyms: strip LTO suffixes from static functions
On Mon, Jun 28, 2021 at 01:31:06PM -0700, Nick Desaulniers wrote:
> 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 v2 -> V3:
> * Un-nest preprocessor checks, as per Nathan.
>
> 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 | 32 +++++++++++++++++++++++++++++---
> 1 file changed, 29 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> index 4067564ec59f..143c69e7e75d 100644
> --- a/kernel/kallsyms.c
> +++ b/kernel/kallsyms.c
> @@ -171,6 +171,26 @@ static unsigned long kallsyms_sym_address(int idx)
> return kallsyms_relative_base - 1 - kallsyms_offsets[idx];
> }
>
> +#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;
> +}
> +#else
> +static bool cleanup_symbol_name_thinlto(char *s) { return false; }
> +#endif /* CONFIG_LTO_CLANG_THIN */
> +
> #if defined(CONFIG_CFI_CLANG) && defined(CONFIG_LTO_CLANG_THIN)
> /*
> * LLVM appends a hash to static function names when ThinLTO and CFI are
> @@ -178,7 +198,7 @@ static unsigned long kallsyms_sym_address(int idx)
> * 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 +209,14 @@ 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 && 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
>
Is there any reason that we cannot eliminate the stubs and combine the
functions, or am I missing something? Completely untested diff.
diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index c851ca0ed357..014b59ad68a3 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -161,26 +161,36 @@ 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)
-/*
- * LLVM appends a hash to static function names when ThinLTO and CFI are
- * both enabled, i.e. foo() becomes foo$707af9a22804d33c81801f27dcfe489b.
- * This causes confusion and potentially breaks user space tools, so we
- * strip the suffix from expanded symbol names.
- */
-static inline bool cleanup_symbol_name(char *s)
+static inline bool cleanup_symbol_name_thinlto(char *s)
{
char *res;
- res = strrchr(s, '$');
+ if (!IS_ENABLED(CONFIG_LTO_CLANG_THIN))
+ return false;
+
+ /*
+ * LLVM appends a suffix for local variables that must be promoted to global
+ * scope as part of ThinLTO. foo() becomes foo.llvm.974640843467629774. This
+ * can break hooking of static functions with kprobes.
+ */
+ res = strstr(s, ".llvm.");
if (res)
*res = '\0';
+ /*
+ * LLVM appends a hash to static function names when ThinLTO and CFI are
+ * both enabled, i.e. foo() becomes foo$707af9a22804d33c81801f27dcfe489b.
+ * This causes confusion and potentially breaks user space tools, so we
+ * strip the suffix from expanded symbol names.
+ */
+ if (IS_ENABLED(CONFIG_CFI_CLANG)) {
+ res = strrchr(s, '$');
+ if (res)
+ *res = '\0';
+ }
+
return res != NULL;
}
-#else
-static inline bool cleanup_symbol_name(char *s) { return false; }
-#endif
/* Lookup the address for this symbol. Returns 0 if not found. */
unsigned long kallsyms_lookup_name(const char *name)
@@ -195,7 +205,7 @@ unsigned long kallsyms_lookup_name(const char *name)
if (strcmp(namebuf, name) == 0)
return kallsyms_sym_address(i);
- if (cleanup_symbol_name(namebuf) && strcmp(namebuf, name) == 0)
+ if (cleanup_symbol_name_thinlto(namebuf) && strcmp(namebuf, name) == 0)
return kallsyms_sym_address(i);
}
return module_kallsyms_lookup_name(name);
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