linux-kernel - Re: [PATCH v4] kallsyms: strip LTO suffixes from static functions

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Date:   Fri, 6 Aug 2021 09:20:54 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Fangrui Song <maskray@...gle.com>, "KE . LI" <like1@...o.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Miguel Ojeda <ojeda@...nel.org>, Joe Perches <joe@...ches.com>,
        Stephen Boyd <swboyd@...omium.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH v4] kallsyms: strip LTO suffixes from static functions

On Wed, Jul 7, 2021 at 11:18 AM Nick Desaulniers
<ndesaulniers@...gle.com> 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>
> Suggested-by: Nathan Chancellor <nathan@...nel.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
> Changes v3 -> v4:
> * Convert this function to use IS_ENABLED rather than provide multiple
>   definitions based on preprocessor checks.
> * Add Nathan's suggested-by.
>
> 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 | 43 ++++++++++++++++++++++++++++++-------------
>  1 file changed, 30 insertions(+), 13 deletions(-)
>
> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> index 4067564ec59f..a10dab216f4f 100644
> --- a/kernel/kallsyms.c
> +++ b/kernel/kallsyms.c
> @@ -171,26 +171,43 @@ 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 bool cleanup_symbol_name(char *s)
>  {
>         char *res;
>
> +       /*
> +        * 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.
> +        */
> +       if (!IS_ENABLED(CONFIG_LTO_CLANG_THIN))
> +               return false;
> +
> +       res = strstr(s, ".llvm.");
> +       if (res) {
> +               *res = '\0';
> +               return true;
> +       }
> +
> +       /*
> +        * LLVM appends a hash to static function names when ThinLTO and CFI
> +        * are both enabled, i.e. foo() becomes
> +        * foo$707af9a22804d33c81801f27dcfe489b. This causes confusion and
> +        * potentially breaks user space tools, so we strip the suffix from
> +        * expanded symbol names.
> +        */
> +       if (!IS_ENABLED(CONFIG_CFI_CLANG))
> +               return false;
> +
>         res = strrchr(s, '$');
> -       if (res)
> +       if (res) {
>                 *res = '\0';
> +               return true;
> +       }

Note that starting with https://reviews.llvm.org/D97484, the hash
separator is '.' instead of '$'. It looks like this change will be in
Clang 13.

Sami