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From: Sami Tolvanen <samitolvanen@google.com>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Kees Cook <keescook@chromium.org>,
	Fangrui Song <maskray@google.com>, "KE . LI" <like1@oppo.com>,
	Nathan Chancellor <nathan@kernel.org>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	Andrew Morton <akpm@linux-foundation.org>,
	Miroslav Benes <mbenes@suse.cz>, Miguel Ojeda <ojeda@kernel.org>,
	Joe Perches <joe@perches.com>, Stephen Boyd <swboyd@chromium.org>,
	"Gustavo A. R. Silva" <gustavoars@kernel.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com
Subject: Re: [PATCH v4] kallsyms: strip LTO suffixes from static functions
Date: Fri, 6 Aug 2021 09:20:54 -0700	[thread overview]
Message-ID: <CABCJKue5Ay6_+8sibzh5wRh3gPzV1g72gJ9m2ot4E1ezj8bpHA@mail.gmail.com> (raw)
In-Reply-To: <20210707181814.365496-1-ndesaulniers@google.com>

On Wed, Jul 7, 2021 at 11:18 AM Nick Desaulniers
<ndesaulniers@google.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@oppo.com>
> Suggested-by: Nathan Chancellor <nathan@kernel.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.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

  parent reply	other threads:[~2021-08-06 16:21 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-22 18:38 [PATCH] " Nick Desaulniers
2021-06-22 20:18 ` Fangrui Song
2021-06-28 17:54   ` Nick Desaulniers
2021-06-28 18:20     ` Nick Desaulniers
2021-06-28 19:05   ` [PATCH v2] " Nick Desaulniers
2021-06-28 19:45     ` Nathan Chancellor
2021-06-28 20:31       ` [PATCH v3] " Nick Desaulniers
2021-06-28 21:19         ` Nathan Chancellor
2021-06-28 22:01           ` Nick Desaulniers
2021-06-28 22:16             ` Nathan Chancellor
2021-07-07 18:18               ` [PATCH v4] " Nick Desaulniers
2021-07-07 18:34                 ` Nathan Chancellor
2021-07-07 18:59                   ` Fāng-ruì Sòng
2021-08-06 16:20                 ` Sami Tolvanen [this message]
2021-10-01 19:58                   ` [PATCH v5] " Nick Desaulniers
2021-10-01 20:05                     ` Sami Tolvanen
2021-10-04 10:46                       ` Padmanabha Srinivasaiah

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