linux-kernel - Re: [PATCH v4] kallsyms: strip CLANG CFI postfix ".cfi

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Date:   Thu, 12 Aug 2021 16:05:25 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Padmanabha Srinivasaiah <treasure4paddy@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Jessica Yu <jeyu@...nel.org>, Kees Cook <keescook@...omium.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Stephen Boyd <swboyd@...omium.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Joe Perches <joe@...ches.com>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v4] kallsyms: strip CLANG CFI postfix ".cfi_jt"

On Thu, Aug 5, 2021 at 4:28 PM Padmanabha Srinivasaiah
<treasure4paddy@...il.com> wrote:
>
> Clang CFI adds a postfix ".cfi_jt" to a symbols of extern functions.
> For e.g. 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 v4:
>   - Remove redundant check; irrespective of LTO type (THIN/FULL),
>     LTO_CLANG will be always enabled. Hence will be used as entry flag
>     to check various postfix patterns.
>   - And prior to stripping postfix ".cfi_jt", added a comment to
>     justify why we are doing so.
>
> Change in v3:
>   - Modified commit message to indicate fix is for Clang CFI postfix
>   - Rebased on recent patch from ndesaulniers@...gle.com.
>     https://lore.kernel.org/lkml/
>         20210707181814.365496-1-ndesaulniers@...gle.com/#t
>   - 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 | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> index 5cabe4dd3ff4..1b40bcf20fe6 100644
> --- a/kernel/kallsyms.c
> +++ b/kernel/kallsyms.c
> @@ -174,13 +174,15 @@ 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))
>                 return false;
>
> -       res = strstr(s, ".llvm.");
> -       if (res) {
> -               *res = '\0';
> -               return true;
> +       if (IS_ENABLED(CONFIG_LTO_CLANG_THIN)) {
> +               res = strstr(s, ".llvm.");
> +               if (res) {
> +                       *res = '\0';
> +                       return true;
> +               }
>         }

I confirmed that LLVM renames these also with full LTO, so the config
check can be dropped here.

>
>         /*
> @@ -194,6 +196,17 @@ static bool cleanup_symbol_name(char *s)
>                 return false;
>
>         res = strrchr(s, '$');
> +       if (!res) {
> +               /*
> +                * In case of non static function symbol <funcsym>,
> +                * the local jump table will have entry as <funcsym>.cfi_jt.
> +                *
> +                * Such expansion breaks some built-in components,
> +                * e.g. syscall tracer. Hence remove postfix ".cfi_jt".
> +                */
> +               res = strstr(s, ".cfi_jt");
> +       }
> +
>         if (res) {
>                 *res = '\0';
>                 return true;

Otherwise, the logic looks pretty good to me. Nick, are you planning
to resend your earlier patch? Should this be just folded into the next
version?

Sami