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SubjectRe: [PATCH v4] kallsyms: strip CLANG CFI postfix ".cfi_jt"
On Thu, Aug 5, 2021 at 4:28 PM Padmanabha Srinivasaiah
<treasure4paddy@gmail.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@gmail.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@google.com.
> https://lore.kernel.org/lkml/
> 20210707181814.365496-1-ndesaulniers@google.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

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