Re: [PATCH kbuild v2] kbuild: Reduce the number of compiler-generated suffixes for clang thin-lto build

[Nathan Chancellor <nathan@kernel.org>]

On Fri, Mar 06, 2026 at 09:02:50PM -0800, Yonghong Song wrote:
> The current clang thin-lto build often produces lots of symbols with
> suffix. The following is a partial list of such function call symbols:
>     ...
>     ethnl_module_fw_flash_ntf.llvm.7631589765585346066
>     __nf_conntrack_alloc.llvm.6438426151906658917
>     tcp_can_early_drop.llvm.11937612064648250727
>     tcp_print_conntrack.llvm.11937612064648250727
>     ...
> 
> In my particular build with current bpf-next, the number of '*.llvm.<hash>'
> function calls is 1212. As the side effect of cross-file inlining,
> some static variables may be promoted with '*.llvm.<hash>' as well.
> In my same setup, the number of variables with such suffixes is 9.
> 
> Such symbols make kernel live patching difficult since
>   - a minor code change will change the hash and then the '*.llvm.<hash>'
>     symbol becomes another one with a different hash. Sometimes, maybe
>     the suffix is gone.
>   - a previous source-level symbol may become a one with suffix after live
>     patching code.
> 
> In [1], Song Liu suggested to reduce the number of '*.llvm.<hash>' functions
> to make live patch easier. In respond of this, I implemented this
> in llvm ([2]). The same thin-lto build with [2] only has two symbols with
> suffix:
>     m_stop.llvm.14460341347352036579
>     m_next.llvm.14460341347352036579
> This should make live patch much easier.
> 
> To support suffix symbol reduction, two lld flags are necessary to enable
> this feature in kernel:
>     - Flag '--lto-whole-program-visibility' is needed as it ensures that all
>       non-assembly files are available in the same thin-lto lld, which is true
>       for kernel.
>     - Flag '-mllvm -always-rename-promoted-locals=false' is needed to enable
>       suffix reduction. Currently in llvm [1], only process mode is supported.
>       There is another distributed mode (across different processes or even
>       different machines) which is not supported yet ([2]). The kernel uses
>       process mode so it should work.
> 
> The assembly files may have some global functions/data which may potentially
> conflict with thin-lto global symbols after the above two flags. But such assembly
> global symbols are limited and tend to be uniquely named for its context.
> Hence the conflict with globals in non-assembly codes is rare. If indeed the
> conflict happens, we can rename either of them to avoid conflicts.
> 
> Nathan Chancellor suggested the following under thin-lto:
>   KBUILD_LDFLAGS += $(call ld-option,--lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false)
> The '-mllvm -always-rename-promoted-locals=false' flag is only available for llvm23.
> So for llvm22 or earlier, the above KBUILD_LDFLAGS will ignore those two flags.
> For llvm23 and later, two flags will be added to KBUILD_LDFLAGS.
> 
>   [1] https://lpc.events/event/19/contributions/2212
>   [2] https://github.com/llvm/llvm-project/pull/178587
> 
> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org> # build

Thanks!

> ---
>  Makefile | 1 +
>  1 file changed, 1 insertion(+)
> 
> Changelog:
>   v1 -> v2:
>     - v1: https://lore.kernel.org/linux-kbuild/20260306034325.3605301-1-yonghong.song@linux.dev/
>     - Removed the new config option and use ld-option to check whether new flags
>       will be used or not.
> 
> diff --git a/Makefile b/Makefile
> index e944c6e71e81..e4385af16985 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1034,6 +1034,7 @@ endif
>  ifdef CONFIG_LTO_CLANG
>  ifdef CONFIG_LTO_CLANG_THIN
>  CC_FLAGS_LTO	:= -flto=thin -fsplit-lto-unit
> +KBUILD_LDFLAGS += $(call ld-option,--lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false)
>  else
>  CC_FLAGS_LTO	:= -flto
>  endif
> -- 
> 2.47.3
>