[PATCH kbuild] kbuild: Allow to reduce the number of suffixes for clang thin-lto build

[Yonghong Song <yonghong.song@linux.dev>]

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. 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 or no hash, and
  - a previous source-level symbol may become an 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, a new config
    LTO_CLANG_THIN_SUFFIX_REDUCTION
is introduced and the config depends on thin-lto and llvm23 or higher.

Two lld flags are necessary to enable this feature in kernel:
    - Flag '--lto-whole-program-visibility' is needed as it ensures that all
      modules are available in the same process, which is true for kernel at
      thin-lto lld.
    - 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]).

  [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>
---
 Makefile     |  3 +++
 arch/Kconfig | 15 +++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/Makefile b/Makefile
index e944c6e71e81..9d6033595615 100644
--- a/Makefile
+++ b/Makefile
@@ -1034,6 +1034,9 @@ endif
 ifdef CONFIG_LTO_CLANG
 ifdef CONFIG_LTO_CLANG_THIN
 CC_FLAGS_LTO	:= -flto=thin -fsplit-lto-unit
+ifdef CONFIG_LTO_CLANG_THIN_SUFFIX_REDUCTION
+KBUILD_LDFLAGS += --lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false
+endif
 else
 CC_FLAGS_LTO	:= -flto
 endif
diff --git a/arch/Kconfig b/arch/Kconfig
index 102ddbd4298e..e1db64a3284e 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -861,8 +861,23 @@ config LTO_CLANG_THIN
 	    https://clang.llvm.org/docs/ThinLTO.html
 
 	  If unsure, say Y.
+
 endchoice
 
+config LTO_CLANG_THIN_SUFFIX_REDUCTION
+	bool "Clang ThinLTO Suffix Reduction (EXPERIMENTAL)"
+	depends on LTO_CLANG_THIN
+	depends on CLANG_VERSION >= 230000
+	default y
+	help
+	  This option allows to reduce the number of symbols with
+	  '.llvm.<hash' suffixes. This can help KLP (kernel living
+	  patch) as symbol name can stay stable in most cases.
+
+	  See https://github.com/llvm/llvm-project/pull/178587
+
+	  If unsure, say N.
+
 config ARCH_SUPPORTS_AUTOFDO_CLANG
 	bool
 
-- 
2.47.3