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 <[email protected]>
---
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
