[PATCH] rust: cfi: disable function merging if CFI is enabled

[Gary Guo <gary@kernel.org>]

From: Gary Guo <gary@garyguo.net>

In Rust doc tests, there is a dummy `__module_firmware_test_init` function
generated by the example in `module_firmware!`'s documentation, which just
returns zero. Many other documentation generates function that produces
zero. LKP test robot reports a `Flags::zeroed` instance; my local
reproduction has a `Bounded::new::<0>`.

LLVM's MergeFunctionsPass incorrectly merge functions with the same KCFI
type, causing `__module_firmware_test_init` being merged into one of the
zero-returning function. As module init is invoked via indirect function
call, KCFI is checked and this produces a KCFI failure.

I've reported this bug to upstream LLVM [1]; in the mean time, disable
function merging if CFI is enabled. No separate treatment is needed for
CONFIG_RUST_INLINE_HELPERS, as Clang does not enable function merging by
default.

Reported-by: kernel test robot <yi1.lai@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202608201017.100a4511-lkp@intel.com
Link: https://github.com/llvm/llvm-project/issues/217629 [1]
Signed-off-by: Gary Guo <gary@garyguo.net>
---
 Makefile | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 4fee5ee9e9f4..0dd9a6eda739 100644
--- a/Makefile
+++ b/Makefile
@@ -1118,7 +1118,8 @@ endif
 ifdef CONFIG_RUST
 	# Always pass -Zsanitizer-cfi-normalize-integers as CONFIG_RUST selects
 	# CONFIG_CFI_ICALL_NORMALIZE_INTEGERS.
-	RUSTC_FLAGS_CFI   := -Zsanitizer=kcfi -Zsanitizer-cfi-normalize-integers
+	# Disable function merging as LLVM incorrectly merge functions with the same KCFI type.
+	RUSTC_FLAGS_CFI   := -Zsanitizer=kcfi -Zsanitizer-cfi-normalize-integers -Zmerge-functions=disabled
 	KBUILD_RUSTFLAGS += $(RUSTC_FLAGS_CFI)
 	export RUSTC_FLAGS_CFI
 endif

base-commit: e6664f2b33db9b6811eb4cec109f06cb2b4f458d
-- 
2.54.0