From 29b03d1de967a177176d12811d970cac7882afcf Mon Sep 17 00:00:00 2001 From: Gary Guo Date: Thu, 20 Aug 2026 14:57:32 +0100 Subject: [PATCH] rust: cfi: disable function merging if CFI is enabled 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 examples generate functions that produce zero. LKP test robot reports [1] a `Flags::zeroed` instance; my local reproduction has a `Bounded::new::<0>`. LLVM's MergeFunctionsPass incorrectly merges functions with different KCFI types, causing `__module_firmware_test_init` to be merged into one of the zero-returning functions. As module init is invoked via an indirect function call, KCFI is checked and this produces a KCFI failure. I've reported this bug to upstream LLVM [2]; in the meantime, 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. [ LLVM already has a pending PR: https://github.com/llvm/llvm-project/pull/217665 which solves the issue. In addition, I asked upstream Rust if the unstable `-Zmerge-functions=disabled` flag will remain around: https://rust-lang.zulipchat.com/#narrow/channel/425075-rust-for-linux/topic/.60-Zmerge-functions.3Ddisabled.60/ and it does indeed look like that will be the case. - Miguel ] Reported-by: kernel test robot Closes: https://lore.kernel.org/oe-lkp/202608201017.100a4511-lkp@intel.com [1] Link: https://github.com/llvm/llvm-project/issues/217629 [2] Signed-off-by: Gary Guo Cc: stable@vger.kernel.org Fixes: ca627e636551 ("rust: cfi: add support for CFI_CLANG with Rust") Reviewed-by: Sami Tolvanen Link: https://patch.msgid.link/20260820135733.37121-1-gary@kernel.org [ Fixed typos as discussed. Reworded slightly for other typos. - Miguel ] Signed-off-by: Miguel Ojeda --- Makefile | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/Makefile b/Makefile index 130926fa592e..c2344755a1bb 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 merges functions with different KCFI types. + RUSTC_FLAGS_CFI := -Zsanitizer=kcfi -Zsanitizer-cfi-normalize-integers -Zmerge-functions=disabled KBUILD_RUSTFLAGS += $(RUSTC_FLAGS_CFI) export RUSTC_FLAGS_CFI endif