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rust: cfi: disable function merging if CFI is enabled

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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:🆕:<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 <yi1.lai@intel.com>
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 <gary@garyguo.net>
Cc: stable@vger.kernel.org
Fixes: ca627e6365 ("rust: cfi: add support for CFI_CLANG with Rust")
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
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 <ojeda@kernel.org>
This commit is contained in:
Gary Guo
2026-08-20 14:57:32 +01:00
committed by Miguel Ojeda
parent dea1bf3814
commit 29b03d1de9
1 changed files with 2 additions and 1 deletions
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3
Makefile
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@@ -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