Makefile | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
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
--
2.54.0On Thu, Aug 20, 2026 at 3:57 PM Gary Guo <gary@kernel.org> wrote:
>
> 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>
[ 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 ]
[ Fixed typos as discussed. Reworded slightly for other typos. - Miguel ]
Link: https://github.com/llvm/llvm-project/issues/217629 [2]
Cc: stable@vger.kernel.org
Fixes: ca627e636551 ("rust: cfi: add support for CFI_CLANG with Rust")
Cheers,
Miguel
On Thu Aug 20, 2026 at 2:57 PM BST, Gary Guo wrote: > 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 This should read "with different KCFI type" (obviously). Best, Gary > 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
On Thu, Aug 20, 2026 at 9:06 AM Gary Guo <gary@garyguo.net> wrote: > > On Thu Aug 20, 2026 at 2:57 PM BST, Gary Guo wrote: > > 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 > > This should read "with different KCFI type" (obviously). Also the comment in the Makefile. With both instances fixed: Reviewed-by: Sami Tolvanen <samitolvanen@google.com> The LLVM fix itself is trivial, and it looks like a pull request for it already exists. I assume we'll want to drop this flag for fixed LLVM versions when they're available. Sami
On Fri, Aug 21, 2026 at 1:43 AM Sami Tolvanen <samitolvanen@google.com> wrote:
>
> The LLVM fix itself is trivial, and it looks like a pull request for
> it already exists. I assume we'll want to drop this flag for fixed
> LLVM versions when they're available.
I will link it in the commit message when I pick it up soon -- already
did in https://github.com/Rust-for-Linux/linux/issues/1132.
I also added the `-Z` flag to https://github.com/Rust-for-Linux/linux/issues/2.
The `-Zmerge-functions` flag is internal but has been there since Rust
1.34.0 in upstream commit:
b91d211b4030 ("Add a target option "merge-functions" taking values
in ("disabled", "trampolines", or "aliases (the default)) to allow
targets to opt out of the MergeFunctions LLVM pass. Also add a
corresponding -Z option with the same name and values.")
so that should be fine, but I will ping upstream Rust so that they are
aware we are using it.
And, yeah, agreed, we probably should remove the flag conditionally
later on, to confirm the bug is gone etc.
Cheers,
Miguel
© 2016 - 2026 Red Hat, Inc.