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 <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: ca627e636551 ("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>

0 files changed, 0 insertions, 0 deletions