kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 45 insertions, 0 deletions

diff --git a/arch/m68k/Kconfig.debug b/arch/m68k/Kconfig.debug
new file mode 100644
index 000000000..d036f9038
--- /dev/null
+++ b/arch/m68k/Kconfig.debug
@@ -0,0 +1,45 @@
+# SPDX-License-Identifier: GPL-2.0
+
+config BOOTPARAM
+	bool "Compiled-in Kernel Boot Parameter"
+
+config BOOTPARAM_STRING
+	string "Kernel Boot Parameter"
+	default "console=ttyS0,19200"
+	depends on BOOTPARAM
+
+config EARLY_PRINTK
+	bool "Early printk"
+	depends on MMU_MOTOROLA
+	help
+	  Write kernel log output directly to a serial port.
+	  Where implemented, output goes to the framebuffer as well.
+	  PROM console functionality on Sun 3x is not affected by this option.
+
+	  Pass "earlyprintk" on the kernel command line to get a
+	  boot console.
+
+	  This is useful for kernel debugging when your machine crashes very
+	  early, i.e. before the normal console driver is loaded.
+	  You should normally say N here, unless you want to debug such a crash.
+
+if !MMU
+
+config FULLDEBUG
+	bool "Full Symbolic/Source Debugging support"
+	help
+	  Enable debugging symbols on kernel build.
+
+config HIGHPROFILE
+	bool "Use fast second timer for profiling"
+	depends on COLDFIRE
+	help
+	  Use a fast secondary clock to produce profiling information.
+
+config BDM_DISABLE
+	bool "Disable BDM signals"
+	depends on COLDFIRE
+	help
+	  Disable the ColdFire CPU's BDM signals.
+
+endif