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, 38 insertions, 0 deletions

diff --git a/arch/alpha/Kconfig.debug b/arch/alpha/Kconfig.debug
new file mode 100644
index 000000000..f85f4281c
--- /dev/null
+++ b/arch/alpha/Kconfig.debug
@@ -0,0 +1,38 @@
+# SPDX-License-Identifier: GPL-2.0
+
+config EARLY_PRINTK
+	bool
+	depends on ALPHA_GENERIC || ALPHA_SRM
+	default y
+
+config ALPHA_LEGACY_START_ADDRESS
+	bool "Legacy kernel start address"
+	depends on ALPHA_GENERIC
+	default n
+	help
+	  The 2.4 kernel changed the kernel start address from 0x310000
+	  to 0x810000 to make room for the Wildfire's larger SRM console.
+	  Recent consoles on Titan and Marvel machines also require the
+	  extra room.
+
+	  If you're using aboot 0.7 or later, the bootloader will examine the
+	  ELF headers to determine where to transfer control. Unfortunately,
+	  most older bootloaders -- APB or MILO -- hardcoded the kernel start
+	  address rather than examining the ELF headers, and the result is a
+	  hard lockup.
+
+	  Say Y if you have a broken bootloader.  Say N if you do not, or if
+	  you wish to run on Wildfire, Titan, or Marvel.
+
+config ALPHA_LEGACY_START_ADDRESS
+	bool
+	depends on !ALPHA_GENERIC && !ALPHA_TITAN && !ALPHA_MARVEL && !ALPHA_WILDFIRE
+	default y
+
+config MATHEMU
+	tristate "Kernel FP software completion" if DEBUG_KERNEL && !SMP
+	default y if !DEBUG_KERNEL || SMP
+	help
+	  This option is required for IEEE compliant floating point arithmetic
+	  on the Alpha. The only time you would ever not say Y is to say M in
+	  order to debug the code. Say Y unless you know what you are doing.