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, 39 insertions, 0 deletions

diff --git a/net/mpls/Kconfig b/net/mpls/Kconfig
new file mode 100644
index 000000000..d672ab72a
--- /dev/null
+++ b/net/mpls/Kconfig
@@ -0,0 +1,39 @@
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# MPLS configuration
+#
+
+menuconfig MPLS
+	bool "MultiProtocol Label Switching"
+	default n
+	help
+	  MultiProtocol Label Switching routes packets through logical
+	  circuits.  Originally conceived as a way of routing packets at
+	  hardware speeds (before hardware was capable of routing ipv4 packets),
+	  MPLS remains a simple way of making tunnels.
+
+	  If you have not heard of MPLS you probably want to say N here.
+
+if MPLS
+
+config NET_MPLS_GSO
+	tristate "MPLS: GSO support"
+	help
+	 This is helper module to allow segmentation of non-MPLS GSO packets
+	 that have had MPLS stack entries pushed onto them and thus
+	 become MPLS GSO packets.
+
+config MPLS_ROUTING
+	tristate "MPLS: routing support"
+	depends on NET_IP_TUNNEL || NET_IP_TUNNEL=n
+	depends on PROC_SYSCTL
+	help
+	 Add support for forwarding of mpls packets.
+
+config MPLS_IPTUNNEL
+	tristate "MPLS: IP over MPLS tunnel support"
+	depends on LWTUNNEL && MPLS_ROUTING
+	help
+	 mpls ip tunnel support.
+
+endif # MPLS