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>

4 files changed, 83 insertions, 0 deletions

diff --git a/tools/testing/kunit/configs/all_tests.config b/tools/testing/kunit/configs/all_tests.config
new file mode 100644
index 000000000..6825c2e85
--- /dev/null
+++ b/tools/testing/kunit/configs/all_tests.config
@@ -0,0 +1,62 @@
+# This config enables as many tests as possible under UML.
+# It is intended for use in continuous integration systems and similar for
+# automated testing of as much as possible.
+# The config is manually maintained, though it uses KUNIT_ALL_TESTS=y to enable
+# any tests whose dependencies are already satisfied. Please feel free to add
+# more options if they any new tests.
+
+CONFIG_KUNIT=y
+CONFIG_KUNIT_EXAMPLE_TEST=y
+CONFIG_KUNIT_ALL_TESTS=y
+
+CONFIG_FORTIFY_SOURCE=y
+
+CONFIG_IIO=y
+
+CONFIG_EXT4_FS=y
+
+CONFIG_MSDOS_FS=y
+CONFIG_VFAT_FS=y
+
+CONFIG_PCI=y
+CONFIG_USB4=y
+CONFIG_I2C=y
+CONFIG_GPIOLIB=y
+
+CONFIG_NET=y
+CONFIG_MCTP=y
+CONFIG_MCTP_FLOWS=y
+
+CONFIG_INET=y
+CONFIG_MPTCP=y
+
+CONFIG_NETDEVICES=y
+CONFIG_WLAN=y
+CONFIG_CFG80211=y
+CONFIG_MAC80211=y
+CONFIG_WLAN_VENDOR_INTEL=y
+CONFIG_IWLWIFI=y
+
+CONFIG_DAMON=y
+CONFIG_DAMON_VADDR=y
+CONFIG_DAMON_PADDR=y
+
+CONFIG_REGMAP_BUILD=y
+
+CONFIG_AUDIT=y
+
+CONFIG_CRYPTO_LIB_ENABLE_ALL_FOR_KUNIT=y
+
+CONFIG_PRIME_NUMBERS=y
+
+CONFIG_CRC_ENABLE_ALL_FOR_KUNIT=y
+
+CONFIG_SECURITY=y
+CONFIG_SECURITY_APPARMOR=y
+CONFIG_SECURITY_LANDLOCK=y
+
+CONFIG_SOUND=y
+CONFIG_SND=y
+CONFIG_SND_SOC=y
+CONFIG_SND_SOC_TOPOLOGY_BUILD=y
+CONFIG_SND_SOC_CS35L56_I2C=y
diff --git a/tools/testing/kunit/configs/arch_uml.config b/tools/testing/kunit/configs/arch_uml.config
new file mode 100644
index 000000000..28edf816a
--- /dev/null
+++ b/tools/testing/kunit/configs/arch_uml.config
@@ -0,0 +1,7 @@
+# Config options which are added to UML builds by default
+
+# Enable pci, as a lot of tests require it.
+CONFIG_KUNIT_UML_PCI=y
+
+# Enable FORTIFY_SOURCE for wider checking.
+CONFIG_FORTIFY_SOURCE=y
diff --git a/tools/testing/kunit/configs/coverage_uml.config b/tools/testing/kunit/configs/coverage_uml.config
new file mode 100644
index 000000000..bacb77664
--- /dev/null
+++ b/tools/testing/kunit/configs/coverage_uml.config
@@ -0,0 +1,11 @@
+# This config fragment enables coverage on UML, which is different from the
+# normal gcov used in other arches (no debugfs).
+# Example usage:
+# ./tools/testing/kunit/kunit.py run \
+#   --kunitconfig=tools/testing/kunit/configs/all_tests_uml.config \
+#   --kunitconfig=tools/testing/kunit/configs/coverage_uml.config
+
+CONFIG_DEBUG_KERNEL=y
+CONFIG_DEBUG_INFO=y
+CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y
+CONFIG_GCOV=y
diff --git a/tools/testing/kunit/configs/default.config b/tools/testing/kunit/configs/default.config
new file mode 100644
index 000000000..e67af7b9f
--- /dev/null
+++ b/tools/testing/kunit/configs/default.config
@@ -0,0 +1,3 @@
+CONFIG_KUNIT=y
+CONFIG_KUNIT_EXAMPLE_TEST=y
+CONFIG_KUNIT_ALL_TESTS=y