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, 177 insertions, 0 deletions

diff --git a/net/wireless/tests/fragmentation.c b/net/wireless/tests/fragmentation.c
new file mode 100644
index 000000000..411fae18c
--- /dev/null
+++ b/net/wireless/tests/fragmentation.c
@@ -0,0 +1,177 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * KUnit tests for element fragmentation
+ *
+ * Copyright (C) 2023-2024 Intel Corporation
+ */
+#include <linux/ieee80211.h>
+#include <net/cfg80211.h>
+#include <kunit/test.h>
+
+static void defragment_0(struct kunit *test)
+{
+	ssize_t ret;
+	static const u8 input[] = {
+		[0] = WLAN_EID_EXTENSION,
+		[1] = 254,
+		[2] = WLAN_EID_EXT_EHT_MULTI_LINK,
+		[27] = 27,
+		[123] = 123,
+		[254 + 2] = WLAN_EID_FRAGMENT,
+		[254 + 3] = 7,
+		[254 + 3 + 7] = 0, /* for size */
+	};
+	u8 *data = kunit_kzalloc(test, sizeof(input), GFP_KERNEL);
+
+	KUNIT_ASSERT_NOT_NULL(test, data);
+
+	ret = cfg80211_defragment_element((void *)input,
+					  input, sizeof(input),
+					  NULL, 0,
+					  WLAN_EID_FRAGMENT);
+	KUNIT_EXPECT_EQ(test, ret, 253);
+	ret = cfg80211_defragment_element((void *)input,
+					  input, sizeof(input),
+					  data, ret,
+					  WLAN_EID_FRAGMENT);
+	KUNIT_EXPECT_EQ(test, ret, 253);
+	KUNIT_EXPECT_MEMEQ(test, data, input + 3, 253);
+}
+
+static void defragment_1(struct kunit *test)
+{
+	ssize_t ret;
+	static const u8 input[] = {
+		[0] = WLAN_EID_EXTENSION,
+		[1] = 255,
+		[2] = WLAN_EID_EXT_EHT_MULTI_LINK,
+		[27] = 27,
+		[123] = 123,
+		[255 + 2] = WLAN_EID_FRAGMENT,
+		[255 + 3] = 7,
+		[255 + 3 + 1] = 0xaa,
+		[255 + 3 + 8] = WLAN_EID_FRAGMENT, /* not used */
+		[255 + 3 + 9] = 1,
+		[255 + 3 + 10] = 0, /* for size */
+	};
+	u8 *data = kunit_kzalloc(test, sizeof(input), GFP_KERNEL);
+	const struct element *elem;
+	int count = 0;
+
+	KUNIT_ASSERT_NOT_NULL(test, data);
+
+	for_each_element(elem, input, sizeof(input))
+		count++;
+
+	/* check the elements are right */
+	KUNIT_ASSERT_EQ(test, count, 3);
+
+	ret = cfg80211_defragment_element((void *)input,
+					  input, sizeof(input),
+					  NULL, 0,
+					  WLAN_EID_FRAGMENT);
+	KUNIT_EXPECT_EQ(test, ret, 254 + 7);
+	ret = cfg80211_defragment_element((void *)input,
+					  input, sizeof(input),
+					  data, ret,
+					  WLAN_EID_FRAGMENT);
+	/* this means the last fragment was not used */
+	KUNIT_EXPECT_EQ(test, ret, 254 + 7);
+	KUNIT_EXPECT_MEMEQ(test, data, input + 3, 254);
+	KUNIT_EXPECT_MEMEQ(test, data + 254, input + 255 + 4, 7);
+}
+
+static void defragment_2(struct kunit *test)
+{
+	ssize_t ret;
+	static const u8 input[] = {
+		[0] = WLAN_EID_EXTENSION,
+		[1] = 255,
+		[2] = WLAN_EID_EXT_EHT_MULTI_LINK,
+		[27] = 27,
+		[123] = 123,
+
+		[257 + 0] = WLAN_EID_FRAGMENT,
+		[257 + 1] = 255,
+		[257 + 20] = 0xaa,
+
+		[2 * 257 + 0] = WLAN_EID_FRAGMENT,
+		[2 * 257 + 1] = 1,
+		[2 * 257 + 2] = 0xcc,
+		[2 * 257 + 3] = WLAN_EID_FRAGMENT, /* not used */
+		[2 * 257 + 4] = 1,
+		[2 * 257 + 5] = 0, /* for size */
+	};
+	u8 *data = kunit_kzalloc(test, sizeof(input), GFP_KERNEL);
+	const struct element *elem;
+	int count = 0;
+
+	KUNIT_ASSERT_NOT_NULL(test, data);
+
+	for_each_element(elem, input, sizeof(input))
+		count++;
+
+	/* check the elements are right */
+	KUNIT_ASSERT_EQ(test, count, 4);
+
+	ret = cfg80211_defragment_element((void *)input,
+					  input, sizeof(input),
+					  NULL, 0,
+					  WLAN_EID_FRAGMENT);
+	/* this means the last fragment was not used */
+	KUNIT_EXPECT_EQ(test, ret, 254 + 255 + 1);
+	ret = cfg80211_defragment_element((void *)input,
+					  input, sizeof(input),
+					  data, ret,
+					  WLAN_EID_FRAGMENT);
+	KUNIT_EXPECT_EQ(test, ret, 254 + 255 + 1);
+	KUNIT_EXPECT_MEMEQ(test, data, input + 3, 254);
+	KUNIT_EXPECT_MEMEQ(test, data + 254, input + 257 + 2, 255);
+	KUNIT_EXPECT_MEMEQ(test, data + 254 + 255, input + 2 * 257 + 2, 1);
+}
+
+static void defragment_at_end(struct kunit *test)
+{
+	ssize_t ret;
+	static const u8 input[] = {
+		[0] = WLAN_EID_EXTENSION,
+		[1] = 255,
+		[2] = WLAN_EID_EXT_EHT_MULTI_LINK,
+		[27] = 27,
+		[123] = 123,
+		[255 + 2] = WLAN_EID_FRAGMENT,
+		[255 + 3] = 7,
+		[255 + 3 + 7] = 0, /* for size */
+	};
+	u8 *data = kunit_kzalloc(test, sizeof(input), GFP_KERNEL);
+
+	KUNIT_ASSERT_NOT_NULL(test, data);
+
+	ret = cfg80211_defragment_element((void *)input,
+					  input, sizeof(input),
+					  NULL, 0,
+					  WLAN_EID_FRAGMENT);
+	KUNIT_EXPECT_EQ(test, ret, 254 + 7);
+	ret = cfg80211_defragment_element((void *)input,
+					  input, sizeof(input),
+					  data, ret,
+					  WLAN_EID_FRAGMENT);
+	KUNIT_EXPECT_EQ(test, ret, 254 + 7);
+	KUNIT_EXPECT_MEMEQ(test, data, input + 3, 254);
+	KUNIT_EXPECT_MEMEQ(test, data + 254, input + 255 + 4, 7);
+}
+
+static struct kunit_case element_fragmentation_test_cases[] = {
+	KUNIT_CASE(defragment_0),
+	KUNIT_CASE(defragment_1),
+	KUNIT_CASE(defragment_2),
+	KUNIT_CASE(defragment_at_end),
+	{}
+};
+
+static struct kunit_suite element_fragmentation = {
+	.name = "cfg80211-element-defragmentation",
+	.test_cases = element_fragmentation_test_cases,
+};
+
+kunit_test_suite(element_fragmentation);