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, 239 insertions, 0 deletions

diff --git a/lib/tests/test_hash.c b/lib/tests/test_hash.c
new file mode 100644
index 000000000..a7af39662
--- /dev/null
+++ b/lib/tests/test_hash.c
@@ -0,0 +1,239 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Test cases for <linux/hash.h> and <linux/stringhash.h>
+ * This just verifies that various ways of computing a hash
+ * produce the same thing and, for cases where a k-bit hash
+ * value is requested, is of the requested size.
+ *
+ * We fill a buffer with a 255-byte null-terminated string,
+ * and use both full_name_hash() and hashlen_string() to hash the
+ * substrings from i to j, where 0 <= i < j < 256.
+ *
+ * The returned values are used to check that __hash_32() and
+ * __hash_32_generic() compute the same thing.  Likewise hash_32()
+ * and hash_64().
+ */
+
+#include <linux/compiler.h>
+#include <linux/types.h>
+#include <linux/module.h>
+#include <linux/hash.h>
+#include <linux/stringhash.h>
+#include <kunit/test.h>
+
+/* 32-bit XORSHIFT generator.  Seed must not be zero. */
+static u32 __attribute_const__
+xorshift(u32 seed)
+{
+	seed ^= seed << 13;
+	seed ^= seed >> 17;
+	seed ^= seed << 5;
+	return seed;
+}
+
+/* Given a non-zero x, returns a non-zero byte. */
+static u8 __attribute_const__
+mod255(u32 x)
+{
+	x = (x & 0xffff) + (x >> 16);	/* 1 <= x <= 0x1fffe */
+	x = (x & 0xff) + (x >> 8);	/* 1 <= x <= 0x2fd */
+	x = (x & 0xff) + (x >> 8);	/* 1 <= x <= 0x100 */
+	x = (x & 0xff) + (x >> 8);	/* 1 <= x <= 0xff */
+	return x;
+}
+
+/* Fill the buffer with non-zero bytes. */
+static void fill_buf(char *buf, size_t len, u32 seed)
+{
+	size_t i;
+
+	for (i = 0; i < len; i++) {
+		seed = xorshift(seed);
+		buf[i] = mod255(seed);
+	}
+}
+
+/* Holds most testing variables for the int test. */
+struct test_hash_params {
+        /* Pointer to integer to be hashed. */
+	unsigned long long *h64;
+        /* Low 32-bits of integer to be hashed. */
+	u32 h0;
+        /* Arch-specific hash result. */
+	u32 h1;
+        /* Generic hash result. */
+	u32 h2;
+        /* ORed hashes of given size (in bits). */
+	u32 (*hash_or)[33];
+};
+
+#ifdef HAVE_ARCH__HASH_32
+static void
+test_int__hash_32(struct kunit *test, struct test_hash_params *params)
+{
+	params->hash_or[1][0] |= params->h2 = __hash_32_generic(params->h0);
+#if HAVE_ARCH__HASH_32 == 1
+	KUNIT_EXPECT_EQ_MSG(test, params->h1, params->h2,
+			    "__hash_32(%#x) = %#x != __hash_32_generic() = %#x",
+			    params->h0, params->h1, params->h2);
+#endif
+}
+#endif
+
+#ifdef HAVE_ARCH_HASH_64
+static void
+test_int_hash_64(struct kunit *test, struct test_hash_params *params, u32 const *m, int *k)
+{
+	params->h2 = hash_64_generic(*params->h64, *k);
+#if HAVE_ARCH_HASH_64 == 1
+	KUNIT_EXPECT_EQ_MSG(test, params->h1, params->h2,
+			    "hash_64(%#llx, %d) = %#x != hash_64_generic() = %#x",
+			    *params->h64, *k, params->h1, params->h2);
+#else
+	KUNIT_EXPECT_LE_MSG(test, params->h1, params->h2,
+			    "hash_64_generic(%#llx, %d) = %#x > %#x",
+			    *params->h64, *k, params->h1, *m);
+#endif
+}
+#endif
+
+/*
+ * Test the various integer hash functions.  h64 (or its low-order bits)
+ * is the integer to hash.  hash_or accumulates the OR of the hash values,
+ * which are later checked to see that they cover all the requested bits.
+ *
+ * Because these functions (as opposed to the string hashes) are all
+ * inline, the code being tested is actually in the module, and you can
+ * recompile and re-test the module without rebooting.
+ */
+static void
+test_int_hash(struct kunit *test, unsigned long long h64, u32 hash_or[2][33])
+{
+	int k;
+	struct test_hash_params params = { &h64, (u32)h64, 0, 0, hash_or };
+
+	/* Test __hash32 */
+	hash_or[0][0] |= params.h1 = __hash_32(params.h0);
+#ifdef HAVE_ARCH__HASH_32
+	test_int__hash_32(test, &params);
+#endif
+
+	/* Test k = 1..32 bits */
+	for (k = 1; k <= 32; k++) {
+		u32 const m = ((u32)2 << (k-1)) - 1;	/* Low k bits set */
+
+		/* Test hash_32 */
+		hash_or[0][k] |= params.h1 = hash_32(params.h0, k);
+		KUNIT_EXPECT_LE_MSG(test, params.h1, m,
+				    "hash_32(%#x, %d) = %#x > %#x",
+				    params.h0, k, params.h1, m);
+
+		/* Test hash_64 */
+		hash_or[1][k] |= params.h1 = hash_64(h64, k);
+		KUNIT_EXPECT_LE_MSG(test, params.h1, m,
+				    "hash_64(%#llx, %d) = %#x > %#x",
+				    h64, k, params.h1, m);
+#ifdef HAVE_ARCH_HASH_64
+		test_int_hash_64(test, &params, &m, &k);
+#endif
+	}
+}
+
+#define SIZE 256	/* Run time is cubic in SIZE */
+
+static void test_string_or(struct kunit *test)
+{
+	char buf[SIZE+1];
+	u32 string_or = 0;
+	int i, j;
+
+	fill_buf(buf, SIZE, 1);
+
+	/* Test every possible non-empty substring in the buffer. */
+	for (j = SIZE; j > 0; --j) {
+		buf[j] = '\0';
+
+		for (i = 0; i <= j; i++) {
+			u32 h0 = full_name_hash(buf+i, buf+i, j-i);
+
+			string_or |= h0;
+		} /* i */
+	} /* j */
+
+	/* The OR of all the hash values should cover all the bits */
+	KUNIT_EXPECT_EQ_MSG(test, string_or, -1u,
+			    "OR of all string hash results = %#x != %#x",
+			    string_or, -1u);
+}
+
+static void test_hash_or(struct kunit *test)
+{
+	char buf[SIZE+1];
+	u32 hash_or[2][33] = { { 0, } };
+	unsigned long long h64 = 0;
+	int i, j;
+
+	fill_buf(buf, SIZE, 1);
+
+	/* Test every possible non-empty substring in the buffer. */
+	for (j = SIZE; j > 0; --j) {
+		buf[j] = '\0';
+
+		for (i = 0; i <= j; i++) {
+			u64 hashlen = hashlen_string(buf+i, buf+i);
+			u32 h0 = full_name_hash(buf+i, buf+i, j-i);
+
+			/* Check that hashlen_string gets the length right */
+			KUNIT_EXPECT_EQ_MSG(test, hashlen_len(hashlen), j-i,
+					    "hashlen_string(%d..%d) returned length %u, expected %d",
+					    i, j, hashlen_len(hashlen), j-i);
+			/* Check that the hashes match */
+			KUNIT_EXPECT_EQ_MSG(test, hashlen_hash(hashlen), h0,
+					    "hashlen_string(%d..%d) = %08x != full_name_hash() = %08x",
+					    i, j, hashlen_hash(hashlen), h0);
+
+			h64 = h64 << 32 | h0;	/* For use with hash_64 */
+			test_int_hash(test, h64, hash_or);
+		} /* i */
+	} /* j */
+
+	KUNIT_EXPECT_EQ_MSG(test, hash_or[0][0], -1u,
+			    "OR of all __hash_32 results = %#x != %#x",
+			    hash_or[0][0], -1u);
+#ifdef HAVE_ARCH__HASH_32
+#if HAVE_ARCH__HASH_32 != 1	/* Test is pointless if results match */
+	KUNIT_EXPECT_EQ_MSG(test, hash_or[1][0], -1u,
+			    "OR of all __hash_32_generic results = %#x != %#x",
+			    hash_or[1][0], -1u);
+#endif
+#endif
+
+	/* Likewise for all the i-bit hash values */
+	for (i = 1; i <= 32; i++) {
+		u32 const m = ((u32)2 << (i-1)) - 1;	/* Low i bits set */
+
+		KUNIT_EXPECT_EQ_MSG(test, hash_or[0][i], m,
+				    "OR of all hash_32(%d) results = %#x (%#x expected)",
+				    i, hash_or[0][i], m);
+		KUNIT_EXPECT_EQ_MSG(test, hash_or[1][i], m,
+				    "OR of all hash_64(%d) results = %#x (%#x expected)",
+				    i, hash_or[1][i], m);
+	}
+}
+
+static struct kunit_case hash_test_cases[] __refdata = {
+	KUNIT_CASE(test_string_or),
+	KUNIT_CASE(test_hash_or),
+	{}
+};
+
+static struct kunit_suite hash_test_suite = {
+	.name = "hash",
+	.test_cases = hash_test_cases,
+};
+
+
+kunit_test_suite(hash_test_suite);
+
+MODULE_DESCRIPTION("Test cases for <linux/hash.h> and <linux/stringhash.h>");
+MODULE_LICENSE("GPL");