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, 330 insertions, 0 deletions

diff --git a/lib/math/test_div64.c b/lib/math/test_div64.c
new file mode 100644
index 000000000..3cd699b65
--- /dev/null
+++ b/lib/math/test_div64.c
@@ -0,0 +1,330 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2021  Maciej W. Rozycki
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/init.h>
+#include <linux/ktime.h>
+#include <linux/module.h>
+#include <linux/printk.h>
+#include <linux/time64.h>
+#include <linux/types.h>
+
+#include <asm/div64.h>
+
+#define TEST_DIV64_N_ITER 1024
+
+static const u64 test_div64_dividends[] = {
+	0x00000000ab275080,
+	0x0000000fe73c1959,
+	0x000000e54c0a74b1,
+	0x00000d4398ff1ef9,
+	0x0000a18c2ee1c097,
+	0x00079fb80b072e4a,
+	0x0072db27380dd689,
+	0x0842f488162e2284,
+	0xf66745411d8ab063,
+	0xfffffffffffffffb,
+	0xfffffffffffffffc,
+	0xffffffffffffffff,
+};
+#define SIZE_DIV64_DIVIDENDS ARRAY_SIZE(test_div64_dividends)
+
+#define TEST_DIV64_DIVISOR_0 0x00000009
+#define TEST_DIV64_DIVISOR_1 0x0000007c
+#define TEST_DIV64_DIVISOR_2 0x00000204
+#define TEST_DIV64_DIVISOR_3 0x0000cb5b
+#define TEST_DIV64_DIVISOR_4 0x00010000
+#define TEST_DIV64_DIVISOR_5 0x0008a880
+#define TEST_DIV64_DIVISOR_6 0x003fd3ae
+#define TEST_DIV64_DIVISOR_7 0x0b658fac
+#define TEST_DIV64_DIVISOR_8 0x80000001
+#define TEST_DIV64_DIVISOR_9 0xdc08b349
+#define TEST_DIV64_DIVISOR_A 0xfffffffe
+#define TEST_DIV64_DIVISOR_B 0xffffffff
+
+static const u32 test_div64_divisors[] = {
+	TEST_DIV64_DIVISOR_0,
+	TEST_DIV64_DIVISOR_1,
+	TEST_DIV64_DIVISOR_2,
+	TEST_DIV64_DIVISOR_3,
+	TEST_DIV64_DIVISOR_4,
+	TEST_DIV64_DIVISOR_5,
+	TEST_DIV64_DIVISOR_6,
+	TEST_DIV64_DIVISOR_7,
+	TEST_DIV64_DIVISOR_8,
+	TEST_DIV64_DIVISOR_9,
+	TEST_DIV64_DIVISOR_A,
+	TEST_DIV64_DIVISOR_B,
+};
+#define SIZE_DIV64_DIVISORS ARRAY_SIZE(test_div64_divisors)
+
+static const struct {
+	u64 quotient;
+	u32 remainder;
+} test_div64_results[SIZE_DIV64_DIVIDENDS][SIZE_DIV64_DIVISORS] = {
+	{
+		{ 0x0000000013045e47, 0x00000001 },
+		{ 0x000000000161596c, 0x00000030 },
+		{ 0x000000000054e9d4, 0x00000130 },
+		{ 0x000000000000d776, 0x0000278e },
+		{ 0x000000000000ab27, 0x00005080 },
+		{ 0x00000000000013c4, 0x0004ce80 },
+		{ 0x00000000000002ae, 0x001e143c },
+		{ 0x000000000000000f, 0x0033e56c },
+		{ 0x0000000000000001, 0x2b27507f },
+		{ 0x0000000000000000, 0xab275080 },
+		{ 0x0000000000000000, 0xab275080 },
+		{ 0x0000000000000000, 0xab275080 },
+	}, {
+		{ 0x00000001c45c02d1, 0x00000000 },
+		{ 0x0000000020d5213c, 0x00000049 },
+		{ 0x0000000007e3d65f, 0x000001dd },
+		{ 0x0000000000140531, 0x000065ee },
+		{ 0x00000000000fe73c, 0x00001959 },
+		{ 0x000000000001d637, 0x0004e5d9 },
+		{ 0x0000000000003fc9, 0x000713bb },
+		{ 0x0000000000000165, 0x029abe7d },
+		{ 0x000000000000001f, 0x673c193a },
+		{ 0x0000000000000012, 0x6e9f7e37 },
+		{ 0x000000000000000f, 0xe73c1977 },
+		{ 0x000000000000000f, 0xe73c1968 },
+	}, {
+		{ 0x000000197a3a0cf7, 0x00000002 },
+		{ 0x00000001d9632e5c, 0x00000021 },
+		{ 0x0000000071c28039, 0x000001cd },
+		{ 0x000000000120a844, 0x0000b885 },
+		{ 0x0000000000e54c0a, 0x000074b1 },
+		{ 0x00000000001a7bb3, 0x00072331 },
+		{ 0x00000000000397ad, 0x0002c61b },
+		{ 0x000000000000141e, 0x06ea2e89 },
+		{ 0x00000000000001ca, 0x4c0a72e7 },
+		{ 0x000000000000010a, 0xab002ad7 },
+		{ 0x00000000000000e5, 0x4c0a767b },
+		{ 0x00000000000000e5, 0x4c0a7596 },
+	}, {
+		{ 0x0000017949e37538, 0x00000001 },
+		{ 0x0000001b62441f37, 0x00000055 },
+		{ 0x0000000694a3391d, 0x00000085 },
+		{ 0x0000000010b2a5d2, 0x0000a753 },
+		{ 0x000000000d4398ff, 0x00001ef9 },
+		{ 0x0000000001882ec6, 0x0005cbf9 },
+		{ 0x000000000035333b, 0x0017abdf },
+		{ 0x00000000000129f1, 0x0ab4520d },
+		{ 0x0000000000001a87, 0x18ff0472 },
+		{ 0x0000000000000f6e, 0x8ac0ce9b },
+		{ 0x0000000000000d43, 0x98ff397f },
+		{ 0x0000000000000d43, 0x98ff2c3c },
+	}, {
+		{ 0x000011f321a74e49, 0x00000006 },
+		{ 0x0000014d8481d211, 0x0000005b },
+		{ 0x0000005025cbd92d, 0x000001e3 },
+		{ 0x00000000cb5e71e3, 0x000043e6 },
+		{ 0x00000000a18c2ee1, 0x0000c097 },
+		{ 0x0000000012a88828, 0x00036c97 },
+		{ 0x000000000287f16f, 0x002c2a25 },
+		{ 0x00000000000e2cc7, 0x02d581e3 },
+		{ 0x0000000000014318, 0x2ee07d7f },
+		{ 0x000000000000bbf4, 0x1ba08c03 },
+		{ 0x000000000000a18c, 0x2ee303af },
+		{ 0x000000000000a18c, 0x2ee26223 },
+	}, {
+		{ 0x0000d8db8f72935d, 0x00000005 },
+		{ 0x00000fbd5aed7a2e, 0x00000002 },
+		{ 0x000003c84b6ea64a, 0x00000122 },
+		{ 0x0000000998fa8829, 0x000044b7 },
+		{ 0x000000079fb80b07, 0x00002e4a },
+		{ 0x00000000e16b20fa, 0x0002a14a },
+		{ 0x000000001e940d22, 0x00353b2e },
+		{ 0x0000000000ab40ac, 0x06fba6ba },
+		{ 0x00000000000f3f70, 0x0af7eeda },
+		{ 0x000000000008debd, 0x72d98365 },
+		{ 0x0000000000079fb8, 0x0b166dba },
+		{ 0x0000000000079fb8, 0x0b0ece02 },
+	}, {
+		{ 0x000cc3045b8fc281, 0x00000000 },
+		{ 0x0000ed1f48b5c9fc, 0x00000079 },
+		{ 0x000038fb9c63406a, 0x000000e1 },
+		{ 0x000000909705b825, 0x00000a62 },
+		{ 0x00000072db27380d, 0x0000d689 },
+		{ 0x0000000d43fce827, 0x00082b09 },
+		{ 0x00000001ccaba11a, 0x0037e8dd },
+		{ 0x000000000a13f729, 0x0566dffd },
+		{ 0x0000000000e5b64e, 0x3728203b },
+		{ 0x000000000085a14b, 0x23d36726 },
+		{ 0x000000000072db27, 0x38f38cd7 },
+		{ 0x000000000072db27, 0x3880b1b0 },
+	}, {
+		{ 0x00eafeb9c993592b, 0x00000001 },
+		{ 0x00110e5befa9a991, 0x00000048 },
+		{ 0x00041947b4a1d36a, 0x000000dc },
+		{ 0x00000a6679327311, 0x0000c079 },
+		{ 0x00000842f488162e, 0x00002284 },
+		{ 0x000000f4459740fc, 0x00084484 },
+		{ 0x0000002122c47bf9, 0x002ca446 },
+		{ 0x00000000b9936290, 0x004979c4 },
+		{ 0x000000001085e910, 0x05a83974 },
+		{ 0x00000000099ca89d, 0x9db446bf },
+		{ 0x000000000842f488, 0x26b40b94 },
+		{ 0x000000000842f488, 0x1e71170c },
+	}, {
+		{ 0x1b60cece589da1d2, 0x00000001 },
+		{ 0x01fcb42be1453f5b, 0x0000004f },
+		{ 0x007a3f2457df0749, 0x0000013f },
+		{ 0x0001363130e3ec7b, 0x000017aa },
+		{ 0x0000f66745411d8a, 0x0000b063 },
+		{ 0x00001c757dfab350, 0x00048863 },
+		{ 0x000003dc4979c652, 0x00224ea7 },
+		{ 0x000000159edc3144, 0x06409ab3 },
+		{ 0x00000001ecce8a7e, 0x30bc25e5 },
+		{ 0x000000011eadfee3, 0xa99c48a8 },
+		{ 0x00000000f6674543, 0x0a593ae9 },
+		{ 0x00000000f6674542, 0x13f1f5a5 },
+	}, {
+		{ 0x1c71c71c71c71c71, 0x00000002 },
+		{ 0x0210842108421084, 0x0000000b },
+		{ 0x007f01fc07f01fc0, 0x000000fb },
+		{ 0x00014245eabf1f9a, 0x0000a63d },
+		{ 0x0000ffffffffffff, 0x0000fffb },
+		{ 0x00001d913cecc509, 0x0007937b },
+		{ 0x00000402c70c678f, 0x0005bfc9 },
+		{ 0x00000016766cb70b, 0x045edf97 },
+		{ 0x00000001fffffffb, 0x80000000 },
+		{ 0x0000000129d84b3a, 0xa2e8fe71 },
+		{ 0x0000000100000001, 0xfffffffd },
+		{ 0x0000000100000000, 0xfffffffb },
+	}, {
+		{ 0x1c71c71c71c71c71, 0x00000003 },
+		{ 0x0210842108421084, 0x0000000c },
+		{ 0x007f01fc07f01fc0, 0x000000fc },
+		{ 0x00014245eabf1f9a, 0x0000a63e },
+		{ 0x0000ffffffffffff, 0x0000fffc },
+		{ 0x00001d913cecc509, 0x0007937c },
+		{ 0x00000402c70c678f, 0x0005bfca },
+		{ 0x00000016766cb70b, 0x045edf98 },
+		{ 0x00000001fffffffc, 0x00000000 },
+		{ 0x0000000129d84b3a, 0xa2e8fe72 },
+		{ 0x0000000100000002, 0x00000000 },
+		{ 0x0000000100000000, 0xfffffffc },
+	}, {
+		{ 0x1c71c71c71c71c71, 0x00000006 },
+		{ 0x0210842108421084, 0x0000000f },
+		{ 0x007f01fc07f01fc0, 0x000000ff },
+		{ 0x00014245eabf1f9a, 0x0000a641 },
+		{ 0x0000ffffffffffff, 0x0000ffff },
+		{ 0x00001d913cecc509, 0x0007937f },
+		{ 0x00000402c70c678f, 0x0005bfcd },
+		{ 0x00000016766cb70b, 0x045edf9b },
+		{ 0x00000001fffffffc, 0x00000003 },
+		{ 0x0000000129d84b3a, 0xa2e8fe75 },
+		{ 0x0000000100000002, 0x00000003 },
+		{ 0x0000000100000001, 0x00000000 },
+	},
+};
+
+static inline bool test_div64_verify(u64 quotient, u32 remainder, int i, int j)
+{
+	return (quotient == test_div64_results[i][j].quotient &&
+		remainder == test_div64_results[i][j].remainder);
+}
+
+/*
+ * This needs to be a macro, because we don't want to rely on the compiler
+ * to do constant propagation, and `do_div' may take a different path for
+ * constants, so we do want to verify that as well.
+ */
+#define test_div64_one(dividend, divisor, i, j) ({			\
+	bool result = true;						\
+	u64 quotient;							\
+	u32 remainder;							\
+									\
+	quotient = dividend;						\
+	remainder = do_div(quotient, divisor);				\
+	if (!test_div64_verify(quotient, remainder, i, j)) {		\
+		pr_err("ERROR: %016llx / %08x => %016llx,%08x\n",	\
+		       dividend, divisor, quotient, remainder);		\
+		pr_err("ERROR: expected value              => %016llx,%08x\n",\
+		       test_div64_results[i][j].quotient,		\
+		       test_div64_results[i][j].remainder);		\
+		result = false;						\
+	}								\
+	result;								\
+})
+
+/*
+ * Run calculation for the same divisor value expressed as a constant
+ * and as a variable, so as to verify the implementation for both cases
+ * should they be handled by different code execution paths.
+ */
+static bool __init test_div64(void)
+{
+	u64 dividend;
+	int i, j;
+
+	for (i = 0; i < SIZE_DIV64_DIVIDENDS; i++) {
+		dividend = test_div64_dividends[i];
+		if (!test_div64_one(dividend, TEST_DIV64_DIVISOR_0, i, 0))
+			return false;
+		if (!test_div64_one(dividend, TEST_DIV64_DIVISOR_1, i, 1))
+			return false;
+		if (!test_div64_one(dividend, TEST_DIV64_DIVISOR_2, i, 2))
+			return false;
+		if (!test_div64_one(dividend, TEST_DIV64_DIVISOR_3, i, 3))
+			return false;
+		if (!test_div64_one(dividend, TEST_DIV64_DIVISOR_4, i, 4))
+			return false;
+		if (!test_div64_one(dividend, TEST_DIV64_DIVISOR_5, i, 5))
+			return false;
+		if (!test_div64_one(dividend, TEST_DIV64_DIVISOR_6, i, 6))
+			return false;
+		if (!test_div64_one(dividend, TEST_DIV64_DIVISOR_7, i, 7))
+			return false;
+		if (!test_div64_one(dividend, TEST_DIV64_DIVISOR_8, i, 8))
+			return false;
+		if (!test_div64_one(dividend, TEST_DIV64_DIVISOR_9, i, 9))
+			return false;
+		if (!test_div64_one(dividend, TEST_DIV64_DIVISOR_A, i, 10))
+			return false;
+		if (!test_div64_one(dividend, TEST_DIV64_DIVISOR_B, i, 11))
+			return false;
+		for (j = 0; j < SIZE_DIV64_DIVISORS; j++) {
+			if (!test_div64_one(dividend, test_div64_divisors[j],
+					    i, j))
+				return false;
+		}
+	}
+	return true;
+}
+
+static int __init test_div64_init(void)
+{
+	struct timespec64 ts, ts0, ts1;
+	int i;
+
+	pr_info("Starting 64bit/32bit division and modulo test\n");
+	ktime_get_ts64(&ts0);
+
+	for (i = 0; i < TEST_DIV64_N_ITER; i++)
+		if (!test_div64())
+			break;
+
+	ktime_get_ts64(&ts1);
+	ts = timespec64_sub(ts1, ts0);
+	pr_info("Completed 64bit/32bit division and modulo test, "
+		"%llu.%09lus elapsed\n", ts.tv_sec, ts.tv_nsec);
+
+	return 0;
+}
+
+static void __exit test_div64_exit(void)
+{
+}
+
+module_init(test_div64_init);
+module_exit(test_div64_exit);
+
+MODULE_AUTHOR("Maciej W. Rozycki <macro@orcam.me.uk>");
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("64bit/32bit division and modulo test module");