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, 300 insertions, 0 deletions

diff --git a/lib/random32.c b/lib/random32.c
new file mode 100644
index 000000000..24e7acd93
--- /dev/null
+++ b/lib/random32.c
@@ -0,0 +1,300 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * This is a maximally equidistributed combined Tausworthe generator
+ * based on code from GNU Scientific Library 1.5 (30 Jun 2004)
+ *
+ * lfsr113 version:
+ *
+ * x_n = (s1_n ^ s2_n ^ s3_n ^ s4_n)
+ *
+ * s1_{n+1} = (((s1_n & 4294967294) << 18) ^ (((s1_n <<  6) ^ s1_n) >> 13))
+ * s2_{n+1} = (((s2_n & 4294967288) <<  2) ^ (((s2_n <<  2) ^ s2_n) >> 27))
+ * s3_{n+1} = (((s3_n & 4294967280) <<  7) ^ (((s3_n << 13) ^ s3_n) >> 21))
+ * s4_{n+1} = (((s4_n & 4294967168) << 13) ^ (((s4_n <<  3) ^ s4_n) >> 12))
+ *
+ * The period of this generator is about 2^113 (see erratum paper).
+ *
+ * From: P. L'Ecuyer, "Maximally Equidistributed Combined Tausworthe
+ * Generators", Mathematics of Computation, 65, 213 (1996), 203--213:
+ * http://www.iro.umontreal.ca/~lecuyer/myftp/papers/tausme.ps
+ * ftp://ftp.iro.umontreal.ca/pub/simulation/lecuyer/papers/tausme.ps
+ *
+ * There is an erratum in the paper "Tables of Maximally Equidistributed
+ * Combined LFSR Generators", Mathematics of Computation, 68, 225 (1999),
+ * 261--269: http://www.iro.umontreal.ca/~lecuyer/myftp/papers/tausme2.ps
+ *
+ *      ... the k_j most significant bits of z_j must be non-zero,
+ *      for each j. (Note: this restriction also applies to the
+ *      computer code given in [4], but was mistakenly not mentioned
+ *      in that paper.)
+ *
+ * This affects the seeding procedure by imposing the requirement
+ * s1 > 1, s2 > 7, s3 > 15, s4 > 127.
+ */
+
+#include <linux/types.h>
+#include <linux/percpu.h>
+#include <linux/export.h>
+#include <linux/jiffies.h>
+#include <linux/prandom.h>
+#include <linux/sched.h>
+#include <linux/bitops.h>
+#include <linux/slab.h>
+#include <linux/unaligned.h>
+
+/**
+ *	prandom_u32_state - seeded pseudo-random number generator.
+ *	@state: pointer to state structure holding seeded state.
+ *
+ *	This is used for pseudo-randomness with no outside seeding.
+ *	For more random results, use get_random_u32().
+ */
+u32 prandom_u32_state(struct rnd_state *state)
+{
+#define TAUSWORTHE(s, a, b, c, d) ((s & c) << d) ^ (((s << a) ^ s) >> b)
+	state->s1 = TAUSWORTHE(state->s1,  6U, 13U, 4294967294U, 18U);
+	state->s2 = TAUSWORTHE(state->s2,  2U, 27U, 4294967288U,  2U);
+	state->s3 = TAUSWORTHE(state->s3, 13U, 21U, 4294967280U,  7U);
+	state->s4 = TAUSWORTHE(state->s4,  3U, 12U, 4294967168U, 13U);
+
+	return (state->s1 ^ state->s2 ^ state->s3 ^ state->s4);
+}
+EXPORT_SYMBOL(prandom_u32_state);
+
+/**
+ *	prandom_bytes_state - get the requested number of pseudo-random bytes
+ *
+ *	@state: pointer to state structure holding seeded state.
+ *	@buf: where to copy the pseudo-random bytes to
+ *	@bytes: the requested number of bytes
+ *
+ *	This is used for pseudo-randomness with no outside seeding.
+ *	For more random results, use get_random_bytes().
+ */
+void prandom_bytes_state(struct rnd_state *state, void *buf, size_t bytes)
+{
+	u8 *ptr = buf;
+
+	while (bytes >= sizeof(u32)) {
+		put_unaligned(prandom_u32_state(state), (u32 *) ptr);
+		ptr += sizeof(u32);
+		bytes -= sizeof(u32);
+	}
+
+	if (bytes > 0) {
+		u32 rem = prandom_u32_state(state);
+		do {
+			*ptr++ = (u8) rem;
+			bytes--;
+			rem >>= BITS_PER_BYTE;
+		} while (bytes > 0);
+	}
+}
+EXPORT_SYMBOL(prandom_bytes_state);
+
+static void prandom_warmup(struct rnd_state *state)
+{
+	/* Calling RNG ten times to satisfy recurrence condition */
+	prandom_u32_state(state);
+	prandom_u32_state(state);
+	prandom_u32_state(state);
+	prandom_u32_state(state);
+	prandom_u32_state(state);
+	prandom_u32_state(state);
+	prandom_u32_state(state);
+	prandom_u32_state(state);
+	prandom_u32_state(state);
+	prandom_u32_state(state);
+}
+
+void prandom_seed_full_state(struct rnd_state __percpu *pcpu_state)
+{
+	int i;
+
+	for_each_possible_cpu(i) {
+		struct rnd_state *state = per_cpu_ptr(pcpu_state, i);
+		u32 seeds[4];
+
+		get_random_bytes(&seeds, sizeof(seeds));
+		state->s1 = __seed(seeds[0],   2U);
+		state->s2 = __seed(seeds[1],   8U);
+		state->s3 = __seed(seeds[2],  16U);
+		state->s4 = __seed(seeds[3], 128U);
+
+		prandom_warmup(state);
+	}
+}
+EXPORT_SYMBOL(prandom_seed_full_state);
+
+#ifdef CONFIG_RANDOM32_SELFTEST
+static struct prandom_test1 {
+	u32 seed;
+	u32 result;
+} test1[] = {
+	{ 1U, 3484351685U },
+	{ 2U, 2623130059U },
+	{ 3U, 3125133893U },
+	{ 4U,  984847254U },
+};
+
+static struct prandom_test2 {
+	u32 seed;
+	u32 iteration;
+	u32 result;
+} test2[] = {
+	/* Test cases against taus113 from GSL library. */
+	{  931557656U, 959U, 2975593782U },
+	{ 1339693295U, 876U, 3887776532U },
+	{ 1545556285U, 961U, 1615538833U },
+	{  601730776U, 723U, 1776162651U },
+	{ 1027516047U, 687U,  511983079U },
+	{  416526298U, 700U,  916156552U },
+	{ 1395522032U, 652U, 2222063676U },
+	{  366221443U, 617U, 2992857763U },
+	{ 1539836965U, 714U, 3783265725U },
+	{  556206671U, 994U,  799626459U },
+	{  684907218U, 799U,  367789491U },
+	{ 2121230701U, 931U, 2115467001U },
+	{ 1668516451U, 644U, 3620590685U },
+	{  768046066U, 883U, 2034077390U },
+	{ 1989159136U, 833U, 1195767305U },
+	{  536585145U, 996U, 3577259204U },
+	{ 1008129373U, 642U, 1478080776U },
+	{ 1740775604U, 939U, 1264980372U },
+	{ 1967883163U, 508U,   10734624U },
+	{ 1923019697U, 730U, 3821419629U },
+	{  442079932U, 560U, 3440032343U },
+	{ 1961302714U, 845U,  841962572U },
+	{ 2030205964U, 962U, 1325144227U },
+	{ 1160407529U, 507U,  240940858U },
+	{  635482502U, 779U, 4200489746U },
+	{ 1252788931U, 699U,  867195434U },
+	{ 1961817131U, 719U,  668237657U },
+	{ 1071468216U, 983U,  917876630U },
+	{ 1281848367U, 932U, 1003100039U },
+	{  582537119U, 780U, 1127273778U },
+	{ 1973672777U, 853U, 1071368872U },
+	{ 1896756996U, 762U, 1127851055U },
+	{  847917054U, 500U, 1717499075U },
+	{ 1240520510U, 951U, 2849576657U },
+	{ 1685071682U, 567U, 1961810396U },
+	{ 1516232129U, 557U,    3173877U },
+	{ 1208118903U, 612U, 1613145022U },
+	{ 1817269927U, 693U, 4279122573U },
+	{ 1510091701U, 717U,  638191229U },
+	{  365916850U, 807U,  600424314U },
+	{  399324359U, 702U, 1803598116U },
+	{ 1318480274U, 779U, 2074237022U },
+	{  697758115U, 840U, 1483639402U },
+	{ 1696507773U, 840U,  577415447U },
+	{ 2081979121U, 981U, 3041486449U },
+	{  955646687U, 742U, 3846494357U },
+	{ 1250683506U, 749U,  836419859U },
+	{  595003102U, 534U,  366794109U },
+	{   47485338U, 558U, 3521120834U },
+	{  619433479U, 610U, 3991783875U },
+	{  704096520U, 518U, 4139493852U },
+	{ 1712224984U, 606U, 2393312003U },
+	{ 1318233152U, 922U, 3880361134U },
+	{  855572992U, 761U, 1472974787U },
+	{   64721421U, 703U,  683860550U },
+	{  678931758U, 840U,  380616043U },
+	{  692711973U, 778U, 1382361947U },
+	{  677703619U, 530U, 2826914161U },
+	{   92393223U, 586U, 1522128471U },
+	{ 1222592920U, 743U, 3466726667U },
+	{  358288986U, 695U, 1091956998U },
+	{ 1935056945U, 958U,  514864477U },
+	{  735675993U, 990U, 1294239989U },
+	{ 1560089402U, 897U, 2238551287U },
+	{   70616361U, 829U,   22483098U },
+	{  368234700U, 731U, 2913875084U },
+	{   20221190U, 879U, 1564152970U },
+	{  539444654U, 682U, 1835141259U },
+	{ 1314987297U, 840U, 1801114136U },
+	{ 2019295544U, 645U, 3286438930U },
+	{  469023838U, 716U, 1637918202U },
+	{ 1843754496U, 653U, 2562092152U },
+	{  400672036U, 809U, 4264212785U },
+	{  404722249U, 965U, 2704116999U },
+	{  600702209U, 758U,  584979986U },
+	{  519953954U, 667U, 2574436237U },
+	{ 1658071126U, 694U, 2214569490U },
+	{  420480037U, 749U, 3430010866U },
+	{  690103647U, 969U, 3700758083U },
+	{ 1029424799U, 937U, 3787746841U },
+	{ 2012608669U, 506U, 3362628973U },
+	{ 1535432887U, 998U,   42610943U },
+	{ 1330635533U, 857U, 3040806504U },
+	{ 1223800550U, 539U, 3954229517U },
+	{ 1322411537U, 680U, 3223250324U },
+	{ 1877847898U, 945U, 2915147143U },
+	{ 1646356099U, 874U,  965988280U },
+	{  805687536U, 744U, 4032277920U },
+	{ 1948093210U, 633U, 1346597684U },
+	{  392609744U, 783U, 1636083295U },
+	{  690241304U, 770U, 1201031298U },
+	{ 1360302965U, 696U, 1665394461U },
+	{ 1220090946U, 780U, 1316922812U },
+	{  447092251U, 500U, 3438743375U },
+	{ 1613868791U, 592U,  828546883U },
+	{  523430951U, 548U, 2552392304U },
+	{  726692899U, 810U, 1656872867U },
+	{ 1364340021U, 836U, 3710513486U },
+	{ 1986257729U, 931U,  935013962U },
+	{  407983964U, 921U,  728767059U },
+};
+
+static void prandom_state_selftest_seed(struct rnd_state *state, u32 seed)
+{
+#define LCG(x)	 ((x) * 69069U)	/* super-duper LCG */
+	state->s1 = __seed(LCG(seed),        2U);
+	state->s2 = __seed(LCG(state->s1),   8U);
+	state->s3 = __seed(LCG(state->s2),  16U);
+	state->s4 = __seed(LCG(state->s3), 128U);
+}
+
+static int __init prandom_state_selftest(void)
+{
+	int i, j, errors = 0, runs = 0;
+	bool error = false;
+
+	for (i = 0; i < ARRAY_SIZE(test1); i++) {
+		struct rnd_state state;
+
+		prandom_state_selftest_seed(&state, test1[i].seed);
+		prandom_warmup(&state);
+
+		if (test1[i].result != prandom_u32_state(&state))
+			error = true;
+	}
+
+	if (error)
+		pr_warn("prandom: seed boundary self test failed\n");
+	else
+		pr_info("prandom: seed boundary self test passed\n");
+
+	for (i = 0; i < ARRAY_SIZE(test2); i++) {
+		struct rnd_state state;
+
+		prandom_state_selftest_seed(&state, test2[i].seed);
+		prandom_warmup(&state);
+
+		for (j = 0; j < test2[i].iteration - 1; j++)
+			prandom_u32_state(&state);
+
+		if (test2[i].result != prandom_u32_state(&state))
+			errors++;
+
+		runs++;
+		cond_resched();
+	}
+
+	if (errors)
+		pr_warn("prandom: %d/%d self tests failed\n", errors, runs);
+	else
+		pr_info("prandom: %d self tests passed\n", runs);
+	return 0;
+}
+core_initcall(prandom_state_selftest);
+#endif