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, 262 insertions, 0 deletions

diff --git a/lib/math/prime_numbers.c b/lib/math/prime_numbers.c
new file mode 100644
index 000000000..95a6f7960
--- /dev/null
+++ b/lib/math/prime_numbers.c
@@ -0,0 +1,262 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/prime_numbers.h>
+#include <linux/slab.h>
+
+#include "prime_numbers_private.h"
+
+#if BITS_PER_LONG == 64
+static const struct primes small_primes = {
+	.last = 61,
+	.sz = 64,
+	.primes = {
+		BIT(2) |
+		BIT(3) |
+		BIT(5) |
+		BIT(7) |
+		BIT(11) |
+		BIT(13) |
+		BIT(17) |
+		BIT(19) |
+		BIT(23) |
+		BIT(29) |
+		BIT(31) |
+		BIT(37) |
+		BIT(41) |
+		BIT(43) |
+		BIT(47) |
+		BIT(53) |
+		BIT(59) |
+		BIT(61)
+	}
+};
+#elif BITS_PER_LONG == 32
+static const struct primes small_primes = {
+	.last = 31,
+	.sz = 32,
+	.primes = {
+		BIT(2) |
+		BIT(3) |
+		BIT(5) |
+		BIT(7) |
+		BIT(11) |
+		BIT(13) |
+		BIT(17) |
+		BIT(19) |
+		BIT(23) |
+		BIT(29) |
+		BIT(31)
+	}
+};
+#else
+#error "unhandled BITS_PER_LONG"
+#endif
+
+static DEFINE_MUTEX(lock);
+static const struct primes __rcu *primes = RCU_INITIALIZER(&small_primes);
+
+#if IS_ENABLED(CONFIG_PRIME_NUMBERS_KUNIT_TEST)
+/*
+ * Calls the callback under RCU lock. The callback must not retain
+ * the primes pointer.
+ */
+void with_primes(void *ctx, primes_fn fn)
+{
+	rcu_read_lock();
+	fn(ctx, rcu_dereference(primes));
+	rcu_read_unlock();
+}
+EXPORT_SYMBOL(with_primes);
+
+EXPORT_SYMBOL(slow_is_prime_number);
+
+#else
+static
+#endif
+bool slow_is_prime_number(unsigned long x)
+{
+	unsigned long y = int_sqrt(x);
+
+	while (y > 1) {
+		if ((x % y) == 0)
+			break;
+		y--;
+	}
+
+	return y == 1;
+}
+
+static unsigned long slow_next_prime_number(unsigned long x)
+{
+	while (x < ULONG_MAX && !slow_is_prime_number(++x))
+		;
+
+	return x;
+}
+
+static unsigned long clear_multiples(unsigned long x,
+				     unsigned long *p,
+				     unsigned long start,
+				     unsigned long end)
+{
+	unsigned long m;
+
+	m = 2 * x;
+	if (m < start)
+		m = roundup(start, x);
+
+	while (m < end) {
+		__clear_bit(m, p);
+		m += x;
+	}
+
+	return x;
+}
+
+static bool expand_to_next_prime(unsigned long x)
+{
+	const struct primes *p;
+	struct primes *new;
+	unsigned long sz, y;
+
+	/* Betrand's Postulate (or Chebyshev's theorem) states that if n > 3,
+	 * there is always at least one prime p between n and 2n - 2.
+	 * Equivalently, if n > 1, then there is always at least one prime p
+	 * such that n < p < 2n.
+	 *
+	 * http://mathworld.wolfram.com/BertrandsPostulate.html
+	 * https://en.wikipedia.org/wiki/Bertrand's_postulate
+	 */
+	sz = 2 * x;
+	if (sz < x)
+		return false;
+
+	sz = round_up(sz, BITS_PER_LONG);
+	new = kmalloc(sizeof(*new) + bitmap_size(sz),
+		      GFP_KERNEL | __GFP_NOWARN);
+	if (!new)
+		return false;
+
+	mutex_lock(&lock);
+	p = rcu_dereference_protected(primes, lockdep_is_held(&lock));
+	if (x < p->last) {
+		kfree(new);
+		goto unlock;
+	}
+
+	/* Where memory permits, track the primes using the
+	 * Sieve of Eratosthenes. The sieve is to remove all multiples of known
+	 * primes from the set, what remains in the set is therefore prime.
+	 */
+	bitmap_fill(new->primes, sz);
+	bitmap_copy(new->primes, p->primes, p->sz);
+	for (y = 2UL; y < sz; y = find_next_bit(new->primes, sz, y + 1))
+		new->last = clear_multiples(y, new->primes, p->sz, sz);
+	new->sz = sz;
+
+	BUG_ON(new->last <= x);
+
+	rcu_assign_pointer(primes, new);
+	if (p != &small_primes)
+		kfree_rcu((struct primes *)p, rcu);
+
+unlock:
+	mutex_unlock(&lock);
+	return true;
+}
+
+static void free_primes(void)
+{
+	const struct primes *p;
+
+	mutex_lock(&lock);
+	p = rcu_dereference_protected(primes, lockdep_is_held(&lock));
+	if (p != &small_primes) {
+		rcu_assign_pointer(primes, &small_primes);
+		kfree_rcu((struct primes *)p, rcu);
+	}
+	mutex_unlock(&lock);
+}
+
+/**
+ * next_prime_number - return the next prime number
+ * @x: the starting point for searching to test
+ *
+ * A prime number is an integer greater than 1 that is only divisible by
+ * itself and 1.  The set of prime numbers is computed using the Sieve of
+ * Eratoshenes (on finding a prime, all multiples of that prime are removed
+ * from the set) enabling a fast lookup of the next prime number larger than
+ * @x. If the sieve fails (memory limitation), the search falls back to using
+ * slow trial-divison, up to the value of ULONG_MAX (which is reported as the
+ * final prime as a sentinel).
+ *
+ * Returns: the next prime number larger than @x
+ */
+unsigned long next_prime_number(unsigned long x)
+{
+	const struct primes *p;
+
+	rcu_read_lock();
+	p = rcu_dereference(primes);
+	while (x >= p->last) {
+		rcu_read_unlock();
+
+		if (!expand_to_next_prime(x))
+			return slow_next_prime_number(x);
+
+		rcu_read_lock();
+		p = rcu_dereference(primes);
+	}
+	x = find_next_bit(p->primes, p->last, x + 1);
+	rcu_read_unlock();
+
+	return x;
+}
+EXPORT_SYMBOL(next_prime_number);
+
+/**
+ * is_prime_number - test whether the given number is prime
+ * @x: the number to test
+ *
+ * A prime number is an integer greater than 1 that is only divisible by
+ * itself and 1. Internally a cache of prime numbers is kept (to speed up
+ * searching for sequential primes, see next_prime_number()), but if the number
+ * falls outside of that cache, its primality is tested using trial-divison.
+ *
+ * Returns: true if @x is prime, false for composite numbers.
+ */
+bool is_prime_number(unsigned long x)
+{
+	const struct primes *p;
+	bool result;
+
+	rcu_read_lock();
+	p = rcu_dereference(primes);
+	while (x >= p->sz) {
+		rcu_read_unlock();
+
+		if (!expand_to_next_prime(x))
+			return slow_is_prime_number(x);
+
+		rcu_read_lock();
+		p = rcu_dereference(primes);
+	}
+	result = test_bit(x, p->primes);
+	rcu_read_unlock();
+
+	return result;
+}
+EXPORT_SYMBOL(is_prime_number);
+
+static void __exit primes_exit(void)
+{
+	free_primes();
+}
+
+module_exit(primes_exit);
+
+MODULE_AUTHOR("Intel Corporation");
+MODULE_DESCRIPTION("Prime number library");
+MODULE_LICENSE("GPL");