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, 311 insertions, 0 deletions

diff --git a/lib/plist.c b/lib/plist.c
new file mode 100644
index 000000000..a5bef38ad
--- /dev/null
+++ b/lib/plist.c
@@ -0,0 +1,311 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * lib/plist.c
+ *
+ * Descending-priority-sorted double-linked list
+ *
+ * (C) 2002-2003 Intel Corp
+ * Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>.
+ *
+ * 2001-2005 (c) MontaVista Software, Inc.
+ * Daniel Walker <dwalker@mvista.com>
+ *
+ * (C) 2005 Linutronix GmbH, Thomas Gleixner <tglx@kernel.org>
+ *
+ * Simplifications of the original code by
+ * Oleg Nesterov <oleg@tv-sign.ru>
+ *
+ * Based on simple lists (include/linux/list.h).
+ *
+ * This file contains the add / del functions which are considered to
+ * be too large to inline. See include/linux/plist.h for further
+ * information.
+ */
+
+#include <linux/bug.h>
+#include <linux/plist.h>
+
+#ifdef CONFIG_DEBUG_PLIST
+
+static struct plist_head test_head;
+
+static void plist_check_prev_next(struct list_head *t, struct list_head *p,
+				  struct list_head *n)
+{
+	WARN(n->prev != p || p->next != n,
+			"top: %p, n: %p, p: %p\n"
+			"prev: %p, n: %p, p: %p\n"
+			"next: %p, n: %p, p: %p\n",
+			 t, t->next, t->prev,
+			p, p->next, p->prev,
+			n, n->next, n->prev);
+}
+
+static void plist_check_list(struct list_head *top)
+{
+	struct list_head *prev = top, *next = top->next;
+
+	plist_check_prev_next(top, prev, next);
+	while (next != top) {
+		prev = next;
+		next = prev->next;
+		plist_check_prev_next(top, prev, next);
+	}
+}
+
+static void plist_check_head(struct plist_head *head)
+{
+	if (!plist_head_empty(head))
+		plist_check_list(&plist_first(head)->prio_list);
+	plist_check_list(&head->node_list);
+}
+
+#else
+# define plist_check_head(h)	do { } while (0)
+#endif
+
+/**
+ * plist_add - add @node to @head
+ *
+ * @node:	&struct plist_node pointer
+ * @head:	&struct plist_head pointer
+ */
+void plist_add(struct plist_node *node, struct plist_head *head)
+{
+	struct plist_node *first, *iter, *prev = NULL, *last, *reverse_iter;
+	struct list_head *node_next = &head->node_list;
+
+	plist_check_head(head);
+	WARN_ON(!plist_node_empty(node));
+	WARN_ON(!list_empty(&node->prio_list));
+
+	if (plist_head_empty(head))
+		goto ins_node;
+
+	first = iter = plist_first(head);
+	last = reverse_iter = list_entry(first->prio_list.prev, struct plist_node, prio_list);
+
+	do {
+		if (node->prio < iter->prio) {
+			node_next = &iter->node_list;
+			break;
+		} else if (node->prio >= reverse_iter->prio) {
+			prev = reverse_iter;
+			iter = list_entry(reverse_iter->prio_list.next,
+				struct plist_node, prio_list);
+			if (likely(reverse_iter != last))
+				node_next = &iter->node_list;
+			break;
+		}
+
+		prev = iter;
+		iter = list_entry(iter->prio_list.next,
+				struct plist_node, prio_list);
+		reverse_iter = list_entry(reverse_iter->prio_list.prev,
+				struct plist_node, prio_list);
+	} while (iter != first);
+
+	if (!prev || prev->prio != node->prio)
+		list_add_tail(&node->prio_list, &iter->prio_list);
+ins_node:
+	list_add_tail(&node->node_list, node_next);
+
+	plist_check_head(head);
+}
+
+/**
+ * plist_del - Remove a @node from plist.
+ *
+ * @node:	&struct plist_node pointer - entry to be removed
+ * @head:	&struct plist_head pointer - list head
+ */
+void plist_del(struct plist_node *node, struct plist_head *head)
+{
+	plist_check_head(head);
+
+	if (!list_empty(&node->prio_list)) {
+		if (node->node_list.next != &head->node_list) {
+			struct plist_node *next;
+
+			next = list_entry(node->node_list.next,
+					struct plist_node, node_list);
+
+			/* add the next plist_node into prio_list */
+			if (list_empty(&next->prio_list))
+				list_add(&next->prio_list, &node->prio_list);
+		}
+		list_del_init(&node->prio_list);
+	}
+
+	list_del_init(&node->node_list);
+
+	plist_check_head(head);
+}
+
+/**
+ * plist_requeue - Requeue @node at end of same-prio entries.
+ *
+ * This is essentially an optimized plist_del() followed by
+ * plist_add().  It moves an entry already in the plist to
+ * after any other same-priority entries.
+ *
+ * @node:	&struct plist_node pointer - entry to be moved
+ * @head:	&struct plist_head pointer - list head
+ */
+void plist_requeue(struct plist_node *node, struct plist_head *head)
+{
+	struct plist_node *iter;
+	struct list_head *node_next = &head->node_list;
+
+	plist_check_head(head);
+	BUG_ON(plist_head_empty(head));
+	BUG_ON(plist_node_empty(node));
+
+	if (node == plist_last(head))
+		return;
+
+	iter = plist_next(node);
+
+	if (node->prio != iter->prio)
+		return;
+
+	plist_del(node, head);
+
+	/*
+	 * After plist_del(), iter is the replacement of the node.  If the node
+	 * was on prio_list, take shortcut to find node_next instead of looping.
+	 */
+	if (!list_empty(&iter->prio_list)) {
+		iter = list_entry(iter->prio_list.next, struct plist_node,
+				  prio_list);
+		node_next = &iter->node_list;
+		goto queue;
+	}
+
+	plist_for_each_continue(iter, head) {
+		if (node->prio != iter->prio) {
+			node_next = &iter->node_list;
+			break;
+		}
+	}
+queue:
+	list_add_tail(&node->node_list, node_next);
+
+	plist_check_head(head);
+}
+
+#ifdef CONFIG_DEBUG_PLIST
+#include <linux/sched.h>
+#include <linux/sched/clock.h>
+#include <linux/module.h>
+#include <linux/init.h>
+
+static struct plist_node __initdata test_node[241];
+
+static void __init plist_test_check(int nr_expect)
+{
+	struct plist_node *first, *prio_pos, *node_pos;
+
+	if (plist_head_empty(&test_head)) {
+		BUG_ON(nr_expect != 0);
+		return;
+	}
+
+	prio_pos = first = plist_first(&test_head);
+	plist_for_each(node_pos, &test_head) {
+		if (nr_expect-- < 0)
+			break;
+		if (node_pos == first)
+			continue;
+		if (node_pos->prio == prio_pos->prio) {
+			BUG_ON(!list_empty(&node_pos->prio_list));
+			continue;
+		}
+
+		BUG_ON(prio_pos->prio > node_pos->prio);
+		BUG_ON(prio_pos->prio_list.next != &node_pos->prio_list);
+		prio_pos = node_pos;
+	}
+
+	BUG_ON(nr_expect != 0);
+	BUG_ON(prio_pos->prio_list.next != &first->prio_list);
+}
+
+static void __init plist_test_requeue(struct plist_node *node)
+{
+	plist_requeue(node, &test_head);
+
+	if (node != plist_last(&test_head))
+		BUG_ON(node->prio == plist_next(node)->prio);
+}
+
+static int  __init plist_test(void)
+{
+	int nr_expect = 0, i, loop;
+	unsigned int r = local_clock();
+
+	printk(KERN_DEBUG "start plist test\n");
+	plist_head_init(&test_head);
+	for (i = 0; i < ARRAY_SIZE(test_node); i++)
+		plist_node_init(test_node + i, 0);
+
+	for (loop = 0; loop < 1000; loop++) {
+		r = r * 193939 % 47629;
+		i = r % ARRAY_SIZE(test_node);
+		if (plist_node_empty(test_node + i)) {
+			r = r * 193939 % 47629;
+			test_node[i].prio = r % 99;
+			plist_add(test_node + i, &test_head);
+			nr_expect++;
+		} else {
+			plist_del(test_node + i, &test_head);
+			nr_expect--;
+		}
+		plist_test_check(nr_expect);
+		if (!plist_node_empty(test_node + i)) {
+			plist_test_requeue(test_node + i);
+			plist_test_check(nr_expect);
+		}
+	}
+
+	for (i = 0; i < ARRAY_SIZE(test_node); i++) {
+		if (plist_node_empty(test_node + i))
+			continue;
+		plist_del(test_node + i, &test_head);
+		nr_expect--;
+		plist_test_check(nr_expect);
+	}
+
+	printk(KERN_DEBUG "end plist test\n");
+
+	/* Worst case test for plist_add() */
+	unsigned int test_data[241];
+
+	for (i = 0; i < ARRAY_SIZE(test_data); i++)
+		test_data[i] = i;
+
+	ktime_t start, end, time_elapsed = 0;
+
+	plist_head_init(&test_head);
+
+	for (i = 0; i < ARRAY_SIZE(test_node); i++) {
+		plist_node_init(test_node + i, 0);
+		test_node[i].prio = test_data[i];
+	}
+
+	for (i = 0; i < ARRAY_SIZE(test_node); i++) {
+		if (plist_node_empty(test_node + i)) {
+			start = ktime_get();
+			plist_add(test_node + i, &test_head);
+			end = ktime_get();
+			time_elapsed += (end - start);
+		}
+	}
+
+	pr_debug("plist_add worst case test time elapsed %lld\n", time_elapsed);
+	return 0;
+}
+
+module_init(plist_test);
+
+#endif