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, 215 insertions, 0 deletions

diff --git a/drivers/of/pdt.c b/drivers/of/pdt.c
new file mode 100644
index 000000000..cb0cb374b
--- /dev/null
+++ b/drivers/of/pdt.c
@@ -0,0 +1,215 @@
+// SPDX-License-Identifier: GPL-2.0+
+/* pdt.c: OF PROM device tree support code.
+ *
+ * Paul Mackerras	August 1996.
+ * Copyright (C) 1996-2005 Paul Mackerras.
+ *
+ *  Adapted for 64bit PowerPC by Dave Engebretsen and Peter Bergner.
+ *    {engebret|bergner}@us.ibm.com
+ *
+ *  Adapted for sparc by David S. Miller davem@davemloft.net
+ *  Adapted for multiple architectures by Andres Salomon <dilinger@queued.net>
+ */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/errno.h>
+#include <linux/mutex.h>
+#include <linux/slab.h>
+#include <linux/of.h>
+#include <linux/of_pdt.h>
+
+#include "of_private.h"
+
+static struct of_pdt_ops *of_pdt_prom_ops __initdata;
+
+#if defined(CONFIG_SPARC)
+unsigned int of_pdt_unique_id __initdata;
+
+#define of_pdt_incr_unique_id(p) do { \
+	(p)->unique_id = of_pdt_unique_id++; \
+} while (0)
+
+static char * __init of_pdt_build_full_name(struct device_node *dp)
+{
+	return build_path_component(dp);
+}
+
+#else /* CONFIG_SPARC */
+
+static inline void of_pdt_incr_unique_id(void *p) { }
+static inline void irq_trans_init(struct device_node *dp) { }
+
+static char * __init of_pdt_build_full_name(struct device_node *dp)
+{
+	static int failsafe_id = 0; /* for generating unique names on failure */
+	const char *name;
+	char path[256];
+	char *buf;
+	int len;
+
+	if (!of_pdt_prom_ops->pkg2path(dp->phandle, path, sizeof(path), &len)) {
+		name = kbasename(path);
+		buf = prom_early_alloc(strlen(name) + 1);
+		strcpy(buf, name);
+		return buf;
+	}
+
+	name = of_get_property(dp, "name", &len);
+	buf = prom_early_alloc(len + 16);
+	sprintf(buf, "%s@unknown%i", name, failsafe_id++);
+	pr_err("%s: pkg2path failed; assigning %s\n", __func__, buf);
+	return buf;
+}
+
+#endif /* !CONFIG_SPARC */
+
+static struct property * __init of_pdt_build_one_prop(phandle node, char *prev,
+					       char *special_name,
+					       void *special_val,
+					       int special_len)
+{
+	static struct property *tmp = NULL;
+	struct property *p;
+	int err;
+
+	if (tmp) {
+		p = tmp;
+		memset(p, 0, sizeof(*p) + 32);
+		tmp = NULL;
+	} else {
+		p = prom_early_alloc(sizeof(struct property) + 32);
+		of_pdt_incr_unique_id(p);
+	}
+
+	p->name = (char *) (p + 1);
+	if (special_name) {
+		strcpy(p->name, special_name);
+		p->length = special_len;
+		p->value = prom_early_alloc(special_len);
+		memcpy(p->value, special_val, special_len);
+	} else {
+		err = of_pdt_prom_ops->nextprop(node, prev, p->name);
+		if (err) {
+			tmp = p;
+			return NULL;
+		}
+		p->length = of_pdt_prom_ops->getproplen(node, p->name);
+		if (p->length <= 0) {
+			p->length = 0;
+		} else {
+			int len;
+
+			p->value = prom_early_alloc(p->length + 1);
+			len = of_pdt_prom_ops->getproperty(node, p->name,
+					p->value, p->length);
+			if (len <= 0)
+				p->length = 0;
+			((unsigned char *)p->value)[p->length] = '\0';
+		}
+	}
+	return p;
+}
+
+static struct property * __init of_pdt_build_prop_list(phandle node)
+{
+	struct property *head, *tail;
+
+	head = tail = of_pdt_build_one_prop(node, NULL,
+				     ".node", &node, sizeof(node));
+
+	tail->next = of_pdt_build_one_prop(node, NULL, NULL, NULL, 0);
+	tail = tail->next;
+	while(tail) {
+		tail->next = of_pdt_build_one_prop(node, tail->name,
+					    NULL, NULL, 0);
+		tail = tail->next;
+	}
+
+	return head;
+}
+
+static char * __init of_pdt_get_one_property(phandle node, const char *name)
+{
+	char *buf = "<NULL>";
+	int len;
+
+	len = of_pdt_prom_ops->getproplen(node, name);
+	if (len > 0) {
+		buf = prom_early_alloc(len);
+		len = of_pdt_prom_ops->getproperty(node, name, buf, len);
+	}
+
+	return buf;
+}
+
+static struct device_node * __init of_pdt_create_node(phandle node,
+						    struct device_node *parent)
+{
+	struct device_node *dp;
+
+	if (!node)
+		return NULL;
+
+	dp = prom_early_alloc(sizeof(*dp));
+	of_node_init(dp);
+	of_pdt_incr_unique_id(dp);
+	dp->parent = parent;
+
+	dp->name = of_pdt_get_one_property(node, "name");
+	dp->phandle = node;
+
+	dp->properties = of_pdt_build_prop_list(node);
+
+	dp->full_name = of_pdt_build_full_name(dp);
+
+	irq_trans_init(dp);
+
+	return dp;
+}
+
+static struct device_node * __init of_pdt_build_tree(struct device_node *parent,
+						   phandle node)
+{
+	struct device_node *ret = NULL, *prev_sibling = NULL;
+	struct device_node *dp;
+
+	while (1) {
+		dp = of_pdt_create_node(node, parent);
+		if (!dp)
+			break;
+
+		if (prev_sibling)
+			prev_sibling->sibling = dp;
+
+		if (!ret)
+			ret = dp;
+		prev_sibling = dp;
+
+		dp->child = of_pdt_build_tree(dp, of_pdt_prom_ops->getchild(node));
+
+		node = of_pdt_prom_ops->getsibling(node);
+	}
+
+	return ret;
+}
+
+static void * __init kernel_tree_alloc(u64 size, u64 align)
+{
+	return prom_early_alloc(size);
+}
+
+void __init of_pdt_build_devicetree(phandle root_node, struct of_pdt_ops *ops)
+{
+	BUG_ON(!ops);
+	of_pdt_prom_ops = ops;
+
+	of_root = of_pdt_create_node(root_node, NULL);
+	of_root->full_name = "/";
+
+	of_root->child = of_pdt_build_tree(of_root,
+				of_pdt_prom_ops->getchild(of_root->phandle));
+
+	/* Get pointer to "/chosen" and "/aliases" nodes for use everywhere */
+	of_alias_scan(kernel_tree_alloc);
+}