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, 139 insertions, 0 deletions

diff --git a/tools/perf/util/mem2node.c b/tools/perf/util/mem2node.c
new file mode 100644
index 000000000..51a2292cb
--- /dev/null
+++ b/tools/perf/util/mem2node.c
@@ -0,0 +1,139 @@
+#include <errno.h>
+#include <inttypes.h>
+#include <asm/bug.h>
+#include <linux/bitmap.h>
+#include <linux/kernel.h>
+#include <linux/zalloc.h>
+#include "debug.h"
+#include "env.h"
+#include "mem2node.h"
+
+struct phys_entry {
+	struct rb_node	rb_node;
+	u64	start;
+	u64	end;
+	u64	node;
+};
+
+static void phys_entry__insert(struct phys_entry *entry, struct rb_root *root)
+{
+	struct rb_node **p = &root->rb_node;
+	struct rb_node *parent = NULL;
+	struct phys_entry *e;
+
+	while (*p != NULL) {
+		parent = *p;
+		e = rb_entry(parent, struct phys_entry, rb_node);
+
+		if (entry->start < e->start)
+			p = &(*p)->rb_left;
+		else
+			p = &(*p)->rb_right;
+	}
+
+	rb_link_node(&entry->rb_node, parent, p);
+	rb_insert_color(&entry->rb_node, root);
+}
+
+static void
+phys_entry__init(struct phys_entry *entry, u64 start, u64 bsize, u64 node)
+{
+	entry->start = start;
+	entry->end   = start + bsize;
+	entry->node  = node;
+	RB_CLEAR_NODE(&entry->rb_node);
+}
+
+int mem2node__init(struct mem2node *map, struct perf_env *env)
+{
+	struct memory_node *n, *nodes = &env->memory_nodes[0];
+	struct phys_entry *entries, *tmp_entries;
+	u64 bsize = env->memory_bsize;
+	int i, j = 0, max = 0;
+
+	memset(map, 0x0, sizeof(*map));
+	map->root = RB_ROOT;
+
+	for (i = 0; i < env->nr_memory_nodes; i++) {
+		n = &nodes[i];
+		max += bitmap_weight(n->set, n->size);
+	}
+
+	entries = calloc(max, sizeof(*entries));
+	if (!entries)
+		return -ENOMEM;
+
+	for (i = 0; i < env->nr_memory_nodes; i++) {
+		u64 bit;
+
+		n = &nodes[i];
+
+		for (bit = 0; bit < n->size; bit++) {
+			u64 start;
+
+			if (!test_bit(bit, n->set))
+				continue;
+
+			start = bit * bsize;
+
+			/*
+			 * Merge nearby areas, we walk in order
+			 * through the bitmap, so no need to sort.
+			 */
+			if (j > 0) {
+				struct phys_entry *prev = &entries[j - 1];
+
+				if ((prev->end == start) &&
+				    (prev->node == n->node)) {
+					prev->end += bsize;
+					continue;
+				}
+			}
+
+			phys_entry__init(&entries[j++], start, bsize, n->node);
+		}
+	}
+
+	/* Cut unused entries, due to merging. */
+	tmp_entries = realloc(entries, sizeof(*entries) * j);
+	if (tmp_entries ||
+	    WARN_ONCE(j == 0, "No memory nodes, is CONFIG_MEMORY_HOTPLUG enabled?\n"))
+		entries = tmp_entries;
+
+	for (i = 0; i < j; i++) {
+		pr_debug("mem2node %03" PRIu64 " [0x%016" PRIx64 "-0x%016" PRIx64 "]\n",
+			 entries[i].node, entries[i].start, entries[i].end);
+
+		phys_entry__insert(&entries[i], &map->root);
+	}
+
+	map->entries = entries;
+	return 0;
+}
+
+void mem2node__exit(struct mem2node *map)
+{
+	zfree(&map->entries);
+}
+
+int mem2node__node(struct mem2node *map, u64 addr)
+{
+	struct rb_node **p, *parent = NULL;
+	struct phys_entry *entry;
+
+	p = &map->root.rb_node;
+	while (*p != NULL) {
+		parent = *p;
+		entry = rb_entry(parent, struct phys_entry, rb_node);
+		if (addr < entry->start)
+			p = &(*p)->rb_left;
+		else if (addr >= entry->end)
+			p = &(*p)->rb_right;
+		else
+			goto out;
+	}
+
+	entry = NULL;
+out:
+	return entry ? (int) entry->node : -1;
+}