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, 121 insertions, 0 deletions

diff --git a/scripts/kconfig/util.c b/scripts/kconfig/util.c
new file mode 100644
index 000000000..0809aa061
--- /dev/null
+++ b/scripts/kconfig/util.c
@@ -0,0 +1,121 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2002-2005 Roman Zippel <zippel@linux-m68k.org>
+ * Copyright (C) 2002-2005 Sam Ravnborg <sam@ravnborg.org>
+ */
+
+#include <stdarg.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include <hash.h>
+#include <hashtable.h>
+#include <xalloc.h>
+#include "lkc.h"
+
+/* hash table of all parsed Kconfig files */
+static HASHTABLE_DEFINE(file_hashtable, 1U << 11);
+
+struct file {
+	struct hlist_node node;
+	struct {
+		const char *name;
+		int lineno;
+	} parent;
+	char name[];
+};
+
+static void die_duplicated_include(struct file *file,
+				   const char *parent, int lineno)
+{
+	fprintf(stderr,
+		"%s:%d: error: repeated inclusion of %s\n"
+		"%s:%d: note: location of first inclusion of %s\n",
+		parent, lineno, file->name,
+		file->parent.name, file->parent.lineno, file->name);
+	exit(1);
+}
+
+/* file already present in list? If not add it */
+const char *file_lookup(const char *name,
+			const char *parent_name, int parent_lineno)
+{
+	const char *parent = NULL;
+	struct file *file;
+	size_t len;
+	int hash = hash_str(name);
+
+	if (parent_name)
+		parent = file_lookup(parent_name, NULL, 0);
+
+	hash_for_each_possible(file_hashtable, file, node, hash)
+		if (!strcmp(name, file->name)) {
+			if (!parent_name)
+				return file->name;
+			die_duplicated_include(file, parent, parent_lineno);
+		}
+
+	len = strlen(name);
+	file = xmalloc(sizeof(*file) + len + 1);
+	memset(file, 0, sizeof(*file));
+	memcpy(file->name, name, len);
+	file->name[len] = '\0';
+	file->parent.name = parent;
+	file->parent.lineno = parent_lineno;
+
+	hash_add(file_hashtable, &file->node, hash);
+
+	str_printf(&autoconf_cmd, "\t%s \\\n", name);
+
+	return file->name;
+}
+
+/* Allocate initial growable string */
+struct gstr str_new(void)
+{
+	struct gstr gs;
+	gs.s = xmalloc(sizeof(char) * 64);
+	gs.len = 64;
+	gs.max_width = 0;
+	strcpy(gs.s, "\0");
+	return gs;
+}
+
+/* Free storage for growable string */
+void str_free(struct gstr *gs)
+{
+	free(gs->s);
+	gs->s = NULL;
+	gs->len = 0;
+}
+
+/* Append to growable string */
+void str_append(struct gstr *gs, const char *s)
+{
+	size_t l;
+	if (s) {
+		l = strlen(gs->s) + strlen(s) + 1;
+		if (l > gs->len) {
+			gs->s = xrealloc(gs->s, l);
+			gs->len = l;
+		}
+		strcat(gs->s, s);
+	}
+}
+
+/* Append printf formatted string to growable string */
+void str_printf(struct gstr *gs, const char *fmt, ...)
+{
+	va_list ap;
+	char s[10000]; /* big enough... */
+	va_start(ap, fmt);
+	vsnprintf(s, sizeof(s), fmt, ap);
+	str_append(gs, s);
+	va_end(ap);
+}
+
+/* Retrieve value of growable string */
+char *str_get(const struct gstr *gs)
+{
+	return gs->s;
+}