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, 116 insertions, 0 deletions

diff --git a/fs/proc/nommu.c b/fs/proc/nommu.c
new file mode 100644
index 000000000..c6e7ebc63
--- /dev/null
+++ b/fs/proc/nommu.c
@@ -0,0 +1,116 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* nommu.c: mmu-less memory info files
+ *
+ * Copyright (C) 2004 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/errno.h>
+#include <linux/time.h>
+#include <linux/kernel.h>
+#include <linux/string.h>
+#include <linux/mman.h>
+#include <linux/proc_fs.h>
+#include <linux/mm.h>
+#include <linux/mmzone.h>
+#include <linux/pagemap.h>
+#include <linux/swap.h>
+#include <linux/smp.h>
+#include <linux/seq_file.h>
+#include <linux/hugetlb.h>
+#include <linux/vmalloc.h>
+#include <asm/tlb.h>
+#include <asm/div64.h>
+#include "internal.h"
+
+/*
+ * display a single region to a sequenced file
+ */
+static int nommu_region_show(struct seq_file *m, struct vm_region *region)
+{
+	unsigned long ino = 0;
+	struct file *file;
+	dev_t dev = 0;
+	int flags;
+
+	flags = region->vm_flags;
+	file = region->vm_file;
+
+	if (file) {
+		struct inode *inode = file_inode(region->vm_file);
+		dev = inode->i_sb->s_dev;
+		ino = inode->i_ino;
+	}
+
+	seq_setwidth(m, 25 + sizeof(void *) * 6 - 1);
+	seq_printf(m,
+		   "%08lx-%08lx %c%c%c%c %08llx %02x:%02x %lu ",
+		   region->vm_start,
+		   region->vm_end,
+		   flags & VM_READ ? 'r' : '-',
+		   flags & VM_WRITE ? 'w' : '-',
+		   flags & VM_EXEC ? 'x' : '-',
+		   flags & VM_MAYSHARE ? flags & VM_SHARED ? 'S' : 's' : 'p',
+		   ((loff_t)region->vm_pgoff) << PAGE_SHIFT,
+		   MAJOR(dev), MINOR(dev), ino);
+
+	if (file) {
+		seq_pad(m, ' ');
+		seq_path(m, file_user_path(file), "");
+	}
+
+	seq_putc(m, '\n');
+	return 0;
+}
+
+/*
+ * display a list of all the REGIONs the kernel knows about
+ * - nommu kernels have a single flat list
+ */
+static int nommu_region_list_show(struct seq_file *m, void *_p)
+{
+	struct rb_node *p = _p;
+
+	return nommu_region_show(m, rb_entry(p, struct vm_region, vm_rb));
+}
+
+static void *nommu_region_list_start(struct seq_file *m, loff_t *_pos)
+{
+	struct rb_node *p;
+	loff_t pos = *_pos;
+
+	down_read(&nommu_region_sem);
+
+	for (p = rb_first(&nommu_region_tree); p; p = rb_next(p))
+		if (pos-- == 0)
+			return p;
+	return NULL;
+}
+
+static void nommu_region_list_stop(struct seq_file *m, void *v)
+{
+	up_read(&nommu_region_sem);
+}
+
+static void *nommu_region_list_next(struct seq_file *m, void *v, loff_t *pos)
+{
+	(*pos)++;
+	return rb_next((struct rb_node *) v);
+}
+
+static const struct seq_operations proc_nommu_region_list_seqop = {
+	.start	= nommu_region_list_start,
+	.next	= nommu_region_list_next,
+	.stop	= nommu_region_list_stop,
+	.show	= nommu_region_list_show
+};
+
+static int __init proc_nommu_init(void)
+{
+	proc_create_seq("maps", S_IRUGO, NULL, &proc_nommu_region_list_seqop);
+	return 0;
+}
+
+fs_initcall(proc_nommu_init);