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, 166 insertions, 0 deletions

diff --git a/fs/failfs.c b/fs/failfs.c
new file mode 100644
index 000000000..66a36da3d
--- /dev/null
+++ b/fs/failfs.c
@@ -0,0 +1,166 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2026 Christian Brauner <brauner@kernel.org> */
+#include <linux/fs.h>
+#include <linux/fs/super_types.h>
+#include <linux/fs_context.h>
+#include <linux/fs_struct.h>
+#include <linux/magic.h>
+#include <linux/mount.h>
+
+#include "internal.h"
+
+static struct path failfs_root_path = {};
+
+void failfs_get_root(struct path *path)
+{
+	*path = failfs_root_path;
+	path_get(path);
+}
+
+bool failfs_mnt(const struct vfsmount *mnt)
+{
+	return mnt->mnt_sb == failfs_root_path.mnt->mnt_sb;
+}
+
+static int failfs_permission(struct mnt_idmap *idmap, struct inode *inode,
+			     int mask)
+{
+	return -EOPNOTSUPP;
+}
+
+static struct dentry *failfs_lookup(struct inode *dir, struct dentry *dentry,
+				    unsigned int flags)
+{
+	/* Unreachable: ->permission() already failed the walk. */
+	return ERR_PTR(-EOPNOTSUPP);
+}
+
+static int failfs_getattr(struct mnt_idmap *idmap, const struct path *path,
+			  struct kstat *stat, u32 request_mask,
+			  unsigned int query_flags)
+{
+	return -EOPNOTSUPP;
+}
+
+static const struct inode_operations failfs_dir_inode_operations = {
+	.permission	= failfs_permission,
+	.lookup		= failfs_lookup,
+	.getattr	= failfs_getattr,
+};
+
+static const struct file_operations failfs_dir_operations = {};
+
+static int failfs_d_weak_revalidate(struct dentry *dentry, unsigned int flags)
+{
+	/*
+	 * The root is only ever reached as a path-walk terminal by jumping
+	 * to it: as "/" when it is the caller's root, or through a
+	 * /proc/<pid>/{root,cwd} magic link. ->permission() already fails
+	 * every walk of a component, but a jump lands on the root without
+	 * one. Refuse here too so the root cannot be pinned by an O_PATH
+	 * open or encoded into a file handle.
+	 */
+	return -EOPNOTSUPP;
+}
+
+static char *failfs_dname(struct dentry *dentry, char *buffer, int buflen)
+{
+	return dynamic_dname(buffer, buflen, "failfs:/");
+}
+
+static const struct dentry_operations failfs_dentry_operations = {
+	.d_dname		= failfs_dname,
+	.d_weak_revalidate	= failfs_d_weak_revalidate,
+};
+
+static int failfs_statfs(struct dentry *dentry, struct kstatfs *buf)
+{
+	return -EOPNOTSUPP;
+}
+
+static const struct super_operations failfs_super_operations = {
+	.statfs	= failfs_statfs,
+};
+
+static int failfs_fill_super(struct super_block *s, struct fs_context *fc)
+{
+	struct inode *inode;
+
+	s->s_maxbytes		= MAX_LFS_FILESIZE;
+	s->s_blocksize		= PAGE_SIZE;
+	s->s_blocksize_bits	= PAGE_SHIFT;
+	s->s_magic		= FAIL_FS_MAGIC;
+	s->s_op			= &failfs_super_operations;
+	s->s_export_op		= NULL;
+	s->s_xattr		= NULL;
+	s->s_time_gran		= 1;
+	s->s_d_flags		= 0;
+
+	inode = new_inode(s);
+	if (!inode)
+		return -ENOMEM;
+
+	/* failfs supports no operations... */
+	inode->i_mode	= S_IFDIR;
+	set_nlink(inode, 2);
+	inode->i_op	= &failfs_dir_inode_operations;
+	inode->i_fop	= &failfs_dir_operations;
+	simple_inode_init_ts(inode);
+	inode->i_ino	= 1;
+	/* ... and is immutable. */
+	inode->i_flags |= S_IMMUTABLE;
+
+	set_default_d_op(s, &failfs_dentry_operations);
+	s->s_root = d_make_root(inode);
+	if (!s->s_root)
+		return -ENOMEM;
+
+	return 0;
+}
+
+static int failfs_get_tree(struct fs_context *fc)
+{
+	return get_tree_single(fc, failfs_fill_super);
+}
+
+static const struct fs_context_operations failfs_context_ops = {
+	.get_tree	= failfs_get_tree,
+};
+
+static int failfs_init_fs_context(struct fs_context *fc)
+{
+	fc->ops		= &failfs_context_ops;
+	fc->global	= true;
+	fc->sb_flags	|= SB_NOUSER;
+	fc->s_iflags	|= SB_I_NOEXEC | SB_I_NODEV;
+	return 0;
+}
+
+int failfs_current_chdir(void)
+{
+	struct path path;
+
+	failfs_get_root(&path);
+	set_fs_pwd(current->fs, &path);
+	path_put(&path);
+	return 0;
+}
+
+static struct file_system_type failfs_fs_type = {
+	.name			= "failfs",
+	.init_fs_context	= failfs_init_fs_context,
+	.kill_sb		= kill_anon_super,
+};
+
+void __init failfs_init(void)
+{
+	struct vfsmount *mnt;
+
+	/* A single instance that is member of no mount namespace. */
+	mnt = kern_mount(&failfs_fs_type);
+	if (IS_ERR(mnt))
+		panic("VFS: Failed to create failfs");
+
+	failfs_root_path.mnt	= mnt;
+	failfs_root_path.dentry	= mnt->mnt_root;
+}