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, 337 insertions, 0 deletions

diff --git a/fs/proc_namespace.c b/fs/proc_namespace.c
new file mode 100644
index 000000000..036356c0a
--- /dev/null
+++ b/fs/proc_namespace.c
@@ -0,0 +1,337 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * fs/proc_namespace.c - handling of /proc/<pid>/{mounts,mountinfo,mountstats}
+ *
+ * In fact, that's a piece of procfs; it's *almost* isolated from
+ * the rest of fs/proc, but has rather close relationships with
+ * fs/namespace.c, thus here instead of fs/proc
+ *
+ */
+#include <linux/mnt_namespace.h>
+#include <linux/nsproxy.h>
+#include <linux/security.h>
+#include <linux/fs_struct.h>
+#include <linux/sched/task.h>
+
+#include "proc/internal.h" /* only for get_proc_task() in ->open() */
+
+#include "pnode.h"
+#include "internal.h"
+
+static __poll_t mounts_poll(struct file *file, poll_table *wait)
+{
+	struct seq_file *m = file->private_data;
+	struct proc_mounts *p = m->private;
+	struct mnt_namespace *ns = p->ns;
+	__poll_t res = EPOLLIN | EPOLLRDNORM;
+	int event;
+
+	poll_wait(file, &p->ns->poll, wait);
+
+	event = READ_ONCE(ns->event);
+	if (m->poll_event != event) {
+		m->poll_event = event;
+		res |= EPOLLERR | EPOLLPRI;
+	}
+
+	return res;
+}
+
+struct proc_fs_opts {
+	int flag;
+	const char *str;
+};
+
+static int show_sb_opts(struct seq_file *m, struct super_block *sb)
+{
+	static const struct proc_fs_opts fs_opts[] = {
+		{ SB_SYNCHRONOUS, ",sync" },
+		{ SB_DIRSYNC, ",dirsync" },
+		{ SB_MANDLOCK, ",mand" },
+		{ SB_LAZYTIME, ",lazytime" },
+		{ 0, NULL }
+	};
+	const struct proc_fs_opts *fs_infop;
+
+	for (fs_infop = fs_opts; fs_infop->flag; fs_infop++) {
+		if (sb->s_flags & fs_infop->flag)
+			seq_puts(m, fs_infop->str);
+	}
+
+	return security_sb_show_options(m, sb);
+}
+
+static void show_vfsmnt_opts(struct seq_file *m, struct vfsmount *mnt)
+{
+	static const struct proc_fs_opts mnt_opts[] = {
+		{ MNT_NOSUID, ",nosuid" },
+		{ MNT_NODEV, ",nodev" },
+		{ MNT_NOEXEC, ",noexec" },
+		{ MNT_NOATIME, ",noatime" },
+		{ MNT_NODIRATIME, ",nodiratime" },
+		{ MNT_RELATIME, ",relatime" },
+		{ MNT_NOSYMFOLLOW, ",nosymfollow" },
+		{ 0, NULL }
+	};
+	const struct proc_fs_opts *fs_infop;
+
+	for (fs_infop = mnt_opts; fs_infop->flag; fs_infop++) {
+		if (mnt->mnt_flags & fs_infop->flag)
+			seq_puts(m, fs_infop->str);
+	}
+
+	if (is_idmapped_mnt(mnt))
+		seq_puts(m, ",idmapped");
+}
+
+static inline void mangle(struct seq_file *m, const char *s)
+{
+	seq_escape(m, s, " \t\n\\#");
+}
+
+static void show_type(struct seq_file *m, struct super_block *sb)
+{
+	mangle(m, sb->s_type->name);
+	if (sb->s_subtype) {
+		seq_putc(m, '.');
+		mangle(m, sb->s_subtype);
+	}
+}
+
+static int show_vfsmnt(struct seq_file *m, struct vfsmount *mnt)
+{
+	struct proc_mounts *p = m->private;
+	struct mount *r = real_mount(mnt);
+	struct path mnt_path = { .dentry = mnt->mnt_root, .mnt = mnt };
+	struct super_block *sb = mnt_path.dentry->d_sb;
+	int err;
+
+	if (sb->s_op->show_devname) {
+		err = sb->s_op->show_devname(m, mnt_path.dentry);
+		if (err)
+			goto out;
+	} else {
+		mangle(m, r->mnt_devname);
+	}
+	seq_putc(m, ' ');
+	/* mountpoints outside of chroot jail will give SEQ_SKIP on this */
+	err = seq_path_root(m, &mnt_path, &p->root, " \t\n\\");
+	if (err)
+		goto out;
+	seq_putc(m, ' ');
+	show_type(m, sb);
+	seq_puts(m, __mnt_is_readonly(mnt) ? " ro" : " rw");
+	err = show_sb_opts(m, sb);
+	if (err)
+		goto out;
+	show_vfsmnt_opts(m, mnt);
+	if (sb->s_op->show_options)
+		err = sb->s_op->show_options(m, mnt_path.dentry);
+	seq_puts(m, " 0 0\n");
+out:
+	return err;
+}
+
+static int show_mountinfo(struct seq_file *m, struct vfsmount *mnt)
+{
+	struct proc_mounts *p = m->private;
+	struct mount *r = real_mount(mnt);
+	struct super_block *sb = mnt->mnt_sb;
+	struct path mnt_path = { .dentry = mnt->mnt_root, .mnt = mnt };
+	int err;
+
+	seq_printf(m, "%i %i %u:%u ", r->mnt_id, r->mnt_parent->mnt_id,
+		   MAJOR(sb->s_dev), MINOR(sb->s_dev));
+	err = show_path(m, mnt->mnt_root);
+	if (err)
+		goto out;
+	seq_putc(m, ' ');
+
+	/* mountpoints outside of chroot jail will give SEQ_SKIP on this */
+	err = seq_path_root(m, &mnt_path, &p->root, " \t\n\\");
+	if (err)
+		goto out;
+
+	seq_puts(m, mnt->mnt_flags & MNT_READONLY ? " ro" : " rw");
+	show_vfsmnt_opts(m, mnt);
+
+	/* Tagged fields ("foo:X" or "bar") */
+	if (IS_MNT_SHARED(r))
+		seq_printf(m, " shared:%i", r->mnt_group_id);
+	if (IS_MNT_SLAVE(r)) {
+		int master = r->mnt_master->mnt_group_id;
+		int dom = get_dominating_id(r, &p->root);
+		seq_printf(m, " master:%i", master);
+		if (dom && dom != master)
+			seq_printf(m, " propagate_from:%i", dom);
+	}
+	if (IS_MNT_UNBINDABLE(r))
+		seq_puts(m, " unbindable");
+
+	/* Filesystem specific data */
+	seq_puts(m, " - ");
+	show_type(m, sb);
+	seq_putc(m, ' ');
+	if (sb->s_op->show_devname) {
+		err = sb->s_op->show_devname(m, mnt->mnt_root);
+		if (err)
+			goto out;
+	} else {
+		mangle(m, r->mnt_devname);
+	}
+	seq_puts(m, sb_rdonly(sb) ? " ro" : " rw");
+	err = show_sb_opts(m, sb);
+	if (err)
+		goto out;
+	if (sb->s_op->show_options)
+		err = sb->s_op->show_options(m, mnt->mnt_root);
+	seq_putc(m, '\n');
+out:
+	return err;
+}
+
+static int show_vfsstat(struct seq_file *m, struct vfsmount *mnt)
+{
+	struct proc_mounts *p = m->private;
+	struct mount *r = real_mount(mnt);
+	struct path mnt_path = { .dentry = mnt->mnt_root, .mnt = mnt };
+	struct super_block *sb = mnt_path.dentry->d_sb;
+	int err;
+
+	/* device */
+	seq_puts(m, "device ");
+	if (sb->s_op->show_devname) {
+		err = sb->s_op->show_devname(m, mnt_path.dentry);
+		if (err)
+			goto out;
+	} else {
+		mangle(m, r->mnt_devname);
+	}
+
+	/* mount point */
+	seq_puts(m, " mounted on ");
+	/* mountpoints outside of chroot jail will give SEQ_SKIP on this */
+	err = seq_path_root(m, &mnt_path, &p->root, " \t\n\\");
+	if (err)
+		goto out;
+	seq_putc(m, ' ');
+
+	/* file system type */
+	seq_puts(m, "with fstype ");
+	show_type(m, sb);
+
+	/* optional statistics */
+	if (sb->s_op->show_stats) {
+		seq_putc(m, ' ');
+		err = sb->s_op->show_stats(m, mnt_path.dentry);
+	}
+
+	seq_putc(m, '\n');
+out:
+	return err;
+}
+
+static int mounts_open_common(struct inode *inode, struct file *file,
+			      int (*show)(struct seq_file *, struct vfsmount *))
+{
+	struct task_struct *task = get_proc_task(inode);
+	struct nsproxy *nsp;
+	struct mnt_namespace *ns = NULL;
+	struct path root;
+	struct proc_mounts *p;
+	struct seq_file *m;
+	int ret = -EINVAL;
+
+	if (!task)
+		goto err;
+
+	task_lock(task);
+	nsp = task->nsproxy;
+	if (!nsp || !nsp->mnt_ns) {
+		task_unlock(task);
+		put_task_struct(task);
+		goto err;
+	}
+	ns = nsp->mnt_ns;
+	get_mnt_ns(ns);
+	if (!task->real_fs) {
+		task_unlock(task);
+		put_task_struct(task);
+		ret = -ENOENT;
+		goto err_put_ns;
+	}
+	get_fs_root(task->real_fs, &root);
+	task_unlock(task);
+	put_task_struct(task);
+
+	ret = seq_open_private(file, &mounts_op, sizeof(struct proc_mounts));
+	if (ret)
+		goto err_put_path;
+
+	m = file->private_data;
+	m->poll_event = ns->event;
+
+	p = m->private;
+	p->ns = ns;
+	p->root = root;
+	p->show = show;
+
+	return 0;
+
+ err_put_path:
+	path_put(&root);
+ err_put_ns:
+	put_mnt_ns(ns);
+ err:
+	return ret;
+}
+
+static int mounts_release(struct inode *inode, struct file *file)
+{
+	struct seq_file *m = file->private_data;
+	struct proc_mounts *p = m->private;
+	path_put(&p->root);
+	put_mnt_ns(p->ns);
+	return seq_release_private(inode, file);
+}
+
+static int mounts_open(struct inode *inode, struct file *file)
+{
+	return mounts_open_common(inode, file, show_vfsmnt);
+}
+
+static int mountinfo_open(struct inode *inode, struct file *file)
+{
+	return mounts_open_common(inode, file, show_mountinfo);
+}
+
+static int mountstats_open(struct inode *inode, struct file *file)
+{
+	return mounts_open_common(inode, file, show_vfsstat);
+}
+
+const struct file_operations proc_mounts_operations = {
+	.open		= mounts_open,
+	.read_iter	= seq_read_iter,
+	.splice_read	= copy_splice_read,
+	.llseek		= seq_lseek,
+	.release	= mounts_release,
+	.poll		= mounts_poll,
+};
+
+const struct file_operations proc_mountinfo_operations = {
+	.open		= mountinfo_open,
+	.read_iter	= seq_read_iter,
+	.splice_read	= copy_splice_read,
+	.llseek		= seq_lseek,
+	.release	= mounts_release,
+	.poll		= mounts_poll,
+};
+
+const struct file_operations proc_mountstats_operations = {
+	.open		= mountstats_open,
+	.read_iter	= seq_read_iter,
+	.splice_read	= copy_splice_read,
+	.llseek		= seq_lseek,
+	.release	= mounts_release,
+};