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;
+}