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// 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;
}
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