1 // SPDX-License-Identifier: GPL-2.0-only
2 /* Copyright (c) 2026 Christian Brauner <brauner@kernel.org> */
3 #include <linux/fs.h>
4 #include <linux/fs/super_types.h>
5 #include <linux/fs_context.h>
6 #include <linux/fs_struct.h>
7 #include <linux/magic.h>
8 #include <linux/mount.h>
9
10 #include "internal.h"
11
12 static struct path failfs_root_path = {};
13
failfs_get_root(struct path * path)14 void failfs_get_root(struct path *path)
15 {
16 *path = failfs_root_path;
17 path_get(path);
18 }
19
failfs_mnt(const struct vfsmount * mnt)20 bool failfs_mnt(const struct vfsmount *mnt)
21 {
22 return mnt->mnt_sb == failfs_root_path.mnt->mnt_sb;
23 }
24
failfs_permission(const struct mnt_idmap * idmap,struct inode * inode,int mask)25 static int failfs_permission(const struct mnt_idmap *idmap, struct inode *inode,
26 int mask)
27 {
28 return -EOPNOTSUPP;
29 }
30
failfs_lookup(struct inode * dir,struct dentry * dentry,unsigned int flags)31 static struct dentry *failfs_lookup(struct inode *dir, struct dentry *dentry,
32 unsigned int flags)
33 {
34 /* Unreachable: ->permission() already failed the walk. */
35 return ERR_PTR(-EOPNOTSUPP);
36 }
37
failfs_getattr(const struct mnt_idmap * idmap,const struct path * path,struct kstat * stat,u32 request_mask,unsigned int query_flags)38 static int failfs_getattr(const struct mnt_idmap *idmap, const struct path *path,
39 struct kstat *stat, u32 request_mask,
40 unsigned int query_flags)
41 {
42 return -EOPNOTSUPP;
43 }
44
45 static const struct inode_operations failfs_dir_inode_operations = {
46 .permission = failfs_permission,
47 .lookup = failfs_lookup,
48 .getattr = failfs_getattr,
49 };
50
51 static const struct file_operations failfs_dir_operations = {};
52
failfs_d_weak_revalidate(struct dentry * dentry,unsigned int flags)53 static int failfs_d_weak_revalidate(struct dentry *dentry, unsigned int flags)
54 {
55 /*
56 * The root is only ever reached as a path-walk terminal by jumping
57 * to it: as "/" when it is the caller's root, or through a
58 * /proc/<pid>/{root,cwd} magic link. ->permission() already fails
59 * every walk of a component, but a jump lands on the root without
60 * one. Refuse here too so the root cannot be pinned by an O_PATH
61 * open or encoded into a file handle.
62 */
63 return -EOPNOTSUPP;
64 }
65
failfs_dname(struct dentry * dentry,char * buffer,int buflen)66 static char *failfs_dname(struct dentry *dentry, char *buffer, int buflen)
67 {
68 return dynamic_dname(buffer, buflen, "failfs:/");
69 }
70
71 static const struct dentry_operations failfs_dentry_operations = {
72 .d_dname = failfs_dname,
73 .d_weak_revalidate = failfs_d_weak_revalidate,
74 };
75
failfs_statfs(struct dentry * dentry,struct kstatfs * buf)76 static int failfs_statfs(struct dentry *dentry, struct kstatfs *buf)
77 {
78 return -EOPNOTSUPP;
79 }
80
81 static const struct super_operations failfs_super_operations = {
82 .statfs = failfs_statfs,
83 };
84
failfs_fill_super(struct super_block * s,struct fs_context * fc)85 static int failfs_fill_super(struct super_block *s, struct fs_context *fc)
86 {
87 struct inode *inode;
88
89 s->s_maxbytes = MAX_LFS_FILESIZE;
90 s->s_blocksize = PAGE_SIZE;
91 s->s_blocksize_bits = PAGE_SHIFT;
92 s->s_magic = FAIL_FS_MAGIC;
93 s->s_op = &failfs_super_operations;
94 s->s_export_op = NULL;
95 s->s_xattr = NULL;
96 s->s_time_gran = 1;
97 s->s_d_flags = 0;
98
99 inode = new_inode(s);
100 if (!inode)
101 return -ENOMEM;
102
103 /* failfs supports no operations... */
104 inode->i_mode = S_IFDIR;
105 set_nlink(inode, 2);
106 inode->i_op = &failfs_dir_inode_operations;
107 inode->i_fop = &failfs_dir_operations;
108 simple_inode_init_ts(inode);
109 inode->i_ino = 1;
110 /* ... and is immutable. */
111 inode->i_flags |= S_IMMUTABLE;
112
113 set_default_d_op(s, &failfs_dentry_operations);
114 s->s_root = d_make_root(inode);
115 if (!s->s_root)
116 return -ENOMEM;
117
118 return 0;
119 }
120
failfs_get_tree(struct fs_context * fc)121 static int failfs_get_tree(struct fs_context *fc)
122 {
123 return get_tree_single(fc, failfs_fill_super);
124 }
125
126 static const struct fs_context_operations failfs_context_ops = {
127 .get_tree = failfs_get_tree,
128 };
129
failfs_init_fs_context(struct fs_context * fc)130 static int failfs_init_fs_context(struct fs_context *fc)
131 {
132 fc->ops = &failfs_context_ops;
133 fc->global = true;
134 fc->sb_flags |= SB_NOUSER;
135 fc->s_iflags |= SB_I_NOEXEC | SB_I_NODEV;
136 return 0;
137 }
138
failfs_current_chdir(void)139 int failfs_current_chdir(void)
140 {
141 struct path path;
142
143 failfs_get_root(&path);
144 set_fs_pwd(current->fs, &path);
145 path_put(&path);
146 return 0;
147 }
148
149 static struct file_system_type failfs_fs_type = {
150 .name = "failfs",
151 .init_fs_context = failfs_init_fs_context,
152 .kill_sb = kill_anon_super,
153 };
154
failfs_init(void)155 void __init failfs_init(void)
156 {
157 struct vfsmount *mnt;
158
159 /* A single instance that is member of no mount namespace. */
160 mnt = kern_mount(&failfs_fs_type);
161 if (IS_ERR(mnt))
162 panic("VFS: Failed to create failfs");
163
164 failfs_root_path.mnt = mnt;
165 failfs_root_path.dentry = mnt->mnt_root;
166 }
167