1 // SPDX-License-Identifier: GPL-2.0-only 2 /* Copyright (c) 2026 Christian Brauner <brauner@kernel.org> */ 3 #include <linux/fs/super_types.h> 4 #include <linux/fs_context.h> 5 #include <linux/magic.h> 6 #include <linux/splice.h> 7 #include <linux/uio.h> 8 9 #include "mount.h" 10 11 static const struct super_operations nullfs_super_operations = { 12 .statfs = simple_statfs, 13 }; 14 15 static loff_t nullfs_dir_llseek(struct file *file, loff_t offset, int whence) 16 { 17 /* an empty directory has two entries . and .. at offsets 0 and 1 */ 18 return generic_file_llseek_size(file, offset, whence, 2, 2); 19 } 20 21 static int nullfs_dir_readdir(struct file *file, struct dir_context *ctx) 22 { 23 dir_emit_dots(file, ctx); 24 return 0; 25 } 26 27 /* the one inode of nullfs is shared by every holder, so no locks on it */ 28 static int nullfs_nolock(struct file *file, int cmd, struct file_lock *fl) 29 { 30 return -ENOLCK; 31 } 32 33 /* what libfs gives an empty directory, plus the refusal of file locks */ 34 static const struct file_operations nullfs_dir_operations = { 35 .llseek = nullfs_dir_llseek, 36 .read = generic_read_dir, 37 .iterate_shared = nullfs_dir_readdir, 38 .fsync = noop_fsync, 39 .lock = nullfs_nolock, 40 .flock = nullfs_nolock, 41 .fop_flags = FOP_IMMUTABLE, 42 }; 43 44 static ssize_t nullfs_file_read_iter(struct kiocb *iocb, struct iov_iter *to) 45 { 46 return 0; 47 } 48 49 static ssize_t nullfs_file_splice_read(struct file *in, loff_t *ppos, 50 struct pipe_inode_info *pipe, 51 size_t len, unsigned int flags) 52 { 53 return 0; 54 } 55 56 static ssize_t nullfs_file_write_iter(struct kiocb *iocb, struct iov_iter *from) 57 { 58 size_t count = iov_iter_count(from); 59 60 iov_iter_advance(from, count); 61 return count; 62 } 63 64 static int nullfs_pipe_to_null(struct pipe_inode_info *pipe, 65 struct pipe_buffer *buf, struct splice_desc *sd) 66 { 67 return sd->len; 68 } 69 70 static ssize_t nullfs_file_splice_write(struct pipe_inode_info *pipe, 71 struct file *out, loff_t *ppos, 72 size_t len, unsigned int flags) 73 { 74 return splice_from_pipe(pipe, out, ppos, len, flags, nullfs_pipe_to_null); 75 } 76 77 static int nullfs_file_setattr(struct mnt_idmap *idmap, struct dentry *dentry, 78 struct iattr *attr) 79 { 80 if (attr->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID)) 81 return -EPERM; 82 return 0; 83 } 84 85 static const struct inode_operations nullfs_file_inode_operations = { 86 .setattr = nullfs_file_setattr, 87 }; 88 89 static const struct file_operations nullfs_file_operations = { 90 .llseek = generic_file_llseek, 91 .read_iter = nullfs_file_read_iter, 92 .write_iter = nullfs_file_write_iter, 93 .splice_read = nullfs_file_splice_read, 94 .splice_write = nullfs_file_splice_write, 95 .fsync = noop_fsync, 96 .lock = nullfs_nolock, 97 .flock = nullfs_nolock, 98 }; 99 100 /* 101 * An empty regular file on @sb as a dentry of its own, never hashed under 102 * the root so no lookup finds it. 103 */ 104 struct dentry *nullfs_new_file(struct super_block *sb) 105 { 106 struct dentry *dentry; 107 struct inode *inode; 108 109 inode = new_inode(sb); 110 if (!inode) 111 return ERR_PTR(-ENOMEM); 112 113 /* the root directory is 1 */ 114 inode->i_ino = 2; 115 inode->i_mode = S_IFREG | 0666; 116 inode->i_op = &nullfs_file_inode_operations; 117 inode->i_fop = &nullfs_file_operations; 118 simple_inode_init_ts(inode); 119 inode->i_flags |= S_NOATIME; 120 121 dentry = d_alloc_anon(sb); 122 if (!dentry) { 123 iput(inode); 124 return ERR_PTR(-ENOMEM); 125 } 126 d_instantiate(dentry, inode); 127 return dentry; 128 } 129 130 static int nullfs_fs_fill_super(struct super_block *s, struct fs_context *fc) 131 { 132 struct inode *inode; 133 134 s->s_maxbytes = MAX_LFS_FILESIZE; 135 s->s_blocksize = PAGE_SIZE; 136 s->s_blocksize_bits = PAGE_SHIFT; 137 s->s_magic = NULL_FS_MAGIC; 138 s->s_op = &nullfs_super_operations; 139 s->s_export_op = NULL; 140 s->s_xattr = NULL; 141 s->s_time_gran = 1; 142 s->s_d_flags = 0; 143 144 inode = new_inode(s); 145 if (!inode) 146 return -ENOMEM; 147 148 /* nullfs is permanently empty... */ 149 make_empty_dir_inode(inode); 150 inode->i_fop = &nullfs_dir_operations; 151 simple_inode_init_ts(inode); 152 inode->i_ino = 1; 153 /* ... and immutable, reading it leaves no trace either. */ 154 inode->i_flags |= S_IMMUTABLE | S_NOATIME; 155 156 s->s_root = d_make_root(inode); 157 if (!s->s_root) 158 return -ENOMEM; 159 160 return 0; 161 } 162 163 static int nullfs_fs_get_tree(struct fs_context *fc) 164 { 165 return get_tree_nodev(fc, nullfs_fs_fill_super); 166 } 167 168 static const struct fs_context_operations nullfs_fs_context_ops = { 169 .get_tree = nullfs_fs_get_tree, 170 }; 171 172 static int nullfs_init_fs_context(struct fs_context *fc) 173 { 174 fc->ops = &nullfs_fs_context_ops; 175 fc->sb_flags |= SB_NOUSER; 176 fc->s_iflags |= SB_I_NOEXEC | SB_I_NODEV; 177 return 0; 178 } 179 180 struct file_system_type nullfs_fs_type = { 181 .name = "nullfs", 182 .fs_flags = FS_DISALLOW_NOTIFY, 183 .init_fs_context = nullfs_init_fs_context, 184 .kill_sb = kill_anon_super, 185 }; 186