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 7 static const struct super_operations nullfs_super_operations = { 8 .statfs = simple_statfs, 9 }; 10 11 static int nullfs_fs_fill_super(struct super_block *s, struct fs_context *fc) 12 { 13 struct inode *inode; 14 15 s->s_maxbytes = MAX_LFS_FILESIZE; 16 s->s_blocksize = PAGE_SIZE; 17 s->s_blocksize_bits = PAGE_SHIFT; 18 s->s_magic = NULL_FS_MAGIC; 19 s->s_op = &nullfs_super_operations; 20 s->s_export_op = NULL; 21 s->s_xattr = NULL; 22 s->s_time_gran = 1; 23 s->s_d_flags = 0; 24 25 inode = new_inode(s); 26 if (!inode) 27 return -ENOMEM; 28 29 /* nullfs is permanently empty... */ 30 make_empty_dir_inode(inode); 31 simple_inode_init_ts(inode); 32 inode->i_ino = 1; 33 /* ... and immutable. */ 34 inode->i_flags |= S_IMMUTABLE; 35 36 s->s_root = d_make_root(inode); 37 if (!s->s_root) 38 return -ENOMEM; 39 40 return 0; 41 } 42 43 /* 44 * For now this is a single global instance. If needed we can make it 45 * mountable by userspace at which point we will need to make it 46 * multi-instance. 47 */ 48 static int nullfs_fs_get_tree(struct fs_context *fc) 49 { 50 return get_tree_single(fc, nullfs_fs_fill_super); 51 } 52 53 static const struct fs_context_operations nullfs_fs_context_ops = { 54 .get_tree = nullfs_fs_get_tree, 55 }; 56 57 static int nullfs_init_fs_context(struct fs_context *fc) 58 { 59 fc->ops = &nullfs_fs_context_ops; 60 fc->global = true; 61 fc->sb_flags = SB_NOUSER; 62 fc->s_iflags = SB_I_NOEXEC | SB_I_NODEV; 63 return 0; 64 } 65 66 struct file_system_type nullfs_fs_type = { 67 .name = "nullfs", 68 .init_fs_context = nullfs_init_fs_context, 69 .kill_sb = kill_anon_super, 70 }; 71