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Linux kernel mirror (for testing)
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linux
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
7static const struct super_operations nullfs_super_operations = {
8 .statfs = simple_statfs,
9};
10
11static 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 */
48static int nullfs_fs_get_tree(struct fs_context *fc)
49{
50 return get_tree_single(fc, nullfs_fs_fill_super);
51}
52
53static const struct fs_context_operations nullfs_fs_context_ops = {
54 .get_tree = nullfs_fs_get_tree,
55};
56
57static 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
66struct file_system_type nullfs_fs_type = {
67 .name = "nullfs",
68 .init_fs_context = nullfs_init_fs_context,
69 .kill_sb = kill_anon_super,
70};