// SPDX-License-Identifier: GPL-2.0-only
/* Copyright (c) 2026 Christian Brauner <brauner@kernel.org> */
#include <linux/fs/super_types.h>
#include <linux/fs_context.h>
#include <linux/magic.h>

static const struct super_operations nullfs_super_operations = {
        .statfs = simple_statfs,
};

static int nullfs_fs_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              = NULL_FS_MAGIC;
        s->s_op                 = &nullfs_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;

        /* nullfs is permanently empty... */
        make_empty_dir_inode(inode);
        simple_inode_init_ts(inode);
        inode->i_ino    = 1;
        /* ... and immutable. */
        inode->i_flags |= S_IMMUTABLE;

        s->s_root = d_make_root(inode);
        if (!s->s_root)
                return -ENOMEM;

        return 0;
}

/*
 * For now this is a single global instance. If needed we can make it
 * mountable by userspace at which point we will need to make it
 * multi-instance.
 */
static int nullfs_fs_get_tree(struct fs_context *fc)
{
        return get_tree_single(fc, nullfs_fs_fill_super);
}

static const struct fs_context_operations nullfs_fs_context_ops = {
        .get_tree       = nullfs_fs_get_tree,
};

static int nullfs_init_fs_context(struct fs_context *fc)
{
        fc->ops         = &nullfs_fs_context_ops;
        fc->global      = true;
        fc->sb_flags    = SB_NOUSER;
        fc->s_iflags    = SB_I_NOEXEC | SB_I_NODEV;
        return 0;
}

struct file_system_type nullfs_fs_type = {
        .name                   = "nullfs",
        .init_fs_context        = nullfs_init_fs_context,
        .kill_sb                = kill_anon_super,
};