init

Makefile

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+obj-m += voidfs.o
+
+all: voidfs
+
+voidfs:
+	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

README

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+# Void Filesystem
+
+Just a dummy filesystem for learning.
+
+## Virtual Filesystem (VFS)
+
+Linux VFS allows us to write customize filesystem. The filesystem must be able to mount, other operations such as file read/write or folder creation/deletion can be customized.
+
+A filesystem can be virtual, without a backed-up device. This is the case for, I guess, network file descriptors. For filesystem with a backed-up device, usually a block device, the underlying block device can be accessed by sb_bread to read a block.
+
+Internally, a filesystem must provide a superblock, that will be used as a pointer to read files.
+
+When a file is read, the inode of the file entry can be accessed. If we also keep the internal data of the file in `inode->i_private`, we can fetch that to read the file system through `sb_read(inode->sb)`.
+
+## References
+
+[simplefs](https://github.com/sysprog21/simplefs)
+
+[Linux Kernel Labs - File system drivers](https://linux-kernel-labs.github.io/refs/heads/master/labs/filesystems_part1.html)
+
+[Linux Kernel Documentation](https://docs.kernel.org/filesystems/vfs.html)

voidfs.c

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+#include <linux/blkdev.h>
+#include <linux/buffer_head.h>
+#include <linux/fs.h>
+#include <linux/init.h>
+#include <linux/namei.h>
+#include <linux/module.h>
+#include <linux/parser.h>
+#include <linux/random.h>
+#include <linux/slab.h>
+#include <linux/time.h>
+#include <linux/version.h>
+
+
+struct dentry *voidfs_mount(struct file_system_type *fs_type, int flags, const char *dev_name, void *data);
+
+static int voidfs_fill_super(struct super_block *sb, void *data, int silent);
+void voidfs_kill_superblock(struct super_block *sb);
+
+
+ssize_t voidfs_read(struct file *fp, char __user *buf, size_t len, loff_t *ppos);
+ssize_t voidfs_write(struct file *fp, const char __user *buf, size_t len, loff_t *ppos);
+
+struct file_system_type voidfs_fs_type = {
+    .owner = THIS_MODULE,
+    .name = "voidfs",
+    .mount = voidfs_mount,
+    .kill_sb = voidfs_kill_superblock,
+    .fs_flags = FS_REQUIRES_DEV,
+};
+
+struct super_operations voidfs_super_ops = {
+    // null for now
+}
+
+struct super_operations voidfs_inode_ops = {
+    // null for now
+}
+
+struct file_operations voidfs_dir_ops = {
+    .iterate_shared = voidfs_readdir,
+}
+
+struct file_operations voidfs_file_ops = {
+    .read = voidfs_read,
+    .write = voidfs_write,
+}
+
+int voidfs_readdir(struct file *fp, struct dir_context *ctx) {
+    // populate directory entry maybe
+    struct inode *inode;
+    struct super_block *sb;
+
+    printk(KERN_INFO "readdir");
+
+    inode = file_inode(file);
+    sb = inode->i_sb;
+
+    // if (inode &&
+    //     !dir_emit(ctx, f->filename, SIMPLEFS_FILENAME_LEN, f->inode,
+    //               DT_UNKNOWN))
+    //     break;
+
+    return 0;
+}
+
+ssize_t voidfs_read(struct file *fp, char __user *buf, size_t len, loff_t *ppos) {
+    struct inode *inode;
+    struct super_block *sb;
+
+    inode = file_inode(file);
+    if (!inode) {
+        return -EFAULT;
+    }
+    sb = inode->i_sb;
+
+    // we don't read from the block device for now
+    // so return a dummy string
+    char* buffer = "this is a voidfs file";
+    ssize_t nbytes = 21;
+
+    // get file buffer
+    // move the offset
+    // get as much as we can
+
+    if (copy_to_user(buf, buffer, nbytes)) {
+        brelse(bh);
+        printk(KERN_ERR
+               "Error copying file content to userspace buffer\n");
+        return -EFAULT;
+    }
+}
+
+ssize_t voidfs_write(struct file *fp, const char __user *buf, size_t len, loff_t *ppos) {
+}
+
+struct dentry *voidfs_mount(struct file_system_type *fs_type, int flags, const char *dev_name, void *data) {
+    printk(KERN_INFO "voidfs try to mount %s\n", dev_name);
+
+    struct dentry *ret;
+    ret = mount_bdev(fs_type, flags, dev_name, data, voidfs_fill_super);
+
+    if (unlikely(IS_ERR(ret))) {
+        printk(KERN_ERR "Error mounting voidfs.\n");
+    } else {
+        printk(KERN_INFO "voidfs is succesfully mounted on: %s\n",
+               dev_name);
+    }
+
+    return ret;
+}
+
+// initialize super block metadata
+static int voidfs_fill_super(struct super_block *sb, void *data, int silent) {
+    struct inode *root_inode;
+    struct inode *dummy_body;
+
+    sb->s_magic = 0x44445555;
+    sb->s_op = &voidfs_super_ops;
+    sb_set_blocksize(sb, 1 << 12 /* 4 kilobyte */);
+    sb->s_maxbytes = (1 << 12) * 10;
+
+    // we don't get a read on file for now, because we still have
+    // to define the internal format
+
+    // create a dummy root node that we can access
+    root_inode = new_inode(sb);
+    if (!root_inode) {
+        return -ENOMEM;
+    }
+    inode_init_owner(root_inode, NULL, root_inode->i_mode);
+
+    root_inode->i_sb = sb;
+    root_inode->i_op = &voidfs_inode_ops;
+    root_inode->i_atime = root_inode->i_mtime
+                   = root_inode->i_ctime
+                   = CURRENT_TIME;
+
+    root_inode->i_fop = &voidfs_dir_ops;
+
+    // create a dummy file entry point
+    root_inode->i_fop = &voidfs_file_ops;
+
+    sb->s_root = d_make_root(root_inode);
+
+    return 0;
+}
+
+void voidfs_kill_superblock(struct super_block *sb) {
+    printk(KERN_INFO "voidfs try to unmount\n");
+}
+
+static int __init voidfs_init(void)
+{
+    int ret;
+
+    // hellofs_inode_cache = kmem_cache_create("hellofs_inode_cache",
+    //                                      sizeof(struct hellofs_inode),
+    //                                      0,
+    //                                      (SLAB_RECLAIM_ACCOUNT| SLAB_MEM_SPREAD),
+    //                                      NULL);
+    // if (!hellofs_inode_cache) {
+    //     return -ENOMEM;
+    // }
+
+    ret = register_filesystem(&voidfs_fs_type);
+    if (likely(0 == ret)) {
+        printk(KERN_INFO "Sucessfully registered voidfs\n");
+    } else {
+        printk(KERN_ERR "Failed to register voidfs. Error code: %d\n", ret);
+    }
+
+    return ret;
+}
+
+static void __exit voidfs_exit(void)
+{
+    int ret;
+
+    ret = unregister_filesystem(&voidfs_fs_type);
+    // kmem_cache_destroy(voidfs_inode_cache);
+
+    if (likely(ret == 0)) {
+        printk(KERN_INFO "Sucessfully unregistered voidfs\n");
+    } else {
+        printk(KERN_ERR "Failed to unregister voidfs. Error code: %d\n",
+               ret);
+    }
+}
+
+module_init(voidfs_init);
+module_exit(voidfs_exit);
+
+MODULE_LICENSE("MIT");
+MODULE_AUTHOR("nganhkhoa");