Initial

.gitignore

@@ -0,0 +1,12 @@
+*.cmd
+*.ko
+*.mod
+*.mod.c
+*.mod.o
+*.o
+CMakeLists.txt
+Module.symvers
+modules.order
+.idea
+cmake-build-debug
+build

Makefile

@@ -0,0 +1,10 @@
+obj-m := statsfs.o
+
+KDIR := /lib/modules/$(shell uname -r)/build
+PWD := $(shell pwd)
+
+all:
+	$(MAKE) -C $(KDIR) M=$(PWD) modules
+
+clean:
+	$(MAKE) -C $(KDIR) M=$(PWD) clean

statsfs.c

@@ -0,0 +1,160 @@
+// libfs.c
+#include <linux/fs.h>
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/pagemap.h>
+#include <linux/slab.h>
+#include <linux/namei.h>
+#include <linux/mount.h>
+#include <linux/dcache.h>
+
+#define LIBFS_MAGIC 0xA0B0C0D0
+#define LIBFS_BOOKS_MODE 01777
+#define LIBFS_AUTHORS_MODE 0777
+#define LIBFS_SUBDIR_MODE 01777
+
+static struct super_block *libfs_sb;
+
+// Track books inode to monitor link targets
+static struct inode *books_inode = NULL;
+
+static const struct super_operations libfs_sops = {
+        .statfs = simple_statfs,
+        .drop_inode = generic_delete_inode,
+};
+
+static const struct file_operations libfs_file_ops = {
+        .llseek = no_llseek,
+        .read = NULL,
+        .write = NULL,
+};
+
+static struct inode *libfs_get_inode(struct super_block *sb, umode_t mode) {
+    struct inode *inode = new_inode(sb);
+    if (!inode) return NULL;
+
+    inode->i_ino = get_next_ino();
+    inode->i_sb = sb;
+    inode->i_op = &simple_dir_inode_operations;
+    if (S_ISDIR(mode)) {
+        inode->i_fop = &simple_dir_operations;
+    } else {
+        inode->i_fop = &libfs_file_ops; // fallback
+    }
+    inode->i_mode = mode;
+    return inode;
+}
+
+static bool is_protected_dentry(struct dentry *dentry) {
+    return (strcmp(dentry->d_name.name, "books") == 0 || strcmp(dentry->d_name.name, "authors") == 0);
+}
+
+static int libfs_mkdir(struct mnt_idmap *idmap, struct inode *dir, struct dentry *dentry, umode_t mode) {
+    if (dir == libfs_sb->s_root->d_inode) {
+        return -EPERM; // no mkdir in root
+    }
+    struct inode *inode = libfs_get_inode(dir->i_sb, S_IFDIR | LIBFS_SUBDIR_MODE);
+    if (!inode)
+        return -ENOMEM;
+    inode->i_uid = current_fsuid();
+    d_add(dentry, inode);
+    inc_nlink(dir);
+    return 0;
+}
+
+static int libfs_unlink(struct inode *dir, struct dentry *dentry) {
+    if (dir == libfs_sb->s_root->d_inode && is_protected_dentry(dentry))
+        return -EPERM;
+    return simple_unlink(dir, dentry);
+}
+
+static int libfs_rmdir(struct inode *dir, struct dentry *dentry) {
+    if (dir == libfs_sb->s_root->d_inode && is_protected_dentry(dentry))
+        return -EPERM;
+    return simple_rmdir(dir, dentry);
+}
+
+static int libfs_rename(struct mnt_idmap *idmap, struct inode *old_dir, struct dentry *old_dentry,
+                        struct inode *new_dir, struct dentry *new_dentry, unsigned int flags) {
+    if ((old_dir == libfs_sb->s_root->d_inode || new_dir == libfs_sb->s_root->d_inode) &&
+        (is_protected_dentry(old_dentry) || is_protected_dentry(new_dentry)))
+        return -EPERM;
+    return simple_rename(idmap, old_dir, old_dentry, new_dir, new_dentry, flags);
+}
+
+static int libfs_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_dentry) {
+    if (dir->i_sb != old_dentry->d_inode->i_sb)
+        return -EXDEV;
+    if (!S_ISREG(old_dentry->d_inode->i_mode))
+        return -EPERM;
+    return simple_link(old_dentry, dir, new_dentry);
+}
+
+static const struct inode_operations libfs_dir_iops = {
+        .lookup = simple_lookup,
+        .mkdir = libfs_mkdir,
+        .unlink = libfs_unlink,
+        .rmdir = libfs_rmdir,
+        .rename = libfs_rename,
+        .link = libfs_link,
+};
+
+static int libfs_fill_super(struct super_block *sb, void *data, int silent) {
+    struct inode *inode;
+    struct dentry *root;
+    libfs_sb = sb;
+    sb->s_magic = LIBFS_MAGIC;
+    sb->s_op = &libfs_sops;
+
+    inode = libfs_get_inode(sb, S_IFDIR | 0755);
+    if (!inode)
+        return -ENOMEM;
+    inode->i_op = &libfs_dir_iops;
+    inode->i_fop = &simple_dir_operations;
+
+    root = d_make_root(inode);
+    if (!root)
+        return -ENOMEM;
+    sb->s_root = root;
+
+    // /books
+    {
+        struct inode *books = libfs_get_inode(sb, S_IFDIR | LIBFS_BOOKS_MODE);
+        books_inode = books;
+        struct dentry *d = d_alloc_name(root, "books");
+        d_add(d, books);
+    }
+    // /authors
+    {
+        struct inode *authors = libfs_get_inode(sb, S_IFDIR | LIBFS_AUTHORS_MODE);
+        struct dentry *d = d_alloc_name(root, "authors");
+        d_add(d, authors);
+    }
+    return 0;
+}
+
+static struct dentry *libfs_mount(struct file_system_type *fs_type, int flags, const char *dev_name, void *data) {
+    return mount_nodev(fs_type, flags, data, libfs_fill_super);
+}
+
+static struct file_system_type libfs_type = {
+        .owner = THIS_MODULE,
+        .name = "libfs",
+        .mount = libfs_mount,
+        .kill_sb = kill_litter_super,
+};
+
+static int __init libfs_init(void) {
+    return register_filesystem(&libfs_type);
+}
+
+static void __exit libfs_exit(void) {
+    unregister_filesystem(&libfs_type);
+}
+
+module_init(libfs_init);
+module_exit(libfs_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("you");
+MODULE_DESCRIPTION("Custom library FS with books and authors");