fs/kernfs/mount.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * fs/kernfs/mount.c - kernfs mount implementation
 *
 * Copyright (c) 2001-3 Patrick Mochel
 * Copyright (c) 2007 SUSE Linux Products GmbH
 * Copyright (c) 2007, 2013 Tejun Heo <tj@kernel.org>
 */

#include <linux/fs.h>
#include <linux/mount.h>
#include <linux/init.h>
#include <linux/magic.h>
#include <linux/slab.h>
#include <linux/pagemap.h>
#include <linux/namei.h>
#include <linux/seq_file.h>
#include <linux/exportfs.h>
#include <linux/uuid.h>
#include <linux/statfs.h>

#include "kernfs-internal.h"

struct kmem_cache *kernfs_node_cache __ro_after_init;
struct kmem_cache *kernfs_iattrs_cache __ro_after_init;
struct kernfs_global_locks *kernfs_locks __ro_after_init;

static int kernfs_sop_show_options(struct seq_file *sf, struct dentry *dentry)
{
	struct kernfs_root *root = kernfs_root(kernfs_dentry_node(dentry));
	struct kernfs_syscall_ops *scops = root->syscall_ops;

	if (scops && scops->show_options)
		return scops->show_options(sf, root);
	return 0;
}

static int kernfs_sop_show_path(struct seq_file *sf, struct dentry *dentry)
{
	struct kernfs_node *node = kernfs_dentry_node(dentry);
	struct kernfs_root *root = kernfs_root(node);
	struct kernfs_syscall_ops *scops = root->syscall_ops;

	if (scops && scops->show_path)
		return scops->show_path(sf, node, root);

	seq_dentry(sf, dentry, " \t\n\\");
	return 0;
}

static int kernfs_statfs(struct dentry *dentry, struct kstatfs *buf)
{
	simple_statfs(dentry, buf);
	buf->f_fsid = uuid_to_fsid(dentry->d_sb->s_uuid.b);
	return 0;
}

const struct super_operations kernfs_sops = {
	.statfs		= kernfs_statfs,
	.drop_inode	= inode_just_drop,
	.evict_inode	= kernfs_evict_inode,

	.show_options	= kernfs_sop_show_options,
	.show_path	= kernfs_sop_show_path,

	/*
	 * sysfs is built on top of kernfs and sysfs provides the power
	 * management infrastructure to support suspend/hibernate by
	 * writing to various files in /sys/power/. As filesystems may
	 * be automatically frozen during suspend/hibernate implementing
	 * freeze/thaw support for kernfs generically will cause
	 * deadlocks as the suspending/hibernation initiating task will
	 * hold a VFS lock that it will then wait upon to be released.
	 * If freeze/thaw for kernfs is needed talk to the VFS.
	 */
	.freeze_fs	= NULL,
	.unfreeze_fs	= NULL,
	.freeze_super	= NULL,
	.thaw_super	= NULL,
};

static int kernfs_encode_fh(struct inode *inode, __u32 *fh, int *max_len,
			    struct inode *parent)
{
	struct kernfs_node *kn = inode->i_private;

	if (*max_len < 2) {
		*max_len = 2;
		return FILEID_INVALID;
	}

	*max_len = 2;
	*(u64 *)fh = kn->id;
	return FILEID_KERNFS;
}

static struct dentry *__kernfs_fh_to_dentry(struct super_block *sb,
					    struct fid *fid, int fh_len,
					    int fh_type, bool get_parent)
{
	struct kernfs_super_info *info = kernfs_info(sb);
	struct kernfs_node *kn;
	struct inode *inode;
	u64 id;

	if (fh_len < 2)
		return NULL;

	switch (fh_type) {
	case FILEID_KERNFS:
		id = *(u64 *)fid;
		break;
	case FILEID_INO32_GEN:
	case FILEID_INO32_GEN_PARENT:
		/*
		 * blk_log_action() exposes "LOW32,HIGH32" pair without
		 * type and userland can call us with generic fid
		 * constructed from them.  Combine it back to ID.  See
		 * blk_log_action().
		 */
		id = ((u64)fid->i32.gen << 32) | fid->i32.ino;
		break;
	default:
		return NULL;
	}

	/*
	 * Hold kernfs_rwsem across the lookup as well as kernfs_get_inode().
	 * __kernfs_remove() deactivates the subtree and clears i_nlink on its
	 * inodes under the write lock, so under the read lock either
	 * kernfs_find_and_get_node_by_id() refuses the node, or the inode is
	 * in the inode hash before the ilookup() pass goes looking for it.
	 */
	scoped_guard(rwsem_read, &info->root->kernfs_rwsem) {
		kn = kernfs_find_and_get_node_by_id(info->root, id);
		if (!kn)
			return ERR_PTR(-ESTALE);

		if (get_parent) {
			struct kernfs_node *parent;

			parent = kernfs_get_parent(kn);
			kernfs_put(kn);
			kn = parent;
			if (!kn)
				return ERR_PTR(-ESTALE);
		}

		inode = kernfs_get_inode(sb, kn);
		kernfs_put(kn);
	}

	return d_obtain_alias(inode);
}

static struct dentry *kernfs_fh_to_dentry(struct super_block *sb,
					  struct fid *fid, int fh_len,
					  int fh_type)
{
	return __kernfs_fh_to_dentry(sb, fid, fh_len, fh_type, false);
}

static struct dentry *kernfs_fh_to_parent(struct super_block *sb,
					  struct fid *fid, int fh_len,
					  int fh_type)
{
	return __kernfs_fh_to_dentry(sb, fid, fh_len, fh_type, true);
}

static struct dentry *kernfs_get_parent_dentry(struct dentry *child)
{
	struct kernfs_node *kn = kernfs_dentry_node(child);
	struct kernfs_root *root = kernfs_root(kn);

	guard(rwsem_read)(&root->kernfs_rwsem);
	return d_obtain_alias(kernfs_get_inode(child->d_sb, kernfs_parent(kn)));
}

static const struct export_operations kernfs_export_ops = {
	.encode_fh	= kernfs_encode_fh,
	.fh_to_dentry	= kernfs_fh_to_dentry,
	.fh_to_parent	= kernfs_fh_to_parent,
	.get_parent	= kernfs_get_parent_dentry,
};

/**
 * kernfs_root_from_sb - determine kernfs_root associated with a super_block
 * @sb: the super_block in question
 *
 * Return: the kernfs_root associated with @sb.  If @sb is not a kernfs one,
 * %NULL is returned.
 */
struct kernfs_root *kernfs_root_from_sb(struct super_block *sb)
{
	if (sb->s_op == &kernfs_sops)
		return kernfs_info(sb)->root;
	return NULL;
}

/*
 * find the next ancestor in the path down to @child, where @parent was the
 * ancestor whose descendant we want to find.
 *
 * Say the path is /a/b/c/d.  @child is d, @parent is %NULL.  We return the root
 * node.  If @parent is b, then we return the node for c.
 * Passing in d as @parent is not ok.
 */
static struct kernfs_node *find_next_ancestor(struct kernfs_node *child,
					      struct kernfs_node *parent)
{
	if (child == parent) {
		pr_crit_once("BUG in find_next_ancestor: called with parent == child");
		return NULL;
	}

	while (kernfs_parent(child) != parent) {
		child = kernfs_parent(child);
		if (!child)
			return NULL;
	}

	return child;
}

/**
 * kernfs_node_dentry - get a dentry for the given kernfs_node
 * @kn: kernfs_node for which a dentry is needed
 * @sb: the kernfs super_block
 *
 * Return: the dentry pointer
 */
struct dentry *kernfs_node_dentry(struct kernfs_node *kn,
				  struct super_block *sb)
{
	struct dentry *dentry;
	struct kernfs_node *knparent;
	struct kernfs_root *root;

	BUG_ON(sb->s_op != &kernfs_sops);

	dentry = dget(sb->s_root);

	/* Check if this is the root kernfs_node */
	if (!rcu_access_pointer(kn->__parent))
		return dentry;

	root = kernfs_root(kn);
	/*
	 * As long as kn is valid, its parent can not vanish. This is cgroup's
	 * kn so it can't have its parent replaced. Therefore it is safe to use
	 * the ancestor node outside of the RCU or locked section.
	 */
	if (WARN_ON_ONCE(!(root->flags & KERNFS_ROOT_INVARIANT_PARENT)))
		return ERR_PTR(-EINVAL);
	scoped_guard(rcu) {
		knparent = find_next_ancestor(kn, NULL);
	}
	if (WARN_ON(!knparent)) {
		dput(dentry);
		return ERR_PTR(-EINVAL);
	}

	do {
		struct dentry *dtmp;
		struct kernfs_node *kntmp;
		const char *name;

		if (kn == knparent)
			return dentry;

		scoped_guard(rwsem_read, &root->kernfs_rwsem) {
			kntmp = find_next_ancestor(kn, knparent);
			if (WARN_ON(!kntmp)) {
				dput(dentry);
				return ERR_PTR(-EINVAL);
			}
			name = kstrdup(kernfs_rcu_name(kntmp), GFP_KERNEL);
		}
		if (!name) {
			dput(dentry);
			return ERR_PTR(-ENOMEM);
		}
		dtmp = lookup_noperm_positive_unlocked(&QSTR(name), dentry);
		dput(dentry);
		kfree(name);
		if (IS_ERR(dtmp))
			return dtmp;
		knparent = kntmp;
		dentry = dtmp;
	} while (true);
}

static int kernfs_fill_super(struct super_block *sb, struct kernfs_fs_context *kfc)
{
	struct kernfs_super_info *info = kernfs_info(sb);
	struct kernfs_root *kf_root = kfc->root;
	struct inode *inode;
	struct dentry *root;

	info->sb = sb;
	/* Userspace would break if executables or devices appear on sysfs */
	sb->s_iflags |= SB_I_NOEXEC | SB_I_NODEV;
	sb->s_blocksize = PAGE_SIZE;
	sb->s_blocksize_bits = PAGE_SHIFT;
	sb->s_magic = kfc->magic;
	sb->s_op = &kernfs_sops;
	sb->s_xattr = kernfs_xattr_handlers;
	if (info->root->flags & KERNFS_ROOT_SUPPORT_EXPORTOP)
		sb->s_export_op = &kernfs_export_ops;
	sb->s_time_gran = 1;
	sb->s_maxbytes  = MAX_LFS_FILESIZE;

	/* sysfs dentries and inodes don't require IO to create */
	sb->s_shrink->seeks = 0;

	/* get root inode, initialize and unlock it */
	down_read(&kf_root->kernfs_rwsem);
	inode = kernfs_get_inode(sb, info->root->kn);
	up_read(&kf_root->kernfs_rwsem);
	if (!inode) {
		pr_debug("kernfs: could not get root inode\n");
		return -ENOMEM;
	}

	/* instantiate and link root dentry */
	root = d_make_root(inode);
	if (!root) {
		pr_debug("%s: could not get root dentry!\n", __func__);
		return -ENOMEM;
	}
	sb->s_root = root;
	set_default_d_op(sb, &kernfs_dops);
	return 0;
}

static int kernfs_test_super(struct super_block *sb, struct fs_context *fc)
{
	struct kernfs_super_info *sb_info = kernfs_info(sb);
	struct kernfs_super_info *info = fc->s_fs_info;

	return sb_info->root == info->root && sb_info->ns == info->ns;
}

static int kernfs_set_super(struct super_block *sb, struct fs_context *fc)
{
	struct kernfs_fs_context *kfc = fc->fs_private;

	kfc->ns_tag = NULL;
	return set_anon_super_fc(sb, fc);
}

/**
 * kernfs_super_ns - determine the namespace tag of a kernfs super_block
 * @sb: super_block of interest
 *
 * Return: the namespace tag associated with kernfs super_block @sb.
 */
const struct ns_common *kernfs_super_ns(struct super_block *sb)
{
	struct kernfs_super_info *info = kernfs_info(sb);

	return info->ns;
}

/**
 * kernfs_get_tree - kernfs filesystem access/retrieval helper
 * @fc: The filesystem context.
 *
 * This is to be called from each kernfs user's fs_context->ops->get_tree()
 * implementation, which should set the specified ->@fs_type and ->@flags, and
 * specify the hierarchy and namespace tag to mount via ->@root and ->@ns,
 * respectively.
 *
 * Return: %0 on success, -errno on failure.
 */
int kernfs_get_tree(struct fs_context *fc)
{
	struct kernfs_fs_context *kfc = fc->fs_private;
	struct super_block *sb;
	struct kernfs_super_info *info;
	int error;

	info = kzalloc_obj(*info);
	if (!info)
		return -ENOMEM;

	info->root = kfc->root;
	info->ns = kfc->ns_tag;
	INIT_LIST_HEAD(&info->node);

	fc->s_fs_info = info;
	sb = sget_fc(fc, kernfs_test_super, kernfs_set_super);
	if (IS_ERR(sb))
		return PTR_ERR(sb);

	if (!sb->s_root) {
		struct kernfs_super_info *info = kernfs_info(sb);
		struct kernfs_root *root = kfc->root;

		kfc->new_sb_created = true;

		error = kernfs_fill_super(sb, kfc);
		if (error) {
			deactivate_locked_super(sb);
			return error;
		}
		sb->s_flags |= SB_ACTIVE;

		uuid_t uuid;
		uuid_gen(&uuid);
		super_set_uuid(sb, uuid.b, sizeof(uuid));

		down_write(&root->kernfs_supers_rwsem);
		list_add(&info->node, &info->root->supers);
		up_write(&root->kernfs_supers_rwsem);
	}

	fc->root = dget(sb->s_root);
	return 0;
}

void kernfs_free_fs_context(struct fs_context *fc)
{
	/* Note that we don't deal with kfc->ns_tag here. */
	kfree(fc->s_fs_info);
	fc->s_fs_info = NULL;
}

/**
 * kernfs_kill_sb - kill_sb for kernfs
 * @sb: super_block being killed
 *
 * This can be used directly for file_system_type->kill_sb().  If a kernfs
 * user needs extra cleanup, it can implement its own kill_sb() and call
 * this function at the end.
 */
void kernfs_kill_sb(struct super_block *sb)
{
	struct kernfs_super_info *info = kernfs_info(sb);
	struct kernfs_root *root = info->root;

	down_write(&root->kernfs_supers_rwsem);
	list_del(&info->node);
	up_write(&root->kernfs_supers_rwsem);

	/*
	 * Mark the superblock dead so sget_fc() can't find it,
	 * before freeing kernfs_super_info.
	 */
	kill_anon_super(sb);
	kfree(info);
}

static void __init kernfs_mutex_init(void)
{
	int count;

	for (count = 0; count < NR_KERNFS_LOCKS; count++)
		mutex_init(&kernfs_locks->node_mutex[count]);
}

static void __init kernfs_lock_init(void)
{
	kernfs_locks = kmalloc_obj(struct kernfs_global_locks);
	WARN_ON(!kernfs_locks);

	kernfs_mutex_init();
}

void __init kernfs_init(void)
{
	kernfs_node_cache = kmem_cache_create("kernfs_node_cache",
					      sizeof(struct kernfs_node),
					      0, SLAB_PANIC, NULL);

	/* Creates slab cache for kernfs inode attributes */
	kernfs_iattrs_cache  = kmem_cache_create("kernfs_iattrs_cache",
					      sizeof(struct kernfs_iattrs),
					      0, SLAB_PANIC, NULL);

	kernfs_lock_init();
}