symlink.c

  1// SPDX-License-Identifier: GPL-2.0-or-later
  2/*
  3 * symlink.c - operations for configfs symlinks.
  4 *
  5 * Based on sysfs:
  6 * 	sysfs is Copyright (C) 2001, 2002, 2003 Patrick Mochel
  7 *
  8 * configfs Copyright (C) 2005 Oracle.  All rights reserved.
  9 */
 10
 11#include <linux/fs.h>
 12#include <linux/module.h>
 13#include <linux/namei.h>
 14#include <linux/slab.h>
 15
 16#include <linux/configfs.h>
 17#include "configfs_internal.h"
 18
 19/* Protects attachments of new symlinks */
 20DEFINE_MUTEX(configfs_symlink_mutex);
 21
 22static int item_depth(struct config_item * item)
 23{
 24	struct config_item * p = item;
 25	int depth = 0;
 26	do { depth++; } while ((p = p->ci_parent) && !configfs_is_root(p));
 27	return depth;
 28}
 29
 30static int item_path_length(struct config_item * item)
 31{
 32	struct config_item * p = item;
 33	int length = 1;
 34	do {
 35		length += strlen(config_item_name(p)) + 1;
 36		p = p->ci_parent;
 37	} while (p && !configfs_is_root(p));
 38	return length;
 39}
 40
 41static void fill_item_path(struct config_item * item, char * buffer, int length)
 42{
 43	struct config_item * p;
 44
 45	--length;
 46	for (p = item; p && !configfs_is_root(p); p = p->ci_parent) {
 47		int cur = strlen(config_item_name(p));
 48
 49		/* back up enough to print this bus id with '/' */
 50		length -= cur;
 51		memcpy(buffer + length, config_item_name(p), cur);
 52		*(buffer + --length) = '/';
 53	}
 54}
 55
 56static int configfs_get_target_path(struct config_item *item,
 57		struct config_item *target, char *path)
 58{
 59	int depth, size;
 60	char *s;
 61
 62	depth = item_depth(item);
 63	size = item_path_length(target) + depth * 3 - 1;
 64	if (size > PATH_MAX)
 65		return -ENAMETOOLONG;
 66
 67	pr_debug("%s: depth = %d, size = %d\n", __func__, depth, size);
 68
 69	for (s = path; depth--; s += 3)
 70		strcpy(s,"../");
 71
 72	fill_item_path(target, path, size);
 73	pr_debug("%s: path = '%s'\n", __func__, path);
 74	return 0;
 75}
 76
 77static int create_link(struct config_item *parent_item,
 78		       struct config_item *item,
 79		       struct dentry *dentry)
 80{
 81	struct configfs_dirent *target_sd = item->ci_dentry->d_fsdata;
 82	char *body;
 83	int ret;
 84
 85	if (!configfs_dirent_is_ready(target_sd))
 86		return -ENOENT;
 87
 88	body = kzalloc(PAGE_SIZE, GFP_KERNEL);
 89	if (!body)
 90		return -ENOMEM;
 91
 92	configfs_get(target_sd);
 93	spin_lock(&configfs_dirent_lock);
 94	if (target_sd->s_type & CONFIGFS_USET_DROPPING) {
 95		spin_unlock(&configfs_dirent_lock);
 96		configfs_put(target_sd);
 97		kfree(body);
 98		return -ENOENT;
 99	}
100	target_sd->s_links++;
101	spin_unlock(&configfs_dirent_lock);
102	ret = configfs_get_target_path(parent_item, item, body);
103	if (!ret)
104		ret = configfs_create_link(target_sd, parent_item->ci_dentry,
105					   dentry, body);
106	if (ret) {
107		spin_lock(&configfs_dirent_lock);
108		target_sd->s_links--;
109		spin_unlock(&configfs_dirent_lock);
110		configfs_put(target_sd);
111		kfree(body);
112	}
113	return ret;
114}
115
116
117static int get_target(const char *symname, struct config_item **target,
118		      struct super_block *sb)
119{
120	struct path path __free(path_put) = {};
121	int ret;
122
123	ret = kern_path(symname, LOOKUP_FOLLOW|LOOKUP_DIRECTORY, &path);
124	if (ret)
125		return ret;
126	if (path.dentry->d_sb != sb)
127		return -EPERM;
128	*target = configfs_get_config_item(path.dentry);
129	if (!*target)
130		return -ENOENT;
131	return 0;
132}
133
134
135int configfs_symlink(struct mnt_idmap *idmap, struct inode *dir,
136		     struct dentry *dentry, const char *symname)
137{
138	int ret;
139	struct configfs_dirent *sd;
140	struct config_item *parent_item;
141	struct config_item *target_item = NULL;
142	const struct config_item_type *type;
143
144	sd = dentry->d_parent->d_fsdata;
145	/*
146	 * Fake invisibility if dir belongs to a group/default groups hierarchy
147	 * being attached
148	 */
149	if (!configfs_dirent_is_ready(sd))
150		return -ENOENT;
151
152	parent_item = configfs_get_config_item(dentry->d_parent);
153	type = parent_item->ci_type;
154
155	ret = -EPERM;
156	if (!type || !type->ct_item_ops ||
157	    !type->ct_item_ops->allow_link)
158		goto out_put;
159
160	/*
161	 * This is really sick.  What they wanted was a hybrid of
162	 * link(2) and symlink(2) - they wanted the target resolved
163	 * at syscall time (as link(2) would've done), be a directory
164	 * (which link(2) would've refused to do) *AND* be a deep
165	 * fucking magic, making the target busy from rmdir POV.
166	 * symlink(2) is nothing of that sort, and the locking it
167	 * gets matches the normal symlink(2) semantics.  Without
168	 * attempts to resolve the target (which might very well
169	 * not even exist yet) done prior to locking the parent
170	 * directory.  This perversion, OTOH, needs to resolve
171	 * the target, which would lead to obvious deadlocks if
172	 * attempted with any directories locked.
173	 *
174	 * Unfortunately, that garbage is userland ABI and we should've
175	 * said "no" back in 2005.  Too late now, so we get to
176	 * play very ugly games with locking.
177	 *
178	 * Try *ANYTHING* of that sort in new code, and you will
179	 * really regret it.  Just ask yourself - what could a BOFH
180	 * do to me and do I want to find it out first-hand?
181	 *
182	 *  AV, a thoroughly annoyed bastard.
183	 */
184	inode_unlock(dir);
185	ret = get_target(symname, &target_item, dentry->d_sb);
186	inode_lock(dir);
187	if (ret)
188		goto out_put;
189
190	if (dentry->d_inode || d_unhashed(dentry))
191		ret = -EEXIST;
192	else
193		ret = inode_permission(&nop_mnt_idmap, dir,
194				       MAY_WRITE | MAY_EXEC);
195	if (!ret)
196		ret = type->ct_item_ops->allow_link(parent_item, target_item);
197	if (!ret) {
198		mutex_lock(&configfs_symlink_mutex);
199		ret = create_link(parent_item, target_item, dentry);
200		mutex_unlock(&configfs_symlink_mutex);
201		if (ret && type->ct_item_ops->drop_link)
202			type->ct_item_ops->drop_link(parent_item,
203						     target_item);
204	}
205
206	config_item_put(target_item);
207
208out_put:
209	config_item_put(parent_item);
210	return ret;
211}
212
213int configfs_unlink(struct inode *dir, struct dentry *dentry)
214{
215	struct configfs_dirent *sd = dentry->d_fsdata, *target_sd;
216	struct config_item *parent_item;
217	const struct config_item_type *type;
218	int ret;
219
220	ret = -EPERM;  /* What lack-of-symlink returns */
221	if (!(sd->s_type & CONFIGFS_ITEM_LINK))
222		goto out;
223
224	target_sd = sd->s_element;
225
226	parent_item = configfs_get_config_item(dentry->d_parent);
227	type = parent_item->ci_type;
228
229	spin_lock(&configfs_dirent_lock);
230	list_del_init(&sd->s_sibling);
231	spin_unlock(&configfs_dirent_lock);
232	configfs_drop_dentry(sd, dentry->d_parent);
233	dput(dentry);
234	configfs_put(sd);
235
236	/*
237	 * drop_link() must be called before
238	 * decrementing target's ->s_links, so that the order of
239	 * drop_link(this, target) and drop_item(target) is preserved.
240	 */
241	if (type && type->ct_item_ops &&
242	    type->ct_item_ops->drop_link)
243		type->ct_item_ops->drop_link(parent_item,
244					       target_sd->s_element);
245
246	spin_lock(&configfs_dirent_lock);
247	target_sd->s_links--;
248	spin_unlock(&configfs_dirent_lock);
249	configfs_put(target_sd);
250
251	config_item_put(parent_item);
252
253	ret = 0;
254
255out:
256	return ret;
257}
258
259const struct inode_operations configfs_symlink_inode_operations = {
260	.get_link = simple_get_link,
261	.setattr = configfs_setattr,
262};
263