fs/configfs/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 */
  20 DEFINE_MUTEX(configfs_symlink_mutex);
  21
  22 static 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
  30 static 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
  41 static 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
  56 static 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
  77 static 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
 117 static int get_target(const char *symname, struct path *path,
 118                       struct config_item **target, struct super_block *sb)
 119 {
 120         int ret;
 121
 122         ret = kern_path(symname, LOOKUP_FOLLOW|LOOKUP_DIRECTORY, path);
 123         if (!ret) {
 124                 if (path->dentry->d_sb == sb) {
 125                         *target = configfs_get_config_item(path->dentry);
 126                         if (!*target) {
 127                                 ret = -ENOENT;
 128                                 path_put(path);
 129                         }
 130                 } else {
 131                         ret = -EPERM;
 132                         path_put(path);
 133                 }
 134         }
 135
 136         return ret;
 137 }
 138
 139
 140 int configfs_symlink(struct user_namespace *mnt_userns, struct inode *dir,
 141                      struct dentry *dentry, const char *symname)
 142 {
 143         int ret;
 144         struct path path;
 145         struct configfs_dirent *sd;
 146         struct config_item *parent_item;
 147         struct config_item *target_item = NULL;
 148         const struct config_item_type *type;
 149
 150         sd = dentry->d_parent->d_fsdata;
 151         /*
 152          * Fake invisibility if dir belongs to a group/default groups hierarchy
 153          * being attached
 154          */
 155         if (!configfs_dirent_is_ready(sd))
 156                 return -ENOENT;
 157
 158         parent_item = configfs_get_config_item(dentry->d_parent);
 159         type = parent_item->ci_type;
 160
 161         ret = -EPERM;
 162         if (!type || !type->ct_item_ops ||
 163             !type->ct_item_ops->allow_link)
 164                 goto out_put;
 165
 166         /*
 167          * This is really sick.  What they wanted was a hybrid of
 168          * link(2) and symlink(2) - they wanted the target resolved
 169          * at syscall time (as link(2) would've done), be a directory
 170          * (which link(2) would've refused to do) *AND* be a deep
 171          * fucking magic, making the target busy from rmdir POV.
 172          * symlink(2) is nothing of that sort, and the locking it
 173          * gets matches the normal symlink(2) semantics.  Without
 174          * attempts to resolve the target (which might very well
 175          * not even exist yet) done prior to locking the parent
 176          * directory.  This perversion, OTOH, needs to resolve
 177          * the target, which would lead to obvious deadlocks if
 178          * attempted with any directories locked.
 179          *
 180          * Unfortunately, that garbage is userland ABI and we should've
 181          * said "no" back in 2005.  Too late now, so we get to
 182          * play very ugly games with locking.
 183          *
 184          * Try *ANYTHING* of that sort in new code, and you will
 185          * really regret it.  Just ask yourself - what could a BOFH
 186          * do to me and do I want to find it out first-hand?
 187          *
 188          *  AV, a thoroughly annoyed bastard.
 189          */
 190         inode_unlock(dir);
 191         ret = get_target(symname, &path, &target_item, dentry->d_sb);
 192         inode_lock(dir);
 193         if (ret)
 194                 goto out_put;
 195
 196         if (dentry->d_inode || d_unhashed(dentry))
 197                 ret = -EEXIST;
 198         else
 199                 ret = inode_permission(&init_user_ns, dir,
 200                                        MAY_WRITE | MAY_EXEC);
 201         if (!ret)
 202                 ret = type->ct_item_ops->allow_link(parent_item, target_item);
 203         if (!ret) {
 204                 mutex_lock(&configfs_symlink_mutex);
 205                 ret = create_link(parent_item, target_item, dentry);
 206                 mutex_unlock(&configfs_symlink_mutex);
 207                 if (ret && type->ct_item_ops->drop_link)
 208                         type->ct_item_ops->drop_link(parent_item,
 209                                                      target_item);
 210         }
 211
 212         config_item_put(target_item);
 213         path_put(&path);
 214
 215 out_put:
 216         config_item_put(parent_item);
 217         return ret;
 218 }
 219
 220 int configfs_unlink(struct inode *dir, struct dentry *dentry)
 221 {
 222         struct configfs_dirent *sd = dentry->d_fsdata, *target_sd;
 223         struct config_item *parent_item;
 224         const struct config_item_type *type;
 225         int ret;
 226
 227         ret = -EPERM;  /* What lack-of-symlink returns */
 228         if (!(sd->s_type & CONFIGFS_ITEM_LINK))
 229                 goto out;
 230
 231         target_sd = sd->s_element;
 232
 233         parent_item = configfs_get_config_item(dentry->d_parent);
 234         type = parent_item->ci_type;
 235
 236         spin_lock(&configfs_dirent_lock);
 237         list_del_init(&sd->s_sibling);
 238         spin_unlock(&configfs_dirent_lock);
 239         configfs_drop_dentry(sd, dentry->d_parent);
 240         dput(dentry);
 241         configfs_put(sd);
 242
 243         /*
 244          * drop_link() must be called before
 245          * decrementing target's ->s_links, so that the order of
 246          * drop_link(this, target) and drop_item(target) is preserved.
 247          */
 248         if (type && type->ct_item_ops &&
 249             type->ct_item_ops->drop_link)
 250                 type->ct_item_ops->drop_link(parent_item,
 251                                                target_sd->s_element);
 252
 253         spin_lock(&configfs_dirent_lock);
 254         target_sd->s_links--;
 255         spin_unlock(&configfs_dirent_lock);
 256         configfs_put(target_sd);
 257
 258         config_item_put(parent_item);
 259
 260         ret = 0;
 261
 262 out:
 263         return ret;
 264 }
 265
 266 const struct inode_operations configfs_symlink_inode_operations = {
 267         .get_link = simple_get_link,
 268         .setattr = configfs_setattr,
 269 };
 270