linux/fs/configfs/symlink.c
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   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 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
 140int 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
 215out_put:
 216        config_item_put(parent_item);
 217        return ret;
 218}
 219
 220int 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
 262out:
 263        return ret;
 264}
 265
 266const struct inode_operations configfs_symlink_inode_operations = {
 267        .get_link = simple_get_link,
 268        .setattr = configfs_setattr,
 269};
 270
 271
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