fs/kernfs/symlink.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * fs/kernfs/symlink.c - kernfs symlink implementation
   4  *
   5  * Copyright (c) 2001-3 Patrick Mochel
   6  * Copyright (c) 2007 SUSE Linux Products GmbH
   7  * Copyright (c) 2007, 2013 Tejun Heo <tj@kernel.org>
   8  */
   9
  10 #include <linux/fs.h>
  11 #include <linux/gfp.h>
  12 #include <linux/namei.h>
  13
  14 #include "kernfs-internal.h"
  15
  16 /**
  17  * kernfs_create_link - create a symlink
  18  * @parent: directory to create the symlink in
  19  * @name: name of the symlink
  20  * @target: target node for the symlink to point to
  21  *
  22  * Returns the created node on success, ERR_PTR() value on error.
  23  * Ownership of the link matches ownership of the target.
  24  */
  25 struct kernfs_node *kernfs_create_link(struct kernfs_node *parent,
  26                                        const char *name,
  27                                        struct kernfs_node *target)
  28 {
  29         struct kernfs_node *kn;
  30         int error;
  31         kuid_t uid = GLOBAL_ROOT_UID;
  32         kgid_t gid = GLOBAL_ROOT_GID;
  33
  34         if (target->iattr) {
  35                 uid = target->iattr->ia_uid;
  36                 gid = target->iattr->ia_gid;
  37         }
  38
  39         kn = kernfs_new_node(parent, name, S_IFLNK|S_IRWXUGO, uid, gid,
  40                              KERNFS_LINK);
  41         if (!kn)
  42                 return ERR_PTR(-ENOMEM);
  43
  44         if (kernfs_ns_enabled(parent))
  45                 kn->ns = target->ns;
  46         kn->symlink.target_kn = target;
  47         kernfs_get(target);     /* ref owned by symlink */
  48
  49         error = kernfs_add_one(kn);
  50         if (!error)
  51                 return kn;
  52
  53         kernfs_put(kn);
  54         return ERR_PTR(error);
  55 }
  56
  57 static int kernfs_get_target_path(struct kernfs_node *parent,
  58                                   struct kernfs_node *target, char *path)
  59 {
  60         struct kernfs_node *base, *kn;
  61         char *s = path;
  62         int len = 0;
  63
  64         /* go up to the root, stop at the base */
  65         base = parent;
  66         while (base->parent) {
  67                 kn = target->parent;
  68                 while (kn->parent && base != kn)
  69                         kn = kn->parent;
  70
  71                 if (base == kn)
  72                         break;
  73
  74                 if ((s - path) + 3 >= PATH_MAX)
  75                         return -ENAMETOOLONG;
  76
  77                 strcpy(s, "../");
  78                 s += 3;
  79                 base = base->parent;
  80         }
  81
  82         /* determine end of target string for reverse fillup */
  83         kn = target;
  84         while (kn->parent && kn != base) {
  85                 len += strlen(kn->name) + 1;
  86                 kn = kn->parent;
  87         }
  88
  89         /* check limits */
  90         if (len < 2)
  91                 return -EINVAL;
  92         len--;
  93         if ((s - path) + len >= PATH_MAX)
  94                 return -ENAMETOOLONG;
  95
  96         /* reverse fillup of target string from target to base */
  97         kn = target;
  98         while (kn->parent && kn != base) {
  99                 int slen = strlen(kn->name);
 100
 101                 len -= slen;
 102                 memcpy(s + len, kn->name, slen);
 103                 if (len)
 104                         s[--len] = '/';
 105
 106                 kn = kn->parent;
 107         }
 108
 109         return 0;
 110 }
 111
 112 static int kernfs_getlink(struct inode *inode, char *path)
 113 {
 114         struct kernfs_node *kn = inode->i_private;
 115         struct kernfs_node *parent = kn->parent;
 116         struct kernfs_node *target = kn->symlink.target_kn;
 117         int error;
 118
 119         down_read(&kernfs_rwsem);
 120         error = kernfs_get_target_path(parent, target, path);
 121         up_read(&kernfs_rwsem);
 122
 123         return error;
 124 }
 125
 126 static const char *kernfs_iop_get_link(struct dentry *dentry,
 127                                        struct inode *inode,
 128                                        struct delayed_call *done)
 129 {
 130         char *body;
 131         int error;
 132
 133         if (!dentry)
 134                 return ERR_PTR(-ECHILD);
 135         body = kzalloc(PAGE_SIZE, GFP_KERNEL);
 136         if (!body)
 137                 return ERR_PTR(-ENOMEM);
 138         error = kernfs_getlink(inode, body);
 139         if (unlikely(error < 0)) {
 140                 kfree(body);
 141                 return ERR_PTR(error);
 142         }
 143         set_delayed_call(done, kfree_link, body);
 144         return body;
 145 }
 146
 147 const struct inode_operations kernfs_symlink_iops = {
 148         .listxattr      = kernfs_iop_listxattr,
 149         .get_link       = kernfs_iop_get_link,
 150         .setattr        = kernfs_iop_setattr,
 151         .getattr        = kernfs_iop_getattr,
 152         .permission     = kernfs_iop_permission,
 153 };