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  */
kernfs_create_link(struct kernfs_node * parent,const char * name,struct kernfs_node * target)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|0777, uid, gid, KERNFS_LINK);
40 	if (!kn)
41 		return ERR_PTR(-ENOMEM);
42 
43 	if (kernfs_ns_enabled(parent))
44 		kn->ns = target->ns;
45 	kn->symlink.target_kn = target;
46 	kernfs_get(target);	/* ref owned by symlink */
47 
48 	error = kernfs_add_one(kn);
49 	if (!error)
50 		return kn;
51 
52 	kernfs_put(kn);
53 	return ERR_PTR(error);
54 }
55 
kernfs_get_target_path(struct kernfs_node * parent,struct kernfs_node * target,char * path)56 static int kernfs_get_target_path(struct kernfs_node *parent,
57 				  struct kernfs_node *target, char *path)
58 {
59 	struct kernfs_node *base, *kn;
60 	char *s = path;
61 	int len = 0;
62 
63 	/* go up to the root, stop at the base */
64 	base = parent;
65 	while (base->parent) {
66 		kn = target->parent;
67 		while (kn->parent && base != kn)
68 			kn = kn->parent;
69 
70 		if (base == kn)
71 			break;
72 
73 		if ((s - path) + 3 >= PATH_MAX)
74 			return -ENAMETOOLONG;
75 
76 		strcpy(s, "../");
77 		s += 3;
78 		base = base->parent;
79 	}
80 
81 	/* determine end of target string for reverse fillup */
82 	kn = target;
83 	while (kn->parent && kn != base) {
84 		len += strlen(kn->name) + 1;
85 		kn = kn->parent;
86 	}
87 
88 	/* check limits */
89 	if (len < 2)
90 		return -EINVAL;
91 	len--;
92 	if ((s - path) + len >= PATH_MAX)
93 		return -ENAMETOOLONG;
94 
95 	/* reverse fillup of target string from target to base */
96 	kn = target;
97 	while (kn->parent && kn != base) {
98 		int slen = strlen(kn->name);
99 
100 		len -= slen;
101 		memcpy(s + len, kn->name, slen);
102 		if (len)
103 			s[--len] = '/';
104 
105 		kn = kn->parent;
106 	}
107 
108 	return 0;
109 }
110 
kernfs_getlink(struct inode * inode,char * path)111 static int kernfs_getlink(struct inode *inode, char *path)
112 {
113 	struct kernfs_node *kn = inode->i_private;
114 	struct kernfs_node *parent = kn->parent;
115 	struct kernfs_node *target = kn->symlink.target_kn;
116 	int error;
117 
118 	down_read(&kernfs_rwsem);
119 	error = kernfs_get_target_path(parent, target, path);
120 	up_read(&kernfs_rwsem);
121 
122 	return error;
123 }
124 
kernfs_iop_get_link(struct dentry * dentry,struct inode * inode,struct delayed_call * done)125 static const char *kernfs_iop_get_link(struct dentry *dentry,
126 				       struct inode *inode,
127 				       struct delayed_call *done)
128 {
129 	char *body;
130 	int error;
131 
132 	if (!dentry)
133 		return ERR_PTR(-ECHILD);
134 	body = kzalloc(PAGE_SIZE, GFP_KERNEL);
135 	if (!body)
136 		return ERR_PTR(-ENOMEM);
137 	error = kernfs_getlink(inode, body);
138 	if (unlikely(error < 0)) {
139 		kfree(body);
140 		return ERR_PTR(error);
141 	}
142 	set_delayed_call(done, kfree_link, body);
143 	return body;
144 }
145 
146 const struct inode_operations kernfs_symlink_iops = {
147 	.listxattr	= kernfs_iop_listxattr,
148 	.get_link	= kernfs_iop_get_link,
149 	.setattr	= kernfs_iop_setattr,
150 	.getattr	= kernfs_iop_getattr,
151 	.permission	= kernfs_iop_permission,
152 };
153