xref: /kernel/linux/linux-5.10/fs/kernfs/symlink.c (revision 8c2ecf20) 
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 */
25struct 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
57static 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
112static 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	mutex_lock(&kernfs_mutex);
120	error = kernfs_get_target_path(parent, target, path);
121	mutex_unlock(&kernfs_mutex);
122
123	return error;
124}
125
126static 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
147const 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};
154