[linux-2.6-block.git] / fs / d_path.c

/* SPDX-License-Identifier: GPL-2.0 */
#include <linux/syscalls.h>
#include <linux/export.h>
#include <linux/uaccess.h>
#include <linux/fs_struct.h>
#include <linux/fs.h>
#include <linux/slab.h>
#include <linux/prefetch.h>
#include "mount.h"

static int prepend(char **buffer, int *buflen, const char *str, int namelen)
{
	*buflen -= namelen;
	if (*buflen < 0)
		return -ENAMETOOLONG;
	*buffer -= namelen;
	memcpy(*buffer, str, namelen);
	return 0;
}

/**
 * prepend_name - prepend a pathname in front of current buffer pointer
 * @buffer: buffer pointer
 * @buflen: allocated length of the buffer
 * @name:   name string and length qstr structure
 *
 * With RCU path tracing, it may race with d_move(). Use READ_ONCE() to
 * make sure that either the old or the new name pointer and length are
 * fetched. However, there may be mismatch between length and pointer.
 * The length cannot be trusted, we need to copy it byte-by-byte until
 * the length is reached or a null byte is found. It also prepends "/" at
 * the beginning of the name. The sequence number check at the caller will
 * retry it again when a d_move() does happen. So any garbage in the buffer
 * due to mismatched pointer and length will be discarded.
 *
 * Load acquire is needed to make sure that we see that terminating NUL.
 */
static int prepend_name(char **buffer, int *buflen, const struct qstr *name)
{
	const char *dname = smp_load_acquire(&name->name); /* ^^^ */
	u32 dlen = READ_ONCE(name->len);
	char *p;

	*buflen -= dlen + 1;
	if (*buflen < 0)
		return -ENAMETOOLONG;
	p = *buffer -= dlen + 1;
	*p++ = '/';
	while (dlen--) {
		char c = *dname++;
		if (!c)
			break;
		*p++ = c;
	}
	return 0;
}

/**
 * prepend_path - Prepend path string to a buffer
 * @path: the dentry/vfsmount to report
 * @root: root vfsmnt/dentry
 * @buffer: pointer to the end of the buffer
 * @buflen: pointer to buffer length
 *
 * The function will first try to write out the pathname without taking any
 * lock other than the RCU read lock to make sure that dentries won't go away.
 * It only checks the sequence number of the global rename_lock as any change
 * in the dentry's d_seq will be preceded by changes in the rename_lock
 * sequence number. If the sequence number had been changed, it will restart
 * the whole pathname back-tracing sequence again by taking the rename_lock.
 * In this case, there is no need to take the RCU read lock as the recursive
 * parent pointer references will keep the dentry chain alive as long as no
 * rename operation is performed.
 */
static int prepend_path(const struct path *path,
			const struct path *root,
			char **buffer, int *buflen)
{
	struct dentry *dentry;
	struct vfsmount *vfsmnt;
	struct mount *mnt;
	int error = 0;
	unsigned seq, m_seq = 0;
	char *bptr;
	int blen;

	rcu_read_lock();
restart_mnt:
	read_seqbegin_or_lock(&mount_lock, &m_seq);
	seq = 0;
	rcu_read_lock();
restart:
	bptr = *buffer;
	blen = *buflen;
	error = 0;
	dentry = path->dentry;
	vfsmnt = path->mnt;
	mnt = real_mount(vfsmnt);
	read_seqbegin_or_lock(&rename_lock, &seq);
	while (dentry != root->dentry || vfsmnt != root->mnt) {
		struct dentry * parent;

		if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
			struct mount *parent = READ_ONCE(mnt->mnt_parent);
			/* Escaped? */
			if (dentry != vfsmnt->mnt_root) {
				bptr = *buffer;
				blen = *buflen;
				error = 3;
				break;
			}
			/* Global root? */
			if (mnt != parent) {
				dentry = READ_ONCE(mnt->mnt_mountpoint);
				mnt = parent;
				vfsmnt = &mnt->mnt;
				continue;
			}
			if (is_mounted(vfsmnt) && !is_anon_ns(mnt->mnt_ns))
				error = 1;	// absolute root
			else
				error = 2;	// detached or not attached yet
			break;
		}
		parent = dentry->d_parent;
		prefetch(parent);
		error = prepend_name(&bptr, &blen, &dentry->d_name);
		if (error)
			break;

		dentry = parent;
	}
	if (!(seq & 1))
		rcu_read_unlock();
	if (need_seqretry(&rename_lock, seq)) {
		seq = 1;
		goto restart;
	}
	done_seqretry(&rename_lock, seq);

	if (!(m_seq & 1))
		rcu_read_unlock();
	if (need_seqretry(&mount_lock, m_seq)) {
		m_seq = 1;
		goto restart_mnt;
	}
	done_seqretry(&mount_lock, m_seq);

	if (error >= 0 && bptr == *buffer) {
		if (--blen < 0)
			error = -ENAMETOOLONG;
		else
			*--bptr = '/';
	}
	*buffer = bptr;
	*buflen = blen;
	return error;
}

/**
 * __d_path - return the path of a dentry
 * @path: the dentry/vfsmount to report
 * @root: root vfsmnt/dentry
 * @buf: buffer to return value in
 * @buflen: buffer length
 *
 * Convert a dentry into an ASCII path name.
 *
 * Returns a pointer into the buffer or an error code if the
 * path was too long.
 *
 * "buflen" should be positive.
 *
 * If the path is not reachable from the supplied root, return %NULL.
 */
char *__d_path(const struct path *path,
	       const struct path *root,
	       char *buf, int buflen)
{
	char *res = buf + buflen;
	int error;

	prepend(&res, &buflen, "\0", 1);
	error = prepend_path(path, root, &res, &buflen);

	if (error < 0)
		return ERR_PTR(error);
	if (error > 0)
		return NULL;
	return res;
}

char *d_absolute_path(const struct path *path,
	       char *buf, int buflen)
{
	struct path root = {};
	char *res = buf + buflen;
	int error;

	prepend(&res, &buflen, "\0", 1);
	error = prepend_path(path, &root, &res, &buflen);

	if (error > 1)
		error = -EINVAL;
	if (error < 0)
		return ERR_PTR(error);
	return res;
}

/*
 * same as __d_path but appends "(deleted)" for unlinked files.
 */
static int path_with_deleted(const struct path *path,
			     const struct path *root,
			     char **buf, int *buflen)
{
	prepend(buf, buflen, "\0", 1);
	if (d_unlinked(path->dentry)) {
		int error = prepend(buf, buflen, " (deleted)", 10);
		if (error)
			return error;
	}

	return prepend_path(path, root, buf, buflen);
}

static int prepend_unreachable(char **buffer, int *buflen)
{
	return prepend(buffer, buflen, "(unreachable)", 13);
}

static void get_fs_root_rcu(struct fs_struct *fs, struct path *root)
{
	unsigned seq;

	do {
		seq = read_seqcount_begin(&fs->seq);
		*root = fs->root;
	} while (read_seqcount_retry(&fs->seq, seq));
}

/**
 * d_path - return the path of a dentry
 * @path: path to report
 * @buf: buffer to return value in
 * @buflen: buffer length
 *
 * Convert a dentry into an ASCII path name. If the entry has been deleted
 * the string " (deleted)" is appended. Note that this is ambiguous.
 *
 * Returns a pointer into the buffer or an error code if the path was
 * too long. Note: Callers should use the returned pointer, not the passed
 * in buffer, to use the name! The implementation often starts at an offset
 * into the buffer, and may leave 0 bytes at the start.
 *
 * "buflen" should be positive.
 */
char *d_path(const struct path *path, char *buf, int buflen)
{
	char *res = buf + buflen;
	struct path root;
	int error;

	/*
	 * We have various synthetic filesystems that never get mounted.  On
	 * these filesystems dentries are never used for lookup purposes, and
	 * thus don't need to be hashed.  They also don't need a name until a
	 * user wants to identify the object in /proc/pid/fd/.  The little hack
	 * below allows us to generate a name for these objects on demand:
	 *
	 * Some pseudo inodes are mountable.  When they are mounted
	 * path->dentry == path->mnt->mnt_root.  In that case don't call d_dname
	 * and instead have d_path return the mounted path.
	 */
	if (path->dentry->d_op && path->dentry->d_op->d_dname &&
	    (!IS_ROOT(path->dentry) || path->dentry != path->mnt->mnt_root))
		return path->dentry->d_op->d_dname(path->dentry, buf, buflen);

	rcu_read_lock();
	get_fs_root_rcu(current->fs, &root);
	error = path_with_deleted(path, &root, &res, &buflen);
	rcu_read_unlock();

	if (error < 0)
		res = ERR_PTR(error);
	return res;
}
EXPORT_SYMBOL(d_path);

/*
 * Helper function for dentry_operations.d_dname() members
 */
char *dynamic_dname(struct dentry *dentry, char *buffer, int buflen,
			const char *fmt, ...)
{
	va_list args;
	char temp[64];
	int sz;

	va_start(args, fmt);
	sz = vsnprintf(temp, sizeof(temp), fmt, args) + 1;
	va_end(args);

	if (sz > sizeof(temp) || sz > buflen)
		return ERR_PTR(-ENAMETOOLONG);

	buffer += buflen - sz;
	return memcpy(buffer, temp, sz);
}

char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
{
	char *end = buffer + buflen;
	/* these dentries are never renamed, so d_lock is not needed */
	if (prepend(&end, &buflen, " (deleted)", 11) ||
	    prepend(&end, &buflen, dentry->d_name.name, dentry->d_name.len) ||
	    prepend(&end, &buflen, "/", 1))  
		end = ERR_PTR(-ENAMETOOLONG);
	return end;
}

/*
 * Write full pathname from the root of the filesystem into the buffer.
 */
static char *__dentry_path(struct dentry *d, char *buf, int buflen)
{
	struct dentry *dentry;
	char *end, *retval;
	int len, seq = 0;
	int error = 0;

	if (buflen < 2)
		goto Elong;

	rcu_read_lock();
restart:
	dentry = d;
	end = buf + buflen;
	len = buflen;
	prepend(&end, &len, "\0", 1);
	/* Get '/' right */
	retval = end-1;
	*retval = '/';
	read_seqbegin_or_lock(&rename_lock, &seq);
	while (!IS_ROOT(dentry)) {
		struct dentry *parent = dentry->d_parent;

		prefetch(parent);
		error = prepend_name(&end, &len, &dentry->d_name);
		if (error)
			break;

		retval = end;
		dentry = parent;
	}
	if (!(seq & 1))
		rcu_read_unlock();
	if (need_seqretry(&rename_lock, seq)) {
		seq = 1;
		goto restart;
	}
	done_seqretry(&rename_lock, seq);
	if (error)
		goto Elong;
	return retval;
Elong:
	return ERR_PTR(-ENAMETOOLONG);
}

char *dentry_path_raw(struct dentry *dentry, char *buf, int buflen)
{
	return __dentry_path(dentry, buf, buflen);
}
EXPORT_SYMBOL(dentry_path_raw);

char *dentry_path(struct dentry *dentry, char *buf, int buflen)
{
	char *p = NULL;
	char *retval;

	if (d_unlinked(dentry)) {
		p = buf + buflen;
		if (prepend(&p, &buflen, "//deleted", 10) != 0)
			goto Elong;
		buflen++;
	}
	retval = __dentry_path(dentry, buf, buflen);
	if (!IS_ERR(retval) && p)
		*p = '/';	/* restore '/' overriden with '\0' */
	return retval;
Elong:
	return ERR_PTR(-ENAMETOOLONG);
}

static void get_fs_root_and_pwd_rcu(struct fs_struct *fs, struct path *root,
				    struct path *pwd)
{
	unsigned seq;

	do {
		seq = read_seqcount_begin(&fs->seq);
		*root = fs->root;
		*pwd = fs->pwd;
	} while (read_seqcount_retry(&fs->seq, seq));
}

/*
 * NOTE! The user-level library version returns a
 * character pointer. The kernel system call just
 * returns the length of the buffer filled (which
 * includes the ending '\0' character), or a negative
 * error value. So libc would do something like
 *
 *	char *getcwd(char * buf, size_t size)
 *	{
 *		int retval;
 *
 *		retval = sys_getcwd(buf, size);
 *		if (retval >= 0)
 *			return buf;
 *		errno = -retval;
 *		return NULL;
 *	}
 */
SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
{
	int error;
	struct path pwd, root;
	char *page = __getname();

	if (!page)
		return -ENOMEM;

	rcu_read_lock();
	get_fs_root_and_pwd_rcu(current->fs, &root, &pwd);

	error = -ENOENT;
	if (!d_unlinked(pwd.dentry)) {
		unsigned long len;
		char *cwd = page + PATH_MAX;
		int buflen = PATH_MAX;

		prepend(&cwd, &buflen, "\0", 1);
		error = prepend_path(&pwd, &root, &cwd, &buflen);
		rcu_read_unlock();

		if (error < 0)
			goto out;

		/* Unreachable from current root */
		if (error > 0) {
			error = prepend_unreachable(&cwd, &buflen);
			if (error)
				goto out;
		}

		error = -ERANGE;
		len = PATH_MAX + page - cwd;
		if (len <= size) {
			error = len;
			if (copy_to_user(buf, cwd, len))
				error = -EFAULT;
		}
	} else {
		rcu_read_unlock();
	}

out:
	__putname(page);
	return error;
}
Commit	Line	Data
7a5cf791 AV	1	/* SPDX-License-Identifier: GPL-2.0 */
	2	#include <linux/syscalls.h>
	3	#include <linux/export.h>
	4	#include <linux/uaccess.h>
	5	#include <linux/fs_struct.h>
	6	#include <linux/fs.h>
	7	#include <linux/slab.h>
	8	#include <linux/prefetch.h>
	9	#include "mount.h"
	10
	11	static int prepend(char *buffer, int buflen, const char *str, int namelen)
	12	{
	13	*buflen -= namelen;
	14	if (*buflen < 0)
	15	return -ENAMETOOLONG;
	16	*buffer -= namelen;
	17	memcpy(*buffer, str, namelen);
	18	return 0;
	19	}
	20
	21	/**
	22	* prepend_name - prepend a pathname in front of current buffer pointer
	23	* @buffer: buffer pointer
	24	* @buflen: allocated length of the buffer
	25	* @name: name string and length qstr structure
	26	*
	27	* With RCU path tracing, it may race with d_move(). Use READ_ONCE() to
	28	* make sure that either the old or the new name pointer and length are
	29	* fetched. However, there may be mismatch between length and pointer.
	30	* The length cannot be trusted, we need to copy it byte-by-byte until
	31	* the length is reached or a null byte is found. It also prepends "/" at
	32	* the beginning of the name. The sequence number check at the caller will
	33	* retry it again when a d_move() does happen. So any garbage in the buffer
	34	* due to mismatched pointer and length will be discarded.
	35	*
	36	* Load acquire is needed to make sure that we see that terminating NUL.
	37	*/
	38	static int prepend_name(char *buffer, int buflen, const struct qstr *name)
	39	{
	40	const char dname = smp_load_acquire(&name->name); / ^^^ */
	41	u32 dlen = READ_ONCE(name->len);
	42	char *p;
	43
	44	*buflen -= dlen + 1;
	45	if (*buflen < 0)
	46	return -ENAMETOOLONG;
	47	p = *buffer -= dlen + 1;
	48	*p++ = '/';
	49	while (dlen--) {
	50	char c = *dname++;
	51	if (!c)
	52	break;
	53	*p++ = c;
	54	}
	55	return 0;
	56	}
	57
	58	/**
	59	* prepend_path - Prepend path string to a buffer
	60	* @path: the dentry/vfsmount to report
	61	* @root: root vfsmnt/dentry
	62	* @buffer: pointer to the end of the buffer
	63	* @buflen: pointer to buffer length
	64	*
65	* The function will first try to write out the pathname without taking any
66	* lock other than the RCU read lock to make sure that dentries won't go away.
67	* It only checks the sequence number of the global rename_lock as any change
68	* in the dentry's d_seq will be preceded by changes in the rename_lock
69	* sequence number. If the sequence number had been changed, it will restart
70	* the whole pathname back-tracing sequence again by taking the rename_lock.
71	* In this case, there is no need to take the RCU read lock as the recursive
72	* parent pointer references will keep the dentry chain alive as long as no
73	* rename operation is performed.
74	*/
75	static int prepend_path(const struct path *path,
76	const struct path *root,
77	char *buffer, int buflen)
78	{
79	struct dentry *dentry;
80	struct vfsmount *vfsmnt;
81	struct mount *mnt;
82	int error = 0;
83	unsigned seq, m_seq = 0;
84	char *bptr;
85	int blen;
86
87	rcu_read_lock();
88	restart_mnt:
89	read_seqbegin_or_lock(&mount_lock, &m_seq);
90	seq = 0;
91	rcu_read_lock();
92	restart:
93	bptr = *buffer;
94	blen = *buflen;
95	error = 0;
96	dentry = path->dentry;
97	vfsmnt = path->mnt;
98	mnt = real_mount(vfsmnt);
99	read_seqbegin_or_lock(&rename_lock, &seq);
100	while (dentry != root->dentry \|\| vfsmnt != root->mnt) {
101	struct dentry * parent;
102
103	if (dentry == vfsmnt->mnt_root \|\| IS_ROOT(dentry)) {
104	struct mount *parent = READ_ONCE(mnt->mnt_parent);
105	/* Escaped? */
106	if (dentry != vfsmnt->mnt_root) {
107	bptr = *buffer;
108	blen = *buflen;
109	error = 3;
110	break;
111	}
112	/* Global root? */
113	if (mnt != parent) {
114	dentry = READ_ONCE(mnt->mnt_mountpoint);
115	mnt = parent;
116	vfsmnt = &mnt->mnt;
117	continue;
118	}
f2683bd8 AV	119	if (is_mounted(vfsmnt) && !is_anon_ns(mnt->mnt_ns))
	120	error = 1; // absolute root
	121	else
	122	error = 2; // detached or not attached yet
7a5cf791 AV	123	break;
	124	}
	125	parent = dentry->d_parent;
	126	prefetch(parent);
	127	error = prepend_name(&bptr, &blen, &dentry->d_name);
	128	if (error)
	129	break;
	130
	131	dentry = parent;
	132	}
	133	if (!(seq & 1))
	134	rcu_read_unlock();
	135	if (need_seqretry(&rename_lock, seq)) {
	136	seq = 1;
	137	goto restart;
	138	}
	139	done_seqretry(&rename_lock, seq);
	140
	141	if (!(m_seq & 1))
	142	rcu_read_unlock();
	143	if (need_seqretry(&mount_lock, m_seq)) {
	144	m_seq = 1;
	145	goto restart_mnt;
	146	}
	147	done_seqretry(&mount_lock, m_seq);
	148
	149	if (error >= 0 && bptr == *buffer) {
	150	if (--blen < 0)
	151	error = -ENAMETOOLONG;
	152	else
	153	*--bptr = '/';
	154	}
	155	*buffer = bptr;
	156	*buflen = blen;
	157	return error;
	158	}
	159
	160	/**
	161	* __d_path - return the path of a dentry
	162	* @path: the dentry/vfsmount to report
	163	* @root: root vfsmnt/dentry
	164	* @buf: buffer to return value in
	165	* @buflen: buffer length
	166	*
	167	* Convert a dentry into an ASCII path name.
	168	*
	169	* Returns a pointer into the buffer or an error code if the
	170	* path was too long.
	171	*
	172	* "buflen" should be positive.
	173	*
	174	* If the path is not reachable from the supplied root, return %NULL.
	175	*/
	176	char __d_path(const struct path path,
	177	const struct path *root,
	178	char *buf, int buflen)
	179	{
	180	char *res = buf + buflen;
	181	int error;
	182
	183	prepend(&res, &buflen, "\0", 1);
	184	error = prepend_path(path, root, &res, &buflen);
	185
	186	if (error < 0)
187	return ERR_PTR(error);
188	if (error > 0)
189	return NULL;
190	return res;
191	}
192
193	char d_absolute_path(const struct path path,
194	char *buf, int buflen)
195	{
196	struct path root = {};
197	char *res = buf + buflen;
198	int error;
199
200	prepend(&res, &buflen, "\0", 1);
201	error = prepend_path(path, &root, &res, &buflen);
202
203	if (error > 1)
204	error = -EINVAL;
205	if (error < 0)
206	return ERR_PTR(error);
207	return res;
208	}
209
210	/*
211	* same as __d_path but appends "(deleted)" for unlinked files.
212	*/
213	static int path_with_deleted(const struct path *path,
214	const struct path *root,
215	char *buf, int buflen)
216	{
217	prepend(buf, buflen, "\0", 1);
218	if (d_unlinked(path->dentry)) {
219	int error = prepend(buf, buflen, " (deleted)", 10);
220	if (error)
221	return error;
222	}
223
224	return prepend_path(path, root, buf, buflen);
225	}
226
227	static int prepend_unreachable(char *buffer, int buflen)
228	{
229	return prepend(buffer, buflen, "(unreachable)", 13);
230	}
231
232	static void get_fs_root_rcu(struct fs_struct fs, struct path root)
233	{
234	unsigned seq;
235
236	do {
237	seq = read_seqcount_begin(&fs->seq);
238	*root = fs->root;
239	} while (read_seqcount_retry(&fs->seq, seq));
240	}
241
242	/**
243	* d_path - return the path of a dentry
244	* @path: path to report
245	* @buf: buffer to return value in
246	* @buflen: buffer length
247	*
248	* Convert a dentry into an ASCII path name. If the entry has been deleted
249	* the string " (deleted)" is appended. Note that this is ambiguous.
250	*
251	* Returns a pointer into the buffer or an error code if the path was
252	* too long. Note: Callers should use the returned pointer, not the passed
253	* in buffer, to use the name! The implementation often starts at an offset
254	* into the buffer, and may leave 0 bytes at the start.
255	*
256	* "buflen" should be positive.
257	*/
258	char d_path(const struct path path, char *buf, int buflen)
259	{
260	char *res = buf + buflen;
261	struct path root;
262	int error;
263
264	/*
265	* We have various synthetic filesystems that never get mounted. On
266	* these filesystems dentries are never used for lookup purposes, and
267	* thus don't need to be hashed. They also don't need a name until a
268	* user wants to identify the object in /proc/pid/fd/. The little hack
269	* below allows us to generate a name for these objects on demand:
270	*
271	* Some pseudo inodes are mountable. When they are mounted
272	* path->dentry == path->mnt->mnt_root. In that case don't call d_dname
273	* and instead have d_path return the mounted path.
274	*/
275	if (path->dentry->d_op && path->dentry->d_op->d_dname &&
276	(!IS_ROOT(path->dentry) \|\| path->dentry != path->mnt->mnt_root))
277	return path->dentry->d_op->d_dname(path->dentry, buf, buflen);
278
279	rcu_read_lock();
280	get_fs_root_rcu(current->fs, &root);
281	error = path_with_deleted(path, &root, &res, &buflen);
282	rcu_read_unlock();
283
284	if (error < 0)
285	res = ERR_PTR(error);
286	return res;
287	}
288	EXPORT_SYMBOL(d_path);
289
290	/*
291	* Helper function for dentry_operations.d_dname() members
292	*/
293	char dynamic_dname(struct dentry dentry, char *buffer, int buflen,
294	const char *fmt, ...)
295	{
296	va_list args;
297	char temp[64];
298	int sz;
299
300	va_start(args, fmt);
301	sz = vsnprintf(temp, sizeof(temp), fmt, args) + 1;
302	va_end(args);
303
304	if (sz > sizeof(temp) \|\| sz > buflen)
305	return ERR_PTR(-ENAMETOOLONG);
306
307	buffer += buflen - sz;
308	return memcpy(buffer, temp, sz);
309	}
310
311	char simple_dname(struct dentry dentry, char *buffer, int buflen)
312	{
313	char *end = buffer + buflen;
314	/* these dentries are never renamed, so d_lock is not needed */
315	if (prepend(&end, &buflen, " (deleted)", 11) \|\|
316	prepend(&end, &buflen, dentry->d_name.name, dentry->d_name.len) \|\|
317	prepend(&end, &buflen, "/", 1))
318	end = ERR_PTR(-ENAMETOOLONG);
319	return end;
320	}
7a5cf791 AV	321
	322	/*
	323	* Write full pathname from the root of the filesystem into the buffer.
	324	*/
	325	static char __dentry_path(struct dentry d, char *buf, int buflen)
	326	{
	327	struct dentry *dentry;
	328	char end, retval;
	329	int len, seq = 0;
	330	int error = 0;
	331
	332	if (buflen < 2)
	333	goto Elong;
	334
	335	rcu_read_lock();
	336	restart:
	337	dentry = d;
	338	end = buf + buflen;
	339	len = buflen;
	340	prepend(&end, &len, "\0", 1);
	341	/* Get '/' right */
	342	retval = end-1;
	343	*retval = '/';
	344	read_seqbegin_or_lock(&rename_lock, &seq);
	345	while (!IS_ROOT(dentry)) {
	346	struct dentry *parent = dentry->d_parent;
	347
	348	prefetch(parent);
	349	error = prepend_name(&end, &len, &dentry->d_name);
	350	if (error)
	351	break;
	352
	353	retval = end;
	354	dentry = parent;
	355	}
	356	if (!(seq & 1))
	357	rcu_read_unlock();
	358	if (need_seqretry(&rename_lock, seq)) {
	359	seq = 1;
	360	goto restart;
	361	}
	362	done_seqretry(&rename_lock, seq);
	363	if (error)
	364	goto Elong;
	365	return retval;
	366	Elong:
	367	return ERR_PTR(-ENAMETOOLONG);
	368	}
	369
	370	char dentry_path_raw(struct dentry dentry, char *buf, int buflen)
	371	{
	372	return __dentry_path(dentry, buf, buflen);
	373	}
	374	EXPORT_SYMBOL(dentry_path_raw);
	375
	376	char dentry_path(struct dentry dentry, char *buf, int buflen)
	377	{
	378	char *p = NULL;
	379	char *retval;
	380
	381	if (d_unlinked(dentry)) {
	382	p = buf + buflen;
	383	if (prepend(&p, &buflen, "//deleted", 10) != 0)
	384	goto Elong;
385	buflen++;
386	}
387	retval = __dentry_path(dentry, buf, buflen);
388	if (!IS_ERR(retval) && p)
389	p = '/'; / restore '/' overriden with '\0' */
390	return retval;
391	Elong:
392	return ERR_PTR(-ENAMETOOLONG);
393	}
394
395	static void get_fs_root_and_pwd_rcu(struct fs_struct fs, struct path root,
396	struct path *pwd)
397	{
398	unsigned seq;
399
400	do {
401	seq = read_seqcount_begin(&fs->seq);
402	*root = fs->root;
403	*pwd = fs->pwd;
404	} while (read_seqcount_retry(&fs->seq, seq));
405	}
406
407	/*
408	* NOTE! The user-level library version returns a
409	* character pointer. The kernel system call just
410	* returns the length of the buffer filled (which
411	* includes the ending '\0' character), or a negative
412	* error value. So libc would do something like
413	*
414	* char getcwd(char buf, size_t size)
415	* {
416	* int retval;
417	*
418	* retval = sys_getcwd(buf, size);
419	* if (retval >= 0)
420	* return buf;
421	* errno = -retval;
422	* return NULL;
423	* }
424	*/
425	SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
426	{
427	int error;
428	struct path pwd, root;
429	char *page = __getname();
430
431	if (!page)
432	return -ENOMEM;
433
434	rcu_read_lock();
435	get_fs_root_and_pwd_rcu(current->fs, &root, &pwd);
436
437	error = -ENOENT;
438	if (!d_unlinked(pwd.dentry)) {
439	unsigned long len;
440	char *cwd = page + PATH_MAX;
441	int buflen = PATH_MAX;
442
443	prepend(&cwd, &buflen, "\0", 1);
444	error = prepend_path(&pwd, &root, &cwd, &buflen);
445	rcu_read_unlock();
446
447	if (error < 0)
448	goto out;
449
450	/* Unreachable from current root */
451	if (error > 0) {
452	error = prepend_unreachable(&cwd, &buflen);
453	if (error)
454	goto out;
455	}
456
457	error = -ERANGE;
458	len = PATH_MAX + page - cwd;
459	if (len <= size) {
460	error = len;
461	if (copy_to_user(buf, cwd, len))
462	error = -EFAULT;
463	}
464	} else {
465	rcu_read_unlock();
466	}
467
468	out:
469	__putname(page);
470	return error;
471	}