kernfs_path*() functions always return the length of the full path but
the path content is undefined if the length is larger than the
provided buffer.  This makes its behavior different from strlcpy() and
requires error handling in all its users even when they don't care
about truncation.  In addition, the implementation can actully be
simplified by making it behave properly in strlcpy() style.

* Update kernfs_path_from_node_locked() to always fill up the buffer
  with path.  If the buffer is not large enough, the output is
  truncated and terminated.

* kernfs_path() no longer needs error handling.  Make it a simple
  inline wrapper around kernfs_path_from_node().

* sysfs_warn_dup()'s use of kernfs_path() doesn't need error handling.
  Updated accordingly.

* cgroup_path()'s use of kernfs_path() updated to retain the old
  behavior.

Signed-off-by: Tejun Heo <redacted>
Cc: Serge Hallyn <serge.hallyn-GeWIH/nMZzLQT0dZR+AlfA@public.gmane.org>
Cc: Greg Kroah-Hartman <redacted>
---
 fs/kernfs/dir.c        | 61 ++++++++++++++------------------------------------
 fs/sysfs/dir.c         |  6 ++---
 include/linux/cgroup.h |  7 +++++-
 include/linux/kernfs.h | 21 ++++++++++++-----
 4 files changed, 42 insertions(+), 53 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index e57174d..09242aa 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c

@@ -110,8 +110,9 @@ static struct kernfs_node *kernfs_common_ancestor(struct kernfs_node *a,
  * kn_to:   /n1/n2/n3         [depth=3]
  * result:  /../..
  *
- * return value: length of the string.  If greater than buflen,
- * then contents of buf are undefined.  On error, -1 is returned.
+ * Returns the length of the full path.  If the full length is equal to or
+ * greater than @buflen, @buf contains the truncated path with the trailing
+ * '\0'.  On error, -errno is returned.
  */
 static int kernfs_path_from_node_locked(struct kernfs_node *kn_to,
 					struct kernfs_node *kn_from,

@@ -119,9 +120,8 @@ static int kernfs_path_from_node_locked(struct kernfs_node *kn_to,
 {
 	struct kernfs_node *kn, *common;
 	const char parent_str[] = "/..";
-	size_t depth_from, depth_to, len = 0, nlen = 0;
-	char *p;
-	int i;
+	size_t depth_from, depth_to, len = 0;
+	int i, j;
 
 	if (!kn_from)
 		kn_from = kernfs_root(kn_to)->kn;

@@ -131,7 +131,7 @@ static int kernfs_path_from_node_locked(struct kernfs_node *kn_to,
 
 	common = kernfs_common_ancestor(kn_from, kn_to);
 	if (WARN_ON(!common))
-		return -1;
+		return -EINVAL;
 
 	depth_to = kernfs_depth(common, kn_to);
 	depth_from = kernfs_depth(common, kn_from);

@@ -144,22 +144,16 @@ static int kernfs_path_from_node_locked(struct kernfs_node *kn_to,
 			       len < buflen ? buflen - len : 0);
 
 	/* Calculate how many bytes we need for the rest */
-	for (kn = kn_to; kn != common; kn = kn->parent)
-		nlen += strlen(kn->name) + 1;
-
-	if (len + nlen >= buflen)
-		return len + nlen;
-
-	p = buf + len + nlen;
-	*p = '\0';
-	for (kn = kn_to; kn != common; kn = kn->parent) {
-		size_t tmp = strlen(kn->name);
-		p -= tmp;
-		memcpy(p, kn->name, tmp);
-		*(--p) = '/';
+	for (i = depth_to - 1; i >= 0; i--) {
+		for (kn = kn_to, j = 0; j < i; j++)
+			kn = kn->parent;