kernfs: Convert kernfs_path_from_node_locked() from strlcpy() to strscpy()

One of the last remaining users of strlcpy() in the kernel is
kernfs_path_from_node_locked(), which passes back the problematic "length
we _would_ have copied" return value to indicate truncation.  Convert the
chain of all callers to use the negative return value (some of which
already doing this explicitly). All callers were already also checking
for negative return values, so the risk to missed checks looks very low.

In this analysis, it was found that cgroup1_release_agent() actually
didn't handle the "too large" condition, so this is technically also a
bug fix. :)

Here's the chain of callers, and resolution identifying each one as now
handling the correct return value:

kernfs_path_from_node_locked()
        kernfs_path_from_node()
                pr_cont_kernfs_path()
                        returns void
                kernfs_path()
                        sysfs_warn_dup()
                                return value ignored
                        cgroup_path()
                                blkg_path()
                                        bfq_bic_update_cgroup()
                                                return value ignored
                                TRACE_IOCG_PATH()
                                        return value ignored
                                TRACE_CGROUP_PATH()
                                        return value ignored
                                perf_event_cgroup()
                                        return value ignored
                                task_group_path()
                                        return value ignored
                                damon_sysfs_memcg_path_eq()
                                        return value ignored
                                get_mm_memcg_path()
                                        return value ignored
                                lru_gen_seq_show()
                                        return value ignored
                        cgroup_path_from_kernfs_id()
                                return value ignored
                cgroup_show_path()
                        already converted "too large" error to negative value
                cgroup_path_ns_locked()
                        cgroup_path_ns()
                                bpf_iter_cgroup_show_fdinfo()
                                        return value ignored
                                cgroup1_release_agent()
                                        wasn't checking "too large" error
                        proc_cgroup_show()
                                already converted "too large" to negative value

Cc: Greg Kroah-Hartman <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Zefan Li <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Waiman Long <[email protected]>
Cc:  <[email protected]>
Co-developed-by: Azeem Shaikh <[email protected]>
Signed-off-by: Azeem Shaikh <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: kees

fs/kernfs/dir.c

@@ -127,7 +127,7 @@ static struct kernfs_node *kernfs_common_ancestor(struct kernfs_node *a,
  *
  * [3] when @kn_to is %NULL result will be "(null)"
  *
- * Return: the length of the full path.  If the full length is equal to or
+ * Return: the length of the constructed path.  If the path would have been
  * greater than @buflen, @buf contains the truncated path with the trailing
  * '\0'.  On error, -errno is returned.
  */
@@ -138,16 +138,17 @@ 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;
+	ssize_t copied;
 	int i, j;
 
 	if (!kn_to)
-		return strlcpy(buf, "(null)", buflen);
+		return strscpy(buf, "(null)", buflen);
 
 	if (!kn_from)
 		kn_from = kernfs_root(kn_to)->kn;
 
 	if (kn_from == kn_to)
-		return strlcpy(buf, "/", buflen);
+		return strscpy(buf, "/", buflen);
 
 	common = kernfs_common_ancestor(kn_from, kn_to);
 	if (WARN_ON(!common))
@@ -158,18 +159,19 @@ static int kernfs_path_from_node_locked(struct kernfs_node *kn_to,
 
 	buf[0] = '\0';
 
-	for (i = 0; i < depth_from; i++)
-		len += strlcpy(buf + len, parent_str,
-			       len < buflen ? buflen - len : 0);
+	for (i = 0; i < depth_from; i++) {
+		copied = strscpy(buf + len, parent_str, buflen - len);
+		if (copied < 0)
+			return copied;
+		len += copied;
+	}
 
 	/* Calculate how many bytes we need for the rest */
 	for (i = depth_to - 1; i >= 0; i--) {
 		for (kn = kn_to, j = 0; j < i; j++)
 			kn = kn->parent;
-		len += strlcpy(buf + len, "/",
-			       len < buflen ? buflen - len : 0);
-		len += strlcpy(buf + len, kn->name,
-			       len < buflen ? buflen - len : 0);
+
+		len += scnprintf(buf + len, buflen - len, "/%s", kn->name);
 	}
 
 	return len;
@@ -214,7 +216,7 @@ int kernfs_name(struct kernfs_node *kn, char *buf, size_t buflen)
  * path (which includes '..'s) as needed to reach from @from to @to is
  * returned.
  *
- * Return: the length of the full path.  If the full length is equal to or
+ * Return: the length of the constructed path.  If the path would have been
  * greater than @buflen, @buf contains the truncated path with the trailing
  * '\0'.  On error, -errno is returned.
  */
@@ -265,12 +267,10 @@ void pr_cont_kernfs_path(struct kernfs_node *kn)
 	sz = kernfs_path_from_node(kn, NULL, kernfs_pr_cont_buf,
 				   sizeof(kernfs_pr_cont_buf));
 	if (sz < 0) {
-		pr_cont("(error)");
-		goto out;
-	}
-
-	if (sz >= sizeof(kernfs_pr_cont_buf)) {
-		pr_cont("(name too long)");
+		if (sz == -E2BIG)
+			pr_cont("(name too long)");
+		else
+			pr_cont("(error)");
 		goto out;
 	}
 

kernel/cgroup/cgroup-v1.c

@@ -802,7 +802,7 @@ void cgroup1_release_agent(struct work_struct *work)
 		goto out_free;
 
 	ret = cgroup_path_ns(cgrp, pathbuf, PATH_MAX, &init_cgroup_ns);
-	if (ret < 0 || ret >= PATH_MAX)
+	if (ret < 0)
 		goto out_free;
 
 	argv[0] = agentbuf;

kernel/cgroup/cgroup.c

@@ -1893,7 +1893,7 @@ int cgroup_show_path(struct seq_file *sf, struct kernfs_node *kf_node,
 	len = kernfs_path_from_node(kf_node, ns_cgroup->kn, buf, PATH_MAX);
 	spin_unlock_irq(&css_set_lock);
 
-	if (len >= PATH_MAX)
+	if (len == -E2BIG)
 		len = -ERANGE;
 	else if (len > 0) {
 		seq_escape(sf, buf, " \t\n\\");
@@ -6278,7 +6278,7 @@ int proc_cgroup_show(struct seq_file *m, struct pid_namespace *ns,
 		if (cgroup_on_dfl(cgrp) || !(tsk->flags & PF_EXITING)) {
 			retval = cgroup_path_ns_locked(cgrp, buf, PATH_MAX,
 						current->nsproxy->cgroup_ns);
-			if (retval >= PATH_MAX)
+			if (retval == -E2BIG)
 				retval = -ENAMETOOLONG;
 			if (retval < 0)
 				goto out_unlock;

kernel/cgroup/cpuset.c

@@ -4941,7 +4941,7 @@ int proc_cpuset_show(struct seq_file *m, struct pid_namespace *ns,
 	retval = cgroup_path_ns(css->cgroup, buf, PATH_MAX,
 				current->nsproxy->cgroup_ns);
 	css_put(css);
-	if (retval >= PATH_MAX)
+	if (retval == -E2BIG)
 		retval = -ENAMETOOLONG;
 	if (retval < 0)
 		goto out_free;