[PATCH 1/1] lib/vsprintf: Fix %pfwf when current node refcount == 0

[Herve Codina]

A refcount issue can appeared in __fwnode_link_del() due to the
pr_debug() call:
  WARNING: CPU: 0 PID: 901 at lib/refcount.c:25 refcount_warn_saturate+0xe5/0x110
  Call Trace:
  <TASK>
  ? refcount_warn_saturate+0xe5/0x110
  ? __warn+0x81/0x130
  ? refcount_warn_saturate+0xe5/0x110
  ? report_bug+0x191/0x1c0
  ? srso_alias_return_thunk+0x5/0x7f
  ? prb_read_valid+0x1b/0x30
  ? handle_bug+0x3c/0x80
  ? exc_invalid_op+0x17/0x70
  ? asm_exc_invalid_op+0x1a/0x20
  ? refcount_warn_saturate+0xe5/0x110
  kobject_get+0x68/0x70
  of_node_get+0x1e/0x30
  of_fwnode_get+0x28/0x40
  fwnode_full_name_string+0x34/0x90
  fwnode_string+0xdb/0x140
  vsnprintf+0x17b/0x630
  va_format.isra.0+0x71/0x130
  vsnprintf+0x17b/0x630
  vprintk_store+0x162/0x4d0
  ? srso_alias_return_thunk+0x5/0x7f
  ? srso_alias_return_thunk+0x5/0x7f
  ? srso_alias_return_thunk+0x5/0x7f
  ? try_to_wake_up+0x9c/0x620
  ? rwsem_mark_wake+0x1b2/0x310
  vprintk_emit+0xe4/0x2b0
  _printk+0x5c/0x80
  __dynamic_pr_debug+0x131/0x160
  ? srso_alias_return_thunk+0x5/0x7f
  __fwnode_link_del+0x25/0xa0
  fwnode_links_purge+0x39/0xb0
  of_node_release+0xd9/0x180
  kobject_put+0x7b/0x190
  ...

Indeed, an fwnode (of_node) is being destroyed and so, of_node_release()
is called because the of_node refcount reached 0.
>From of_node_release() several function calls are done and lead to
a pr_debug() calls with %pfwf to print the fwnode full name.
The issue is not present if we change %pfwf to %pfwP.

To print the full name, %pfwf iterates over the current node and its
parents and obtain/drop a reference to all nodes involved.

In order to allow to print the full name (%pfwf) of a node while it is
being destroyed, do not obtain/drop a reference to this current node.

Fixes: a92eb7621b9f ("lib/vsprintf: Make use of fwnode API to obtain node names and separators")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Herve Codina <herve.codina@xxxxxxxxxxx>
---
 lib/vsprintf.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index afb88b24fa74..74ef229c2783 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -2108,8 +2108,8 @@ char *fwnode_full_name_string(struct fwnode_handle *fwnode, char *buf,
 {
 	int depth;
 
-	/* Loop starting from the root node to the current node. */
-	for (depth = fwnode_count_parents(fwnode); depth >= 0; depth--) {
+	/* Loop starting from the root node to the parent of current node. */
+	for (depth = fwnode_count_parents(fwnode); depth > 0; depth--) {
 		struct fwnode_handle *__fwnode =
 			fwnode_get_nth_parent(fwnode, depth);
 
@@ -2121,6 +2121,16 @@ char *fwnode_full_name_string(struct fwnode_handle *fwnode, char *buf,
 		fwnode_handle_put(__fwnode);
 	}
 
+	/* Handle current node without calling fwnode_handle_{get,put}().
+	 * This allows to print the full node name while the current node is
+	 * being destroyed (ie print from a function called because of
+	 * refcount == 0) without any refcount issues.
+	 */
+	buf = string(buf, end, fwnode_get_name_prefix(fwnode),
+		     default_str_spec);
+	buf = string(buf, end, fwnode_get_name(fwnode),
+		     default_str_spec);
+
 	return buf;
 }
 
-- 
2.41.0