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

[Sakari Ailus]

Hi Herve,

Thanks for the patch.

On Tue, Nov 14, 2023 at 12:04:56PM +0100, Herve Codina wrote:
> 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);

How about, without changing the loop:

		/*
		 * Only get a reference for other nodes, fwnode refcount
		 * may be 0 here.
		 */
		struct fwnode_handle *__fwnode =
			depth ? fwnode_get_nth_parent(fwnode, depth) : fwnode;

>  
> @@ -2121,6 +2121,16 @@ char *fwnode_full_name_string(struct fwnode_handle *fwnode, char *buf,
>  		fwnode_handle_put(__fwnode);

And:

		if (__fwnode != fwnode)
			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);

It'd avoid duplicating this part, too, which I find worth the while.

> +
>  	return buf;
>  }
>  

-- 
Kind regards,

Sakari Ailus