Re: [PATCH] software node: replace -EEXIST with -EBUSY

[Sakari Ailus]

Hi Daniel,

Thanks for the patch.

On Sat, Dec 20, 2025 at 04:55:00AM +0100, Daniel Gomez wrote:
> From: Daniel Gomez <da.gomez@xxxxxxxxxxx>
> 
> The -EEXIST error code is reserved by the module loading infrastructure
> to indicate that a module is already loaded. When a module's init
> function returns -EEXIST, userspace tools like kmod interpret this as
> "module already loaded" and treat the operation as successful, returning
> 0 to the user even though the module initialization actually failed.
> 
> This follows the precedent set by commit 54416fd76770 ("netfilter:
> conntrack: helper: Replace -EEXIST by -EBUSY") which fixed the same
> issue in nf_conntrack_helper_register().
> 
> Affected modules:
>   * meraki_mx100 pcengines_apuv2
> 
> Signed-off-by: Daniel Gomez <da.gomez@xxxxxxxxxxx>
> ---
> The error code -EEXIST is reserved by the kernel module loader to
> indicate that a module with the same name is already loaded. When a
> module's init function returns -EEXIST, kmod interprets this as "module
> already loaded" and reports success instead of failure [1].
> 
> The kernel module loader will include a safety net that provides -EEXIST
> to -EBUSY with a warning [2], and a documentation patch has been sent to
> prevent future occurrences [3].
> 
> These affected code paths were identified using a static analysis tool
> [4] that traces -EEXIST returns to module_init(). The tool was developed
> with AI assistance and all findings were manually validated.

This might not be the only case where -EEXIST may be returned by loading a
module. The patch is fine IMO but I'd just change -EEXIST to -EBUSY in e.g.
do_init_module() to avoid this being an actual bug elsewhere.

I wonder what others think.

> 
> Link: https://lore.kernel.org/all/aKEVQhJpRdiZSliu@xxxxxxxxxxxxx/ [1]
> Link: https://lore.kernel.org/all/20251013-module-warn-ret-v1-0-ab65b41af01f@xxxxxxxxx/ [2]
> Link: https://lore.kernel.org/all/20251218-dev-module-init-eexists-modules-docs-v1-0-361569aa782a@xxxxxxxxxxx/ [3]
> Link: https://gitlab.com/-/snippets/4913469 [4]
> ---
>  drivers/base/swnode.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/base/swnode.c b/drivers/base/swnode.c
> index 16a8301c25d6..083593d99a18 100644
> --- a/drivers/base/swnode.c
> +++ b/drivers/base/swnode.c
> @@ -919,7 +919,7 @@ int software_node_register(const struct software_node *node)
>  	struct swnode *parent = software_node_to_swnode(node->parent);
>  
>  	if (software_node_to_swnode(node))
> -		return -EEXIST;
> +		return -EBUSY;
>  
>  	if (node->parent && !parent)
>  		return -EINVAL;
> 

-- 
Regards,

Sakari Ailus