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

[Daniel Gomez <da.gomez@kernel.org>]

On 22/12/2025 09.19, Greg Kroah-Hartman wrote:
> On Sat, Dec 20, 2025 at 04:55:00AM +0100, Daniel Gomez wrote:
>> From: Daniel Gomez <da.gomez@samsung.com>
>>
>> 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@samsung.com>
>> ---
>> 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.
>>
>> Link: https://lore.kernel.org/all/aKEVQhJpRdiZSliu@orbyte.nwl.cc/ [1]
>> Link: https://lore.kernel.org/all/20251013-module-warn-ret-v1-0-ab65b41af01f@intel.com/ [2]
>> Link: https://lore.kernel.org/all/20251218-dev-module-init-eexists-modules-docs-v1-0-361569aa782a@samsung.com/ [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;
> 
> While I understand the want for the module loader to be returning
> -EBUSY, that doesn't really make sense down here in this layer of the
> kernel.
> 
> So why doesn't the module loader turn -EEXIST return values into -EBUSY
> if it wishes to pass that value on to userspace?  Otherwise you are

Indeed, we are planning to do that as well with "[PATCH 0/2] module: Tweak
return and warning":

https://lore.kernel.org/all/20251013-module-warn-ret-v1-0-ab65b41af01f@intel.com/#t

However, we don't consider that as the right fix.

> going to be constantly playing "whack-a-mole" here and have really
> set things up so that NO api can ever return EEXIST as an error value in
> the future.

100%.

For that reason, on top of the series from Lucas, we are documenting this to
make it clear:

https://lore.kernel.org/linux-modules/20251218-dev-module-init-eexists-modules-docs-v1-0-361569aa782a@samsung.com/T/#m2ed6fbffb3f78b9bff53840f6492a198c389cb50

And sending patches where we see modules need fixing. I have already sent 6 out
of 20-ish series (that include a total of 40+ fixes):

https://lore.kernel.org/all/20251220-dev-module-init-eexists-linux-scsi-v1-0-5379db749d54@samsung.com
https://lore.kernel.org/all/20251219-dev-module-init-eexists-netfilter-v1-1-efd3f62412dc@samsung.com
https://lore.kernel.org/all/20251220-dev-module-init-eexists-bpf-v1-1-7f186663dbe7@samsung.com
https://lore.kernel.org/all/20251220-dev-module-init-eexists-keyring-v1-1-a2f23248c300@samsung.com
https://lore.kernel.org/all/20251220-dev-module-init-eexists-dm-devel-v1-1-90ed00444ea0@samsung.com