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

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

On 22/12/2025 12.56, Greg Kroah-Hartman wrote:
> On Mon, Dec 22, 2025 at 09:48:54AM +0100, Daniel Gomez wrote:
>> 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
> 
> Wait, no, that's not what I mean at all :)
> 
>> 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
> 
> Please no, let us keep using -EEXIST in the kernel source, and if your

This is not just random places in the kernel. It's only errors in the module
initialization path.

> usage is going to map this to userspace somehow, do the translation
> there, in the module code, as your original patch above said.
> > Otherwise, again, this is never going to work, let the subsystems use
> this error code how ever they feel they need to.

I considered module_init() (somehow) to be part of the module code, and
replacing the error at the module loading layer felt like a hack to me. My
concern is that a module_init() user expecting -EEXIST to propagate to userspace
won't get it as it will be silently replaced. But without a concrete use case
where that matters, I'll go with the consensus. Note that I'm hinting at we
should remove the warning from Lucas' original patch [1] before merging it.

Link: https://lore.kernel.org/all/20251013-module-warn-ret-v1-1-ab65b41af01f@intel.com/ [1]