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

[Daniel Gomez]

On 21/12/2025 22.59, Sakari Ailus wrote:
> 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.

That is correct. There are 40+ places detected and around 20+ more where
the error is returned but at some point ignored and not propagated back to
userspace. I have all the series ready for the first case, but I've only sent
the first 6.

> 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.

We are planning to merge that too. Link [2] refers to Lucas's "[PATCH 0/2]
module: Tweak return and warning" series, which replaces -EEXIST with -EBUSY at
runtime and warns. However, we do not consider that to be the actual fix. I am
starting to send these fixes out to avoid "spamming" unnecessarily the kernel
log in cases we can already detect.

> 
> I wonder what others think.

Please, find the rest of the series sent:
https://lore.kernel.org/all/20251220-dev-module-init-eexists-linux-scsi-v1-0-5379db749d54@xxxxxxxxxxx
https://lore.kernel.org/all/20251219-dev-module-init-eexists-netfilter-v1-1-efd3f62412dc@xxxxxxxxxxx
https://lore.kernel.org/all/20251220-dev-module-init-eexists-bpf-v1-1-7f186663dbe7@xxxxxxxxxxx
https://lore.kernel.org/all/20251220-dev-module-init-eexists-keyring-v1-1-a2f23248c300@xxxxxxxxxxx
https://lore.kernel.org/all/20251220-dev-module-init-eexists-dm-devel-v1-1-90ed00444ea0@xxxxxxxxxxx

FYI, these docs go on top of Lucas' changes with the hope this is clear in the
docs.

https://lore.kernel.org/all/20251218-dev-module-init-eexists-modules-docs-v1-0-361569aa782a@xxxxxxxxxxx

> 
>>
>> 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]