Re: [PATCH 0/2] scsi: target+fcoe: replace -EEXIST with -EBUSY in module_init() paths

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

On 20/12/2025 05.27, James Bottomley wrote:
> On Sat, 2025-12-20 at 04:37 +0100, Daniel Gomez wrote:
>> 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].
> 
> That reference doesn't sufficiently explain why this error code should
> be unique to modules.

It's unique only to the module initialization. You can find how it's used in the
kernel module code at module_patient_check_exists() in kernel/module/main.c [1].

In addition, init_module(2) man pages indicates this:

man 2 init_module | grep EEXIST
       EEXIST A module with this name is already loaded.

So, a module that is already loaded will be detected by the kernel module loader
and the EEXIST error will be returned. This will be detected by kmod as success
[2]. I think this functionality was added very early on in kmod by commit
5f35147 "libkmod-module: probe: add flag to stop loading on already loaded" [3].
Prior to that, module-init-tools had the same behavior [4]. Even in modutils
[5], we had back then in insmod/insmod.c:2088:

		case EEXIST:
			if (dolock) {
				/*
				 * Assume that we were just invoked
				 * simultaneous with another insmod
				 * and return success.
				 */
				exit_status = 0;
				goto out;
			}
			error("a module named %s already exists", m_name);
			goto out;

Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/module/main.c?h=v6.19-rc1#n3206 [1]
Link: https://github.com/kmod-project/kmod/blob/v34.2/libkmod/libkmod-module.c#L1088 [2]
Link: https://github.com/kmod-project/kmod/commit/5f3514731ef82084c1a24b15445e0f1352681a19 [3]
Link: https://git.kernel.org/pub/scm/utils/kernel/module-init-tools/module-init-tools.git/tree/modprobe.c#n1797 [4]
Link: https://cdn.kernel.org/pub/linux/utils/kernel/modutils/v2.4/modutils-2.4.27.tar.gz [5]

> EEXIST is used all over the kernel to indicate
> that something being attempted has already happened or does already
> exist and that seems perfectly logical .... please explain why you're

That is correct but not all are conflicts within the
module_init()/init_module(2) path. I have detected 40+ cases where this error
is returned and another 20+ where error is returned but in upper layers of
the module itself, not propagated back to userspace. So far, I've only sent just
a few + docs:

https://lore.kernel.org/all/20251220-dev-module-init-eexists-dm-devel-v1-1-90ed00444ea0@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-linux-acpi-v1-1-af59b1a0e217@samsung.com
https://lore.kernel.org/all/20251218-dev-module-init-eexists-modules-docs-v1-0-361569aa782a@samsung.com

> trying to push it back to being a single use case for modules alone.
> 
> Regards,
> 
> James
> 
> 
>