linux-kernel - [PATCH] dm: replace -EEXIST with -EBUSY

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Message-Id: <20251220-dev-module-init-eexists-dm-devel-v1-1-90ed00444ea0@samsung.com>
Date: Sat, 20 Dec 2025 04:49:37 +0100
From: Daniel Gomez <da.gomez@...nel.org>
To: Alasdair Kergon <agk@...hat.com>, Mike Snitzer <snitzer@...nel.org>, 
 Mikulas Patocka <mpatocka@...hat.com>, 
 Benjamin Marzinski <bmarzins@...hat.com>
Cc: Luis Chamberlain <mcgrof@...nel.org>, Petr Pavlu <petr.pavlu@...e.com>, 
 Daniel Gomez <da.gomez@...nel.org>, Sami Tolvanen <samitolvanen@...gle.com>, 
 Aaron Tomlin <atomlin@...mlin.com>, Lucas De Marchi <demarchi@...nel.org>, 
 dm-devel@...ts.linux.dev, linux-modules@...r.kernel.org, 
 linux-kernel@...r.kernel.org, Daniel Gomez <da.gomez@...sung.com>
Subject: [PATCH] dm: replace -EEXIST with -EBUSY

From: Daniel Gomez <da.gomez@...sung.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:
  * dm_cache dm_clone dm_integrity dm_mirror dm_multipath dm_pcache
  * dm_vdo dm-ps-round-robin dm_historical_service_time dm_io_affinity
  * dm_queue_length dm_service_time dm_snapshot

Signed-off-by: Daniel Gomez <da.gomez@...sung.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/md/dm-exception-store.c | 2 +-
 drivers/md/dm-log.c             | 2 +-
 drivers/md/dm-path-selector.c   | 2 +-
 drivers/md/dm-target.c          | 2 +-
 4 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/md/dm-exception-store.c b/drivers/md/dm-exception-store.c
index c3799757bf4a..88f119a0a2ae 100644
--- a/drivers/md/dm-exception-store.c
+++ b/drivers/md/dm-exception-store.c
@@ -116,7 +116,7 @@ int dm_exception_store_type_register(struct dm_exception_store_type *type)
 	if (!__find_exception_store_type(type->name))
 		list_add(&type->list, &_exception_store_types);
 	else
-		r = -EEXIST;
+		r = -EBUSY;
 	spin_unlock(&_lock);
 
 	return r;
diff --git a/drivers/md/dm-log.c b/drivers/md/dm-log.c
index 9d85d045f9d9..bced5a783ee3 100644
--- a/drivers/md/dm-log.c
+++ b/drivers/md/dm-log.c
@@ -121,7 +121,7 @@ int dm_dirty_log_type_register(struct dm_dirty_log_type *type)
 	if (!__find_dirty_log_type(type->name))
 		list_add(&type->list, &_log_types);
 	else
-		r = -EEXIST;
+		r = -EBUSY;
 	spin_unlock(&_lock);
 
 	return r;
diff --git a/drivers/md/dm-path-selector.c b/drivers/md/dm-path-selector.c
index d0b883fabfeb..2b0ac200f1c0 100644
--- a/drivers/md/dm-path-selector.c
+++ b/drivers/md/dm-path-selector.c
@@ -107,7 +107,7 @@ int dm_register_path_selector(struct path_selector_type *pst)
 
 	if (__find_path_selector_type(pst->name)) {
 		kfree(psi);
-		r = -EEXIST;
+		r = -EBUSY;
 	} else
 		list_add(&psi->list, &_path_selectors);
 
diff --git a/drivers/md/dm-target.c b/drivers/md/dm-target.c
index 8fede41adec0..1fd41289de36 100644
--- a/drivers/md/dm-target.c
+++ b/drivers/md/dm-target.c
@@ -88,7 +88,7 @@ int dm_register_target(struct target_type *tt)
 	if (__find_target_type(tt->name)) {
 		DMERR("%s: '%s' target already registered",
 		      __func__, tt->name);
-		rv = -EEXIST;
+		rv = -EBUSY;
 	} else {
 		list_add(&tt->list, &_targets);
 	}

---
base-commit: 8f0b4cce4481fb22653697cced8d0d04027cb1e8
change-id: 20251218-dev-module-init-eexists-dm-devel-2d656f9f2365

Best regards,
--  
Daniel Gomez <da.gomez@...sung.com>