Re: [PATCH v3] lkdtm: Add lockdep related crash tests

[Yunseong Kim <ysk@kzalloc.com>]

Hi Tzung-Bi,

Thanks for the great work!

I'm very interested in this work as it will be helpful for verifying
whether kernel crash dumps are correctly extracted when a deadlock is
detected by lockdep. I really hope to see this merged.

I have a couple of questions regarding the organization of these tests:

Is the intent of this new lockdep.c specifically to validate the lockdep
infrastructure itself rather than providing general deadlock.c code It
feels like "deadlock" might be a broader category. For example, DEPT
(Dependency Tracker) can track wait/event dependencies for PG_locked/writeback.
I'm planning a follow-up contribution to cover these cases that DEPT can detect.

Do you think it would be better to implement deadlock detection for LKDTM
(like the ones DEPT handles) in a separate deadlock.c instead of
lockdep.c? I'd love to hear your thoughts on the scope.

Best regards,
Yunseong

On 12/15/25 4:25 PM, Tzung-Bi Shih wrote:
> Introduce various lockdep related crash tests.
> 
> Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
> ---
> v3:
> - Add some entries in tools/testing/selftests/lkdtm/tests.txt.
> 
> v2: https://lore.kernel.org/all/20251117033337.3730681-1-tzungbi@kernel.org/
> - Fix "warning: suggest braces around empty body in an 'else' statement [-Wempty-body]"
>   reported by 0day test robot.
> 
> v1: https://lore.kernel.org/lkml/20251114062535.1827309-1-tzungbi@kernel.org/T/#u
> 
>  drivers/misc/lkdtm/Makefile             |  1 +
>  drivers/misc/lkdtm/core.c               |  1 +
>  drivers/misc/lkdtm/lkdtm.h              |  1 +
>  drivers/misc/lkdtm/lockdep.c            | 98 +++++++++++++++++++++++++
>  tools/testing/selftests/lkdtm/tests.txt |  8 ++
>  5 files changed, 109 insertions(+)
>  create mode 100644 drivers/misc/lkdtm/lockdep.c
> 
> diff --git a/drivers/misc/lkdtm/Makefile b/drivers/misc/lkdtm/Makefile
> index 03ebe33185f9..830b71c8e6a0 100644
> --- a/drivers/misc/lkdtm/Makefile
> +++ b/drivers/misc/lkdtm/Makefile
> @@ -11,6 +11,7 @@ lkdtm-$(CONFIG_LKDTM)		+= usercopy.o
>  lkdtm-$(CONFIG_LKDTM)		+= kstack_erase.o
>  lkdtm-$(CONFIG_LKDTM)		+= cfi.o
>  lkdtm-$(CONFIG_LKDTM)		+= fortify.o
> +lkdtm-$(CONFIG_LKDTM)		+= lockdep.o
>  lkdtm-$(CONFIG_PPC_64S_HASH_MMU)	+= powerpc.o
>  
>  KASAN_SANITIZE_stackleak.o	:= n
> diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
> index 5732fd59a227..43e91388940f 100644
> --- a/drivers/misc/lkdtm/core.c
> +++ b/drivers/misc/lkdtm/core.c
> @@ -96,6 +96,7 @@ static const struct crashtype_category *crashtype_categories[] = {
>  	&stackleak_crashtypes,
>  	&cfi_crashtypes,
>  	&fortify_crashtypes,
> +	&lockdep_crashtypes,
>  #ifdef CONFIG_PPC_64S_HASH_MMU
>  	&powerpc_crashtypes,
>  #endif
> diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
> index 015e0484026b..d2d97e6f323e 100644
> --- a/drivers/misc/lkdtm/lkdtm.h
> +++ b/drivers/misc/lkdtm/lkdtm.h
> @@ -84,6 +84,7 @@ extern struct crashtype_category usercopy_crashtypes;
>  extern struct crashtype_category stackleak_crashtypes;
>  extern struct crashtype_category cfi_crashtypes;
>  extern struct crashtype_category fortify_crashtypes;
> +extern struct crashtype_category lockdep_crashtypes;
>  extern struct crashtype_category powerpc_crashtypes;
>  
>  /* Each category's init/exit routines. */
> diff --git a/drivers/misc/lkdtm/lockdep.c b/drivers/misc/lkdtm/lockdep.c
> new file mode 100644
> index 000000000000..e029e9e60ce6
> --- /dev/null
> +++ b/drivers/misc/lkdtm/lockdep.c
> @@ -0,0 +1,98 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright 2025 Google LLC
> + *
> + * Tests related to lockdep warnings.
> + */
> +#include "lkdtm.h"
> +#include <linux/cleanup.h>
> +#include <linux/irqflags.h>
> +#include <linux/mutex.h>
> +#include <linux/spinlock.h>
> +#include <linux/srcu.h>
> +
> +static DEFINE_SPINLOCK(lock_A);
> +static DEFINE_SPINLOCK(lock_B);
> +
> +/* For "WARNING: possible circular locking dependency detected". */
> +static void lkdtm_LOCKDEP_CIRCULAR_LOCK(void)
> +{
> +	scoped_guard(spinlock, &lock_A)
> +		scoped_guard(spinlock, &lock_B) {}
> +	scoped_guard(spinlock, &lock_B)
> +		scoped_guard(spinlock, &lock_A) {}
> +}
> +
> +/* For "WARNING: possible recursive locking detected". */
> +static void lkdtm_LOCKDEP_RECURSIVE_LOCK(void)
> +{
> +	guard(spinlock)(&lock_A);
> +	guard(spinlock)(&lock_A);
> +}
> +
> +/* For "WARNING: inconsistent lock state". */
> +static void lkdtm_LOCKDEP_INCONSISTENT_LOCK(void)
> +{
> +	lockdep_softirq_enter();
> +	scoped_guard(spinlock, &lock_A) {}
> +	lockdep_softirq_exit();
> +
> +	scoped_guard(spinlock, &lock_A) {}
> +}
> +
> +/* For "WARNING: Nested lock was not taken". */
> +static void lkdtm_LOCKDEP_NESTED_LOCK_NOT_HELD(void)
> +{
> +	spin_lock_nest_lock(&lock_B, &lock_A);
> +}
> +
> +/* For "WARNING: bad unlock balance detected!". */
> +static void lkdtm_LOCKDEP_BAD_UNLOCK_BALANCE(void)
> +{
> +	spin_unlock(&lock_A);
> +}
> +
> +/* For "WARNING: held lock freed!". */
> +static void lkdtm_LOCKDEP_HELD_LOCK_FREED(void)
> +{
> +	spin_lock(&lock_A);
> +	spin_lock_init(&lock_A);
> +}
> +
> +/* For "WARNING: lock held when returning to user space!". */
> +static void lkdtm_LOCKDEP_HELD_LOCK(void)
> +{
> +	spin_lock(&lock_A);
> +}
> +
> +/* For "WARNING: suspicious RCU usage". */
> +static void lkdtm_LOCKDEP_SUSPICIOUS_RCU(void)
> +{
> +	struct srcu_struct srcu;
> +	void __rcu *res = NULL;
> +	int idx;
> +
> +	init_srcu_struct(&srcu);
> +
> +	idx = srcu_read_lock(&srcu);
> +	rcu_dereference(res);
> +	srcu_read_unlock(&srcu, idx);
> +
> +	cleanup_srcu_struct(&srcu);
> +}
> +
> +static struct crashtype crashtypes[] = {
> +	CRASHTYPE(LOCKDEP_CIRCULAR_LOCK),
> +	CRASHTYPE(LOCKDEP_RECURSIVE_LOCK),
> +	CRASHTYPE(LOCKDEP_INCONSISTENT_LOCK),
> +	CRASHTYPE(LOCKDEP_NESTED_LOCK_NOT_HELD),
> +	CRASHTYPE(LOCKDEP_BAD_UNLOCK_BALANCE),
> +	CRASHTYPE(LOCKDEP_HELD_LOCK_FREED),
> +	CRASHTYPE(LOCKDEP_HELD_LOCK),
> +	CRASHTYPE(LOCKDEP_SUSPICIOUS_RCU),
> +};
> +
> +struct crashtype_category lockdep_crashtypes = {
> +	.crashtypes = crashtypes,
> +	.len	    = ARRAY_SIZE(crashtypes),
> +};
> diff --git a/tools/testing/selftests/lkdtm/tests.txt b/tools/testing/selftests/lkdtm/tests.txt
> index cff124c1eddd..7b32dece3d3a 100644
> --- a/tools/testing/selftests/lkdtm/tests.txt
> +++ b/tools/testing/selftests/lkdtm/tests.txt
> @@ -83,3 +83,11 @@ FORTIFY_STR_MEMBER detected buffer overflow
>  FORTIFY_MEM_OBJECT detected buffer overflow
>  FORTIFY_MEM_MEMBER detected field-spanning write
>  PPC_SLB_MULTIHIT Recovered
> +#LOCKDEP_CIRCULAR_LOCK Lockdep can only trigger once
> +#LOCKDEP_RECURSIVE_LOCK Hangs the system
> +#LOCKDEP_INCONSISTENT_LOCK Lockdep can only trigger once
> +#LOCKDEP_NESTED_LOCK_NOT_HELD Cause a deadlock in subsequent lockdep tests
> +#LOCKDEP_BAD_UNLOCK_BALANCE Lockdep can only trigger once
> +#LOCKDEP_HELD_LOCK_FREED Lockdep can only trigger once
> +#LOCKDEP_HELD_LOCK Cause a deadlock in subsequent lockdep tests
> +#LOCKDEP_SUSPICIOUS_RCU Lockdep can only trigger once