Re: [PATCH v3 08/12] kasan/um: select ARCH_DEFER_KASAN and call kasan_init_generic

[Andrey Ryabinin <ryabinin.a.a@gmail.com>]

On 7/22/25 4:17 PM, Sabyrzhan Tasbolatov wrote:
> On Tue, Jul 22, 2025 at 4:00 AM Andrey Ryabinin <ryabinin.a.a@gmail.com> wrote:
>>
>>
>>
>> On 7/17/25 4:27 PM, Sabyrzhan Tasbolatov wrote:
>>> UserMode Linux needs deferred KASAN initialization as it has a custom
>>> kasan_arch_is_ready() implementation that tracks shadow memory readiness
>>> via the kasan_um_is_ready flag.
>>>
>>> Select ARCH_DEFER_KASAN to enable the unified static key mechanism
>>> for runtime KASAN control. Call kasan_init_generic() which handles
>>> Generic KASAN initialization and enables the static key.
>>>
>>> Delete the key kasan_um_is_ready in favor of the unified kasan_enabled()
>>> interface.
>>>
>>> Note that kasan_init_generic has __init macro, which is called by
>>> kasan_init() which is not marked with __init in arch/um code.
>>>
>>> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217049
>>> Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@gmail.com>
>>> ---
>>> Changes in v3:
>>> - Added CONFIG_ARCH_DEFER_KASAN selection for proper runtime control
>>> ---
>>>  arch/um/Kconfig             | 1 +
>>>  arch/um/include/asm/kasan.h | 5 -----
>>>  arch/um/kernel/mem.c        | 4 ++--
>>>  3 files changed, 3 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/arch/um/Kconfig b/arch/um/Kconfig
>>> index f08e8a7fac9..fd6d78bba52 100644
>>> --- a/arch/um/Kconfig
>>> +++ b/arch/um/Kconfig
>>> @@ -8,6 +8,7 @@ config UML
>>>       select ARCH_WANTS_DYNAMIC_TASK_STRUCT
>>>       select ARCH_HAS_CPU_FINALIZE_INIT
>>>       select ARCH_HAS_FORTIFY_SOURCE
>>> +     select ARCH_DEFER_KASAN
>>>       select ARCH_HAS_GCOV_PROFILE_ALL
>>>       select ARCH_HAS_KCOV
>>>       select ARCH_HAS_STRNCPY_FROM_USER
>>> diff --git a/arch/um/include/asm/kasan.h b/arch/um/include/asm/kasan.h
>>> index f97bb1f7b85..81bcdc0f962 100644
>>> --- a/arch/um/include/asm/kasan.h
>>> +++ b/arch/um/include/asm/kasan.h
>>> @@ -24,11 +24,6 @@
>>>
>>>  #ifdef CONFIG_KASAN
>>>  void kasan_init(void);
>>> -extern int kasan_um_is_ready;
>>> -
>>> -#ifdef CONFIG_STATIC_LINK
>>> -#define kasan_arch_is_ready() (kasan_um_is_ready)
>>> -#endif
>>>  #else
>>>  static inline void kasan_init(void) { }
>>>  #endif /* CONFIG_KASAN */
>>> diff --git a/arch/um/kernel/mem.c b/arch/um/kernel/mem.c
>>> index 76bec7de81b..058cb70e330 100644
>>> --- a/arch/um/kernel/mem.c
>>> +++ b/arch/um/kernel/mem.c
>>> @@ -21,9 +21,9 @@
>>>  #include <os.h>
>>>  #include <um_malloc.h>
>>>  #include <linux/sched/task.h>
>>> +#include <linux/kasan.h>
>>>
>>>  #ifdef CONFIG_KASAN
>>> -int kasan_um_is_ready;
>>>  void kasan_init(void)
>>>  {
>>>       /*
>>> @@ -32,7 +32,7 @@ void kasan_init(void)
>>>        */
>>>       kasan_map_memory((void *)KASAN_SHADOW_START, KASAN_SHADOW_SIZE);
>>>       init_task.kasan_depth = 0;
>>> -     kasan_um_is_ready = true;
>>> +     kasan_init_generic();
>>
>> I think this runs before jump_label_init(), and static keys shouldn't be switched before that.>  }
> 
> I got the warning in my local compilation and from kernel CI [1].
> 
> arch/um places kasan_init() in own `.kasan_init` section, while
> kasan_init_generic() is called from __init.

No, kasan_init() is in text section as the warning says. It's kasan_init_ptr in .kasan_init.
Adding __init to kasan_init() should fix the warning.


> Could you suggest a way how I can verify the functions call order?
> 

By code inspection? or run uder gdb.

kasan_init() is initialization routine called before main().
jump_label_init() called from start_kernel()<-start_kernel_proc()<-... main()

> I need to familiarize myself with how to run arch/um locally 

It's as simple as:
ARCH=um  make 
./linux rootfstype=hostfs ro init=/bin/bash