[PATCH] mm: kfence: export kfence_enabled as global variables

[Zhenhua Huang]

Thanks Marco!

On 2023/2/7 15:19, Marco Elver wrote:
> On Tue, 7 Feb 2023 at 07:15, Zhenhua Huang <quic_zhenhuah at quicinc.com> wrote:
>>
>> Export the variable to ease the judgement of whether kfence enabled
>> at runtime. It should be more precise than through kernel config
>> "CONFIG_KFENCE".
>>
>> For example We can disable kfence at runtime using bootargs
>> "kfence.sample_interval=0" but CONFIG_KFENCE enabled.
>> It was false positive.
>>
>> Signed-off-by: Zhenhua Huang <quic_zhenhuah at quicinc.com>
>> ---
>>   arch/arm64/mm/pageattr.c | 3 ++-
>>   include/linux/kfence.h   | 2 ++
>>   mm/kfence/core.c         | 2 +-
>>   3 files changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
>> index 79dd201..208d780 100644
>> --- a/arch/arm64/mm/pageattr.c
>> +++ b/arch/arm64/mm/pageattr.c
>> @@ -7,6 +7,7 @@
>>   #include <linux/module.h>
>>   #include <linux/sched.h>
>>   #include <linux/vmalloc.h>
>> +#include <linux/kfence.h>
>>
>>   #include <asm/cacheflush.h>
>>   #include <asm/set_memory.h>
>> @@ -27,7 +28,7 @@ bool can_set_direct_map(void)
>>           * protect/unprotect single pages.
>>           */
>>          return (rodata_enabled && rodata_full) || debug_pagealloc_enabled() ||
>> -               IS_ENABLED(CONFIG_KFENCE);
>> +               kfence_enabled;
> 
> Unfortunately this won't work, because it's possible to enable KFENCE
> after the kernel has booted with e.g.: echo 100 >
> /sys/module/kfence/parameters/sample_interval
Yeah, got it. Thanks for catching it.
> 
> What is the problem you have encountered? Is the page-granular direct
> map causing issues?
We're working on a low memory target, page-granular mapping costed more 
(2M per 1GB) memory. Due to GKI constraints, it is not easy to disable 
CONFIG_KFENCE. So my intention was to move the judgement to runtime 
configurable w/ CONFIG_KFENCE on...

Do you have any further suggestion/proposal on this? Many Thanks!

Thanks,
Zhenhua