Re: [PATCH] tools/sched_ext: Improve BPF verifier arena detection workaround

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Fri Feb 6, 2026 at 2:03 AM EST, Andrea Righi wrote:
> Hi,
>
> On Fri, Feb 06, 2026 at 12:18:08PM +0800, zhidao su wrote:
>> Replace bpf_printk() with volatile access in scx_sdt scheduler's
>> BPF verifier workaround to eliminate console output while maintaining
>> the required LD.IMM instruction generation for arena detection.
>> 
>> This addresses the side effect issue of the previous hack while
>> preserving the essential functionality needed by the BPF verifier.
>> 
>> Signed-off-by: zhidao su <suzhidao@xiaomi.com>
>
> Adding Emil in cc.
>

This still doesn't pass verification for me. @suzhidao is this working for you?
If so please let me know your setup so I can replicate this. I am
running this on for-6.20.

>> ---
>>  tools/sched_ext/scx_sdt.bpf.c | 18 +++++++++---------
>>  1 file changed, 9 insertions(+), 9 deletions(-)
>> 
>> diff --git a/tools/sched_ext/scx_sdt.bpf.c b/tools/sched_ext/scx_sdt.bpf.c
>> index 31b09958e8d5..13d3060c99ff 100644
>> --- a/tools/sched_ext/scx_sdt.bpf.c
>> +++ b/tools/sched_ext/scx_sdt.bpf.c
>> @@ -64,14 +64,10 @@ DEFINE_SDT_STAT(select_busy_cpu);
>>  static __u64 zero = 0;
>>  
>>  /*
>> - * XXX Hack to get the verifier to find the arena for sdt_exit_task.
>> - * As of 6.12-rc5, The verifier associates arenas with programs by
>> - * checking LD.IMM instruction operands for an arena and populating
>> - * the program state with the first instance it finds. This requires
>> - * accessing our global arena variable, but scx methods do not necessarily
>> - * do so while still using pointers from that arena. Insert a bpf_printk
>> - * statement that triggers at most once to generate an LD.IMM instruction
>> - * to access the arena and help the verifier.
>> + * Workaround to help BPF verifier track arena usage.
>> + * The verifier needs to see an explicit reference to the arena variable
>> + * to properly track arena memory usage. This generates the required
>> + * LD.IMM instruction without producing unnecessary output.
>>   */
>>  static volatile bool scx_arena_verify_once;
>>  
>> @@ -80,7 +76,11 @@ __hidden void scx_arena_subprog_init(void)
>>  	if (scx_arena_verify_once)
>>  		return;
>>  
>> -	bpf_printk("%s: arena pointer %p", __func__, &arena);
>> +	/*
>> +	 * Use volatile access to generate LD.IMM instruction without
>> +	 * producing console output like bpf_printk does.
>> +	 */
>> +	(void)*(volatile void **)&arena;
>
> Makes sense to me.
>
> If we want to be extra picky we can do something like this:
>
>   volatile void *arena_ref = &arena;
>   (void)arena_ref;
>
> In this way we take the address of the arena map descriptor and store it
> in a volatile void * variable, so the compiler can't optimize away and we
> never read the content of the map descriptor, we only use its address. So,
> we're not reinterpreting the bytes of the struct as another type (no type
> punning, no strict-aliasing violation). Even if it's probably just a
> theoretical thing.
>
>>  	scx_arena_verify_once = true;
>>  }
>>  
>> -- 
>> 2.43.0
>> 
>
> Thanks,
> -Andrea