Re: [PATCH bpf-next v3 1/5] selftests/bpf: libarena: Fix can-loop zero variable definition

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

On Sun Jul 5, 2026 at 2:34 AM EDT, Kumar Kartikeya Dwivedi wrote:
> On Thu Jul 2, 2026 at 12:50 AM CEST, Emil Tsalapatis wrote:
>> BPF can_loop based loops require the index variable to stay imprecise.
>> This means we must initialize them from a currently imprecise variable
>> instead of directly assigning 0 to them, like so:
>>
>> static volatile u32 zero = 0;
>>
>> for (i = zero; i < NUM_LOOPS; i++) {
>> 	/* loop body */
>> }
>>
>> The libarena implementation of this technique is currently faulty. For
>> the technique to work, the variable must not be in a map. This includes
>> the .rodata DATASEC map used for const variables. However, libarena
>> still defines the zero variable as constant.
>>
>> Modify the zero variable definition into a volatile variable. This
>> change adds a complication caused by the compiler optimizing array
>> derefences from
>>
>> for (i = zero; i < NUM_LOOPS; i++) {
>> 	val = *(ptr + i);
>> }
>>
>> into
>>
>> for (i = zero; i < NUM_LOOPS; i++) {
>> 	val = *ptr++;
>> }
>>
>> and causing verification failures. Use the barrier_var() clobber macro
>> to prevent this optimization from taking place. Using barrier_var() is
>> the only way to break the optimization, as annotating the index as
>> volatile does not suffice.
>>
>> After that, remove the bpf_for() invocations introduced in libarena for
>> parallel spmc testing.
>>
>> Reported-by: Eduard Zingerman <eddyz87@gmail.com>
>> Acked-by: Eduard Zingerman <eddyz87@gmail.com>
>> Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
>> ---
>>  .../selftests/bpf/libarena/include/libarena/common.h     | 2 +-
>>  .../bpf/libarena/selftests/test_asan_buddy.bpf.c         | 8 ++++++-
>
> No such file exists, did you forget to send the renaming patch from
> st_asan_buddy to test_asan_buddy ordered before this one?
>
> Please respin. Looks good otherwise.

Yes patch 1 is missing from v3. Will do a resend.

>
> pw-bot: cr