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

["Kumar Kartikeya Dwivedi" <memxor@gmail.com>]

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.

pw-bot: cr