Re: [PATCH bpf-next v3 7/9] selftests/bpf: Add struct_ops __arena and __arena_nullable argument tests

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2026-08-03 at 14:51 +0200, Kumar Kartikeya Dwivedi wrote:
> From: Tejun Heo <tj@kernel.org>
> 
> Add test_arena and test_arena_nullable members to bpf_testmod_ops3 with
> arena-tagged stub arguments and kfuncs that forward a caller-provided
> pointer to them. The kfuncs take arena-tagged arguments, so each round
> trip exercises both conversion directions end to end: the kfunc receives
> a kernel arena address and the trampoline converts it back to an arena
> pointer for the callback.
> 
> The non-nullable callback dereferences its argument with no NULL branch
> and captures the raw ctx value, which the trigger program compares
> against the arena offset of the passed object, pinning the exact
> (u32)(kaddr - kern_vm_start) conversion. The nullable callback verifies
> that only a true kernel NULL arrives as NULL. Failure coverage: a
> program with no arena is rejected when it loads. The tests run on x86-64
> and skip elsewhere, as the programs fail verification where the JIT
> lacks arena argument support.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

...

> diff --git a/tools/testing/selftests/bpf/progs/arena_kfunc.c b/tools/testing/selftests/bpf/progs/arena_kfunc.c
> index e7250c5197ab..15d48151797d 100644
> --- a/tools/testing/selftests/bpf/progs/arena_kfunc.c
> +++ b/tools/testing/selftests/bpf/progs/arena_kfunc.c
> @@ -235,16 +235,26 @@ int arena_arg_bad_reg(void *ctx)
>  	return 0;
>  }
>  
> +#if defined(__BPF_FEATURE_ADDR_SPACE_CAST) && \
> +	defined(__BPF_FEATURE_STACK_ARGUMENT)
>  SEC("syscall")
>  __arch_x86_64
>  __failure __msg("arena pointer cannot be a stack argument")
>  int arena_arg_stack(void *ctx)
>  {
> -#if defined(__BPF_FEATURE_ADDR_SPACE_CAST)
>  	bpf_arena_alloc_pages(&arena, NULL, 1, NUMA_NO_NODE, 0);
>  	bpf_kfunc_arena_stack_arg_test(1, 2, 3, 4, 5, (u64 *)1);
> -#endif
>  	return 0;
>  }
> +#else
> +SEC("syscall")
> +__arch_x86_64
> +__description("arena_arg_stack: not supported, dummy test")
> +__success
> +int arena_arg_stack(void *ctx)
> +{
> +	return 0;
> +}
> +#endif

Nit: should this be a part of the first selftests patch?

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