Re: [PATCH bpf-next v2 2/2] selftests/bpf: Test global subprog callback contexts

[Eduard Zingerman <eddyz87@gmail.com>]

On Sat, 2026-09-05 at 07:12 +0200, Kumar Kartikeya Dwivedi wrote:
> Exercise global subprogram verification from workqueue and task-work
> callbacks. Both callback types can run in a sleepable context even when the
> containing program is not sleepable, so an unprotected callback must not let
> the global subprogram use implicit RCU protection inherited from the program.
> 
> Add negative cases which load an RCU-protected task kptr in a global
> subprogram reached from each callback type. The tests fail on an unfixed
> kernel because the programs are incorrectly accepted.
> 
> Also cover a workqueue callback protected by an explicit RCU read-side
> critical section. Finally, call the same harmless global subprogram directly
> from the main program and from an unprotected callback. This requires both
> non-sleepable and sleepable verification roots and proves that global calls
> from callbacks are not rejected wholesale.
> 
> Keep the harmless global opaque to LLVM with barrier() so both call sites

__weak is another useful trick for such things.

> remain in the generated BPF object and the positive case cannot pass
> vacuously. Have workqueue callbacks return zero explicitly after calling a
> global subprogram, as required by their callback contract, instead of relying
> on interprocedural return-value optimization.
> 
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---

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

...

> @@ -179,3 +285,29 @@ int task_work_sleepable_prog(void *ctx)
>  	bpf_task_work_schedule_resume(task, &val->tw, &task_work_map, task_work_cb);
>  	return 0;
>  }
> +
> +static int task_work_global_rcu_cb(struct bpf_map *map, void *key, void *value)
> +{
> +	return wq_global_acquire();
> +}
> +
> +SEC("fentry/bpf_fentry_test1")
> +__failure __msg("R1 must be a rcu pointer")
> +int task_work_global_rcu_prog(void *ctx)
> +{
> +	struct task_work_elem *val;
> +	struct task_struct *task;
> +	int key = 0;
> +
> +	val = bpf_map_lookup_elem(&task_work_map, &key);
> +	if (!val)
> +		return 0;
> +
> +	task = bpf_get_current_task_btf();
> +	if (!task)
> +		return 0;
> +
> +	bpf_task_work_schedule_resume(task, &val->tw, &task_work_map,
> +				      task_work_global_rcu_cb);
> +	return 0;
> +}

I think this test is redundant.