Re: [PATCH bpf-next v2 2/2] selftests/bpf: Test RCU iterator state pruning

> selftests/bpf: Test RCU iterator state pruning
>
> Add a path where RCU protection reaches zero and then starts again.
> The iterator is untrusted after this gap and must be rejected.
>
> Signed-off-by: Ning Ding <[email protected]>

This isn't a bug, but could the changelog mention why BPF_F_TEST_STATE_FREQ
is needed and that both arms deliberately converge with the same RCU lock
depth, so a future reader knows what keeps this test meaningful?

> diff --git a/tools/testing/selftests/bpf/progs/iters_task_failure.c b/tools/testing/selftests/bpf/progs/iters_task_failure.c
> index fe3663dedbe1..566a1d3dffea 100644
> --- a/tools/testing/selftests/bpf/progs/iters_task_failure.c
> +++ b/tools/testing/selftests/bpf/progs/iters_task_failure.c
> @@ -61,6 +61,52 @@ int BPF_PROG(iter_tasks_lock_and_unlock)
>  	return 0;
>  }
>
> +SEC("?fentry.s/" SYS_PREFIX "sys_getpgid")
> +__failure __msg("expected an RCU CS when using bpf_iter_task_next")
> +__flag(BPF_F_TEST_STATE_FREQ)
> +int BPF_PROG(iter_tasks_rcu_state_pruning)
> +{
> +	struct bpf_iter_task it;
> +
> +	asm volatile (
> +		"call %[bpf_rcu_read_lock];"
> +		"r1 = %[it];"
> +		"r2 = 0;"
> +		"r3 = 0;" /* BPF_TASK_ITER_ALL_PROCS */
> +		"call %[bpf_iter_task_new];"
> +
> +		"call %[bpf_get_prandom_u32];"
> +		"if w0 == 0 goto unprotected_%=;"
> +
> +		/* Keep the outer RCU lock active on the straight-line path. */
> +		"call %[bpf_rcu_read_lock];"
> +		"call %[bpf_rcu_read_unlock];"

This isn't a bug, but would it help to say what the nested lock/unlock
pair is for here - e.g. that it scratches the caller-saved registers so
the only state difference at merge_ is the iterator's MEM_RCU vs
PTR_UNTRUSTED type?

> +		"goto merge_%=;"
> +
> +	"unprotected_%=:"
> +		/* Create an unprotected gap on the taken path. */
> +		"call %[bpf_rcu_read_unlock];"
> +		"call %[bpf_rcu_read_lock];"
> +
> +	"merge_%=: r1 = %[it];"
> +		"call %[bpf_iter_task_next];"
> +		"r1 = %[it];"
> +		"call %[bpf_iter_task_destroy];"
> +		"call %[bpf_rcu_read_unlock];"
> +		:
> +		: __imm_ptr(it),
> +		  __imm(bpf_get_prandom_u32),
> +		  __imm(bpf_iter_task_new),
> +		  __imm(bpf_iter_task_next),
> +		  __imm(bpf_iter_task_destroy),
> +		  __imm(bpf_rcu_read_lock),
> +		  __imm(bpf_rcu_read_unlock)
> +		: __clobber_common
> +	);
> +
> +	return 0;
> +}
> +


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/31457825387