Re: [PATCH v2 1/2] bpf: reject stack-argument callback subprograms

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index e421ea2b80c3..6f6e35bc7c97 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9785,6 +9785,8 @@ static int push_callback_call(struct bpf_verifier_env *env, struct bpf_insn *ins
>  	err = btf_check_subprog_call(env, subprog, caller->regs);
>  	if (err == -EFAULT)
>  		return err;
> +	if (bpf_in_stack_arg_cnt(&env->subprog_info[subprog]))
> +		return -EINVAL;

Should this rejection include a verbose() message explaining why the
callback was rejected? Currently this returns a bare -EINVAL, which means
bpf_loop(), bpf_for_each_map_elem(), bpf_timer_set_callback(), bpf_find_vma(),
and bpf_user_ringbuf_drain() with a callback that has more than five
arguments will fail with EINVAL but nothing in the verifier log shows why.

check_helper_call() propagates the error silently for all five of those
helpers. Only the three kfunc callback sites (in check_kfunc_call()) add
"kfunc %s#%d failed callback verification".

Every other stack-argument rejection in this feature logs a message before
returning: check_stack_arg_read() reports "invalid read from stack arg off
%d depth %d", check_stack_arg_write() reports "stack arg write offset %d
exceeds max %d stack args", check_outgoing_stack_args() reports "callee
expects %d args, stack arg%d is not initialized", and bpf_fixup_call_args()
reports "func#%d writes %u stack arg slots, but calls only require %u".

This is the path the patch is meant to reject. For the other outcome (the
caller's outgoing slots are not initialized), btf_check_subprog_call()
returns -EFAULT and the existing message is printed, and push_callback_call()
returns before this new check.

All other error paths in push_callback_call() also log: verifier_bug() for
the two internal consistency checks, and verbose() inside
setup_func_entry() or push_stack().

>
>  	/* set_callee_state is used for direct subprog calls, but we are
>  	 * interested in validating only BPF helpers that can call subprogs as


---
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/32068993990