Re: [RFC PATCH v9 01/11] bpf: Support getting referenced kptr from struct_ops argument

On Tue, Jul 23, 2024 at 5:32 PM Martin KaFai Lau [off-list ref] wrote:

On 7/14/24 10:51 AM, Amery Hung wrote:

quoted

@@ -21004,6 +21025,13 @@ static int do_check_common(struct bpf_verifier_env *env, int subprog)
              mark_reg_known_zero(env, regs, BPF_REG_1);
      }

+     if (env->prog->type == BPF_PROG_TYPE_STRUCT_OPS) {
+             ctx_arg_info = (struct bpf_ctx_arg_aux *)env->prog->aux->ctx_arg_info;
+             for (i = 0; i < env->prog->aux->ctx_arg_info_size; i++)
+                     if (ctx_arg_info[i].refcounted)
+                             ctx_arg_info[i].ref_obj_id = acquire_reference_state(env, 0);
+     }
+

I think this will miss a case when passing the struct_ops prog ctx (i.e. "__u64
*ctx") to a global subprog. Something like this:

__noinline int subprog_release(__u64 *ctx __arg_ctx)
{
        struct task_struct *task = (struct task_struct *)ctx[1];
        int dummy = (int)ctx[0];

        bpf_task_release(task);

        return dummy + 1;
}

SEC("struct_ops/subprog_ref")
__failure
int test_subprog_ref(__u64 *ctx)
{
        struct task_struct *task = (struct task_struct *)ctx[1];

        bpf_task_release(task);

        return subprog_release(ctx);;
}

SEC(".struct_ops.link")
struct bpf_testmod_ops subprog_ref = {
        .test_refcounted = (void *)test_subprog_ref,
};

Thanks for pointing this out. The test did failed.

A quick thought is, I think tracking the ctx's ref id in the env->cur_state may
not be the correct place.

I think it is a bit tricky because subprogs are checked independently
and their state is folded (i.e., there can be multiple edges from the
main program to a subprog).

Maybe the verifier can rewrite the program: set the refcounted ctx to
NULL when releasing reference. Then, in do_check_common(), if it is a
global subprog, we mark refcounted ctx as PTR_MAYBE_NULL to force a
runtime check. How does it sound?

[ Just want to bring up what I have noticed so far. I will stop at here for
today and will continue. ]