linux-kernel - Re: [PATCH 1/3] bpf: verifier: Accept dynptr mem as mem in helpers

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Date:   Thu, 6 Apr 2023 15:35:15 -0700
From:   Daniel Rosenberg <drosen@...gle.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Shuah Khan <shuah@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Joanne Koong <joannelkoong@...il.com>,
        Mykola Lysenko <mykolal@...com>, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 1/3] bpf: verifier: Accept dynptr mem as mem in helpers

On Thu, Apr 6, 2023 at 1:55 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> Something feels off here. Can you paste a bit of verifier log for the
> failure you were getting. And let's have a selftest for this situation
> as well.
>
> ARG_PTR_TO_MEM shouldn't be qualified with the DYNPTR_TYPE flag, it's
> just memory, there is no need to know what type of dynptr it was
> derived from. So if that happens, the problem is somewhere else. Let's
> root cause and fix that. Having a selftest that demonstrates the
> problem will help with that.
>
>
This label is added by dynptr_slice(_rdwr)

if (meta.func_id == special_kfunc_list[KF_bpf_dynptr_slice] ||
   meta.func_id == special_kfunc_list[KF_bpf_dynptr_slice_rdwr]) {
enum bpf_type_flag type_flag =
get_dynptr_type_flag(meta.initialized_dynptr.type);
...
regs[BPF_REG_0].type = PTR_TO_MEM | type_flag;

That extra flag was causing the type to be unexpected later on.
I'll add a selftest for this as well.