linux-kernel - Re: [PATCH 2/3] bpf: Allow NULL buffers in bpf_dynptr_slice(

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Date:   Wed, 3 May 2023 11:34:17 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Daniel Rosenberg <drosen@...gle.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 2/3] bpf: Allow NULL buffers in bpf_dynptr_slice(_rw)

On Fri, Apr 28, 2023 at 6:58 PM Daniel Rosenberg <drosen@...gle.com> wrote:
>
> On Thu, Apr 6, 2023 at 2:09 PM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > would this work correctly if someone passes a non-null buffer with too
> > small size? Can you please add a test for this use case.
> >
> Working on a test case for this, but the test case I wrote fails
> without my patches.
> I'm just declaring a buffer of size 9 on the stack, and then passing
> in bpf_dynptr_slice that buffer, and size 10. That's passing the
> verifier just fine. In fact, it loads successfully up to size 16. I'm
> guessing that's adjusting for alignment? Still feels very strange. Is
> that expected behavior?

pointer to stack is trickier (verifier will just mark part of stack as
overwritten with random data), it's best to use map value pointer as a
source of buffer. So try using ARRAY map with small value_size, do
lookup_elem, check for NULL, and pass non-NULL pointer as a buffer.