netdev - Re: [PATCH bpf-next v6 11/16] bpf, libbpf: support global data/bss/rodata sections

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Message-ID: <10337819-96ae-f6e2-8dae-355fb430c734@iogearbox.net>
Date:   Tue, 23 Apr 2019 10:31:26 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     bpf@...r.kernel.org, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Joe Stringer <joe@...d.net.nz>, Yonghong Song <yhs@...com>,
        Martin Lau <kafai@...com>, Jann Horn <jannh@...gle.com>,
        Andrey Ignatov <rdna@...com>
Subject: Re: [PATCH bpf-next v6 11/16] bpf, libbpf: support global
 data/bss/rodata sections

On 04/23/2019 06:06 AM, Andrii Nakryiko wrote:
> On Mon, Apr 22, 2019 at 5:58 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>> On 04/19/2019 03:18 AM, Andrii Nakryiko wrote:
>>> On Tue, Apr 9, 2019 at 2:20 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>>>>
>> [...]
>>>> +       def->type = BPF_MAP_TYPE_ARRAY;
>>>> +       def->key_size = sizeof(int);
>>>> +       def->value_size = data->d_size;
>>>> +       def->max_entries = 1;
>>>> +       def->map_flags = type == LIBBPF_MAP_RODATA ?
>>>> +                        BPF_F_RDONLY_PROG : 0;
>>>
>>> This is breaking BPF programs (even those that don't use global data,
>>> as they still have .rodata section, though I haven't investigated its
>>> contents) on kernels that don't yet support BPF_F_RDONLY_PROG flag
>>> yet. We probably need to probe support for that flag first, before
>>> using it. Just giving heads up, as I just discovered it trying to sync
>>> libbpf on github.
>>
>> Thanks for reporting! On a quick look test_progs (modulo global data test)
>> seems to pass with an slightly older kernel. I'll retry with a latest LLVM git
>> tree tomorrow with our test suite. Did you see a specific one failing or do you
>> have a reproducer in case it's something not covered where I could look into?
> 
> You need to add something like this to trigger .rodata section
> generation (BPF code doesn't have to use that struct, it just needs to
> be present):
> 
> const struct { int x, y; } bla = {};
> 
> This will cause libbpf to create a map for .rodata and specify
> BPF_F_RDONLY_PROG flag, which on older kernels will be rejected.

Fair enough, working on a fix in that case. Thanks!