linux-kernel - Re: [PATCH bpf 1/2] bpf: fix alignment of netns_dev/netns_ino fields in bpf_{map,prog}

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Date:   Sat, 2 Jun 2018 05:28:46 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Song Liu <liu.song.a23@...il.com>,
        Eugene Syromiatnikov <esyr@...hat.com>
Cc:     netdev@...r.kernel.org, open list <linux-kernel@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Alexei Starovoitov <ast@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jiri Olsa <jolsa@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Lawrence Brakmo <brakmo@...com>, Andrey Ignatov <rdna@...com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        John Fastabend <john.fastabend@...il.com>,
        "Dmitry V. Levin" <ldv@...linux.org>
Subject: Re: [PATCH bpf 1/2] bpf: fix alignment of netns_dev/netns_ino fields
 in bpf_{map,prog}_info

On 05/29/2018 07:17 PM, Song Liu wrote:
> On Sun, May 27, 2018 at 4:28 AM, Eugene Syromiatnikov <esyr@...hat.com> wrote:
>> Recent introduction of netns_dev/netns_ino to bpf_map_info/bpf_prog info
>> has broken compat, as offsets of these fields are different in 32-bit
>> and 64-bit ABIs.  One fix (other than implementing compat support in
>> syscall in order to handle this discrepancy) is to use __aligned_u64
>> instead of __u64 for these fields.
>>
>> Reported-by: Dmitry V. Levin <ldv@...linux.org>
>> Fixes: 52775b33bb507 ("bpf: offload: report device information about
>> offloaded maps")
>> Fixes: 675fc275a3a2d ("bpf: offload: report device information for
>> offloaded programs")
>>
>> Signed-off-by: Eugene Syromiatnikov <esyr@...hat.com>
>> ---
>>  include/uapi/linux/bpf.h       | 8 ++++----
>>  tools/include/uapi/linux/bpf.h | 8 ++++----
>>  2 files changed, 8 insertions(+), 8 deletions(-)
>>
>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> index c5ec897..903010a 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -1017,8 +1017,8 @@ struct bpf_prog_info {
>>         __aligned_u64 map_ids;
>>         char name[BPF_OBJ_NAME_LEN];
>>         __u32 ifindex;
>> -       __u64 netns_dev;
>> -       __u64 netns_ino;
>> +       __aligned_u64 netns_dev;
>> +       __aligned_u64 netns_ino;
>>  } __attribute__((aligned(8)));
> 
> Shall we add a __u32 padding variable before netns_dev? We can use it
> for in the future.

Agree with Song, and definitely prefer that approach since we already use the hole
as a bitfield in net-next; like this https://patchwork.ozlabs.org/patch/924415/.