netdev - Re: [PATCH bpf] bpf: fix bpffs non-array map seq

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Date:   Thu, 9 Aug 2018 08:15:37 -0700
From:   Yonghong Song <yhs@...com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     <ast@...com>, <netdev@...r.kernel.org>, <kernel-team@...com>
Subject: Re: [PATCH bpf] bpf: fix bpffs non-array map seq_show issue



On 8/9/18 7:24 AM, Daniel Borkmann wrote:
> On 08/09/2018 05:55 AM, Yonghong Song wrote:
>> On 8/8/18 7:25 PM, Alexei Starovoitov wrote:
>>> On Wed, Aug 08, 2018 at 06:25:19PM -0700, Yonghong Song wrote:
>>>> In function map_seq_next() of kernel/bpf/inode.c,
>>>> the first key will be the "0" regardless of the map type.
>>>> This works for array. But for hash type, if it happens
>>>> key "0" is in the map, the bpffs map show will miss
>>>> some items if the key "0" is not the first element of
>>>> the first bucket.
>>>>
>>>> This patch fixed the issue by guaranteeing to get
>>>> the first element, if the seq_show is just started,
>>>> by passing NULL pointer key to map_get_next_key() callback.
>>>> This way, no missing elements will occur for
>>>> bpffs hash table show even if key "0" is in the map.
>>
>> Currently, map_seq_show_elem callback is only implemented
>> for arraymap. So the problem actually is not exposed.
>>
>> The issue is discovered when I tried to implement
>> map_seq_show_elem for hash maps, and I will have followup
>> patches for it.
>>
>> So this patch probably should apply to bpf-next or
>> I can include this patch in my later patch set
>> which implements map_seq_show_elem for hash map
>> which can demonstrate the problem.
>>
>> Please let me know.
>>
>>>> Fixes: a26ca7c982cb5 ("bpf: btf: Add pretty print support to the basic arraymap")
>>>> Signed-off-by: Yonghong Song <yhs@...com>
>>>
>>> Acked-by: Alexei Starovoitov <ast@...nel.org>
> 
> Given this doesn't affect any current code, I think bpf-next
> would be fine.
> 
> Anyway, this cannot be used as-is, results in following compile
> warning ...

Thanks and will fix the problem and resubmit the patch set to
bpf-next.

> 
> # make -j4 kernel/bpf/
>    DESCEND  objtool
>    CALL    scripts/checksyscalls.sh
>    CC      kernel/bpf/verifier.o
>    CC      kernel/bpf/inode.o
> kernel/bpf/inode.c: In function ‘map_seq_next’:
> kernel/bpf/inode.c:214:1: warning: label ‘done’ defined but not used [-Wunused-label]
>   done:
>   ^~~~
>    AR      kernel/bpf/built-in.a
>