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

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Message-ID: <c5f1bd74-f411-25c1-7bfb-b709e6746924@fb.com>
Date:   Wed, 8 Aug 2018 20:55:20 -0700
From:   Yonghong Song <yhs@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     <ast@...com>, <daniel@...earbox.net>, <netdev@...r.kernel.org>,
        <kernel-team@...com>
Subject: Re: [PATCH bpf] bpf: fix bpffs non-array map seq_show issue



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>
>