netdev - Re: [PATCH net v3] net: fix pos incrementment in ipv6_route_seq

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Date:   Wed, 14 Oct 2020 16:49:53 -0700
From:   Yonghong Song <yhs@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
CC:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>,
        Andrii Nakryiko <andriin@...com>,
        Vasily Averin <vvs@...tuozzo.com>
Subject: Re: [PATCH net v3] net: fix pos incrementment in ipv6_route_seq_next



On 10/14/20 4:14 PM, Andrii Nakryiko wrote:
> On Wed, Oct 14, 2020 at 2:53 PM Yonghong Song <yhs@...com> wrote:
>>
>> Commit 4fc427e05158 ("ipv6_route_seq_next should increase position index")
>> tried to fix the issue where seq_file pos is not increased
>> if a NULL element is returned with seq_ops->next(). See bug
>>    https://bugzilla.kernel.org/show_bug.cgi?id=206283
>> The commit effectively does:
>>    - increase pos for all seq_ops->start()
>>    - increase pos for all seq_ops->next()
>>
>> For ipv6_route, increasing pos for all seq_ops->next() is correct.
>> But increasing pos for seq_ops->start() is not correct
>> since pos is used to determine how many items to skip during
>> seq_ops->start():
>>    iter->skip = *pos;
>> seq_ops->start() just fetches the *current* pos item.
>> The item can be skipped only after seq_ops->show() which essentially
>> is the beginning of seq_ops->next().
>>
>> For example, I have 7 ipv6 route entries,
>>    root@...h-fb-vm1:~/net-next dd if=/proc/net/ipv6_route bs=4096
>>    00000000000000000000000000000000 40 00000000000000000000000000000000 00 00000000000000000000000000000000 00000400 00000001 00000000 00000001     eth0
>>    fe800000000000000000000000000000 40 00000000000000000000000000000000 00 00000000000000000000000000000000 00000100 00000001 00000000 00000001     eth0
>>    00000000000000000000000000000000 00 00000000000000000000000000000000 00 00000000000000000000000000000000 ffffffff 00000001 00000000 00200200       lo
>>    00000000000000000000000000000001 80 00000000000000000000000000000000 00 00000000000000000000000000000000 00000000 00000003 00000000 80200001       lo
>>    fe800000000000002050e3fffebd3be8 80 00000000000000000000000000000000 00 00000000000000000000000000000000 00000000 00000002 00000000 80200001     eth0
>>    ff000000000000000000000000000000 08 00000000000000000000000000000000 00 00000000000000000000000000000000 00000100 00000004 00000000 00000001     eth0
>>    00000000000000000000000000000000 00 00000000000000000000000000000000 00 00000000000000000000000000000000 ffffffff 00000001 00000000 00200200       lo
>>    0+1 records in
>>    0+1 records out
>>    1050 bytes (1.0 kB, 1.0 KiB) copied, 0.00707908 s, 148 kB/s
>>    root@...h-fb-vm1:~/net-next
>>
>> In the above, I specify buffer size 4096, so all records can be returned
>> to user space with a single trip to the kernel.
>>
>> If I use buffer size 128, since each record size is 149, internally
>> kernel seq_read() will read 149 into its internal buffer and return the data
>> to user space in two read() syscalls. Then user read() syscall will trigger
>> next seq_ops->start(). Since the current implementation increased pos even
>> for seq_ops->start(), it will skip record #2, #4 and #6, assuming the first
>> record is #1.
>>
>>    root@...h-fb-vm1:~/net-next dd if=/proc/net/ipv6_route bs=128
> 
> Did you test with non-zero skip= parameter as well (to force lseek)?
> To make sure we don't break the scenario that original fix tried to
> fix.

I did with skip=1 and it won't show the last line any more. And I
did not really change that logic (increasing pos even when returning 
NULL for seq_ops->next()).

> 
> If that works:
> 
> Acked-by: Andrii Nakryiko <andrii@...nel.org>
> 
> [...]
> 
>> diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
>> index 141c0a4c569a..605cdd38a919 100644
>> --- a/net/ipv6/ip6_fib.c
>> +++ b/net/ipv6/ip6_fib.c
>> @@ -2622,8 +2622,10 @@ static void *ipv6_route_seq_start(struct seq_file *seq, loff_t *pos)
>>          iter->skip = *pos;
>>
>>          if (iter->tbl) {
>> +               loff_t p = 0;
>> +
>>                  ipv6_route_seq_setup_walk(iter, net);
>> -               return ipv6_route_seq_next(seq, NULL, pos);
>> +               return ipv6_route_seq_next(seq, NULL, &p);
> 
> nit: comment here wouldn't hurt for the next guy stumbling upon this
> code and wondering why we ignore p afterwards

Typically you won't increase pos from seq_ops->start(). So I think
we are fine here without comments.

> 
>>          } else {
>>                  return NULL;
>>          }
>> --
>> 2.24.1
>>