Re: [PATCH net] net: fix pos incrementment in ipv6_route_seq_next

[Yonghong Song <yhs@fb.com>]

On 10/12/20 11:56 PM, Vasily Averin wrote:
> Dear Yonghong Song,
> thank you for reporting the problem.
> As far as I understand the problem here is that pos is incremented in .start function.

Yes.

> 
> I do not,like an idea to avoid increment in ipv6_route_seq_next()
> however ipv6_route_seq_start can provide fake argument instead.
> 
> --- a/net/ipv6/ip6_fib.c
> +++ b/net/ipv6/ip6_fib.c
> @@ -2618,8 +2618,9 @@ static void *ipv6_route_seq_start(struct seq_file *seq, loff_t *pos)
>          iter->skip = *pos;
>   
>          if (iter->tbl) {
> +               loff_t p;
>                  ipv6_route_seq_setup_walk(iter, net);
> -               return ipv6_route_seq_next(seq, NULL, pos);
> +               return ipv6_route_seq_next(seq, NULL, &p);
>          } else {
>                  return NULL;
>          }

This should work too.

I am fine with the change. I will wait until tomorrow for
additional comments, if any, about this fake point approach vs.
my approach, before sending v2.

> 
> In this case patch subject should be changed accordingly.
> 
> Thank you,
> 	Vasily Averin
> 
> On 10/13/20 3:09 AM, Yonghong Song 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@arch-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@arch-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@arch-fb-vm1:~/net-next dd if=/proc/net/ipv6_route bs=128
>>    00000000000000000000000000000000 40 00000000000000000000000000000000 00 00000000000000000000000000000000 00000400 00000001 00000000 00000001     eth0
>>    00000000000000000000000000000000 00 00000000000000000000000000000000 00 00000000000000000000000000000000 ffffffff 00000001 00000000 00200200       lo
>>    fe800000000000002050e3fffebd3be8 80 00000000000000000000000000000000 00 00000000000000000000000000000000 00000000 00000002 00000000 80200001     eth0
>>    00000000000000000000000000000000 00 00000000000000000000000000000000 00 00000000000000000000000000000000 ffffffff 00000001 00000000 00200200       lo
>> 4+1 records in
>> 4+1 records out
>> 600 bytes copied, 0.00127758 s, 470 kB/s
>>
>> To fix the problem, do not increase pos for seq_ops->start() and the
>> above `dd` command with `bs=128` will show correct result.
>>
>> Fixes: 4fc427e05158 ("ipv6_route_seq_next should increase position index")
>> Cc: Vasily Averin <vvs@virtuozzo.com>
>> Cc: Andrii Nakryiko <andriin@fb.com>
>> Cc: Alexei Starovoitov <ast@kernel.org>
>> Signed-off-by: Yonghong Song <yhs@fb.com>
>> ---
>>   net/ipv6/ip6_fib.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
>> index 141c0a4c569a..5aac5094bc41 100644
>> --- a/net/ipv6/ip6_fib.c
>> +++ b/net/ipv6/ip6_fib.c
>> @@ -2582,10 +2582,10 @@ static void *ipv6_route_seq_next(struct seq_file *seq, void *v, loff_t *pos)
>>   	struct net *net = seq_file_net(seq);
>>   	struct ipv6_route_iter *iter = seq->private;
>>   
>> -	++(*pos);
>>   	if (!v)
>>   		goto iter_table;
>>   
>> +	++(*pos);
>>   	n = rcu_dereference_bh(((struct fib6_info *)v)->fib6_next);
>>   	if (n)
>>   		return n;
>>