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Message-ID: <fffa4642-e1e6-29c4-6cac-7b4fd1ec1997@fb.com>
Date: Tue, 13 Oct 2020 00:13:39 -0700
From: Yonghong Song <yhs@...com>
To: Vasily Averin <vvs@...tuozzo.com>, <bpf@...r.kernel.org>,
<netdev@...r.kernel.org>
CC: Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>, <kernel-team@...com>,
Andrii Nakryiko <andriin@...com>
Subject: Re: [PATCH net] net: fix pos incrementment in ipv6_route_seq_next
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@...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
>> 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@...tuozzo.com>
>> Cc: Andrii Nakryiko <andriin@...com>
>> Cc: Alexei Starovoitov <ast@...nel.org>
>> Signed-off-by: Yonghong Song <yhs@...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;
>>
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