On 10/14/20 4:14 PM, Andrii Nakryiko wrote:
On Wed, Oct 14, 2020 at 2:53 PM Yonghong Song <yhs@xxxxxx> 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
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@xxxxxxxxxx>
[...]
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
