Subject: Regression: TCP connections fail over wireless: bad cksum?

From: Ted Percival <[email-masked]>
Date: Thu, 4 Sep 2014 14:11:52 -0600

Yesterday's linux-next build introduced a problem with wireless
networking on my machine. ie. next-20140901 worked fine but
next-20140902 does not seem able to sustain a TCP connection over
wireless. Wired networking works fine. I am using the brcmsmac driver
and the hardware is "Broadcom Corporation BCM4313 802.11b/g/n Wireless
LAN Controller (rev 01)".

Pings, even large pings (ping -s 16000) work fine but TCP connections hang.

I looked through the changes between the bad & good commits from the net
& net-next trees and I wonder if some of the changes to checksumming
have surfaced a problem with this driver. When I look at a tcpdump, it
indicates that all the checksums are wrong (although I don't know if
that is just due to hardware offload).

Here is a short trace of the hung connection attempt of
  curl http://lwn.net/

$ sudo tcpdump -vvn -i wlan0 port 80
tcpdump: listening on wlan0, link-type EN10MB (Ethernet), capture size
65535 bytes
13:20:15.120770 IP (tos 0x0, ttl 64, id 22734, offset 0, flags [DF],
proto TCP (6), length 60)
    10.5.51.93.38035 > 72.51.34.34.80: Flags [S], cksum 0xa7e5
(incorrect -> 0x4fac), seq 2861986597, win 29200, options [mss
1460,sackOK,TS val 142315 ecr 0,nop,wscale 7], length 0
13:20:16.121755 IP (tos 0x0, ttl 64, id 22735, offset 0, flags [DF],
proto TCP (6), length 60)
    10.5.51.93.38035 > 72.51.34.34.80: Flags [S], cksum 0xa7e5
(incorrect -> 0x4bc3), seq 2861986597, win 29200, options [mss
1460,sackOK,TS val 143316 ecr 0,nop,wscale 7], length 0
13:20:18.125748 IP (tos 0x0, ttl 64, id 22736, offset 0, flags [DF],
proto TCP (6), length 60)
    10.5.51.93.38035 > 72.51.34.34.80: Flags [S], cksum 0xa7e5
(incorrect -> 0x43ef), seq 2861986597, win 29200, options [mss
1460,sackOK,TS val 145320 ecr 0,nop,wscale 7], length 0
13:20:22.133743 IP (tos 0x0, ttl 64, id 22737, offset 0, flags [DF],
proto TCP (6), length 60)
    10.5.51.93.38035 > 72.51.34.34.80: Flags [S], cksum 0xa7e5
(incorrect -> 0x3447), seq 2861986597, win 29200, options [mss
1460,sackOK,TS val 149328 ecr 0,nop,wscale 7], length 0
13:20:30.149754 IP (tos 0x0, ttl 64, id 22738, offset 0, flags [DF],
proto TCP (6), length 60)
    10.5.51.93.38035 > 72.51.34.34.80: Flags [S], cksum 0xa7e5
(incorrect -> 0x14f7), seq 2861986597, win 29200, options [mss
1460,sackOK,TS val 157344 ecr 0,nop,wscale 7], length 0

I don't see anything that looks related in dmesg. The only brcmsmac
messages I see are:

[  567.122218] brcmsmac bcma0:0: brcmsmac: brcms_ops_bss_info_changed:
associated
[  567.122226] brcmsmac bcma0:0: brcms_ops_bss_info_changed: arp
filtering: 1 addresses (implement)
[  567.122231] brcmsmac bcma0:0: brcms_ops_bss_info_changed: qos
enabled: true (implement)
[  567.192283] brcmsmac bcma0:0: brcms_ops_bss_info_changed: qos
enabled: true (implement)

I am writing to linux-netdev rather than linux-wireless because
according to Next/SHA1s the wireless & wireless-next trees were not
updated between next-20140901 and next-20140902, but the net & net-next
trees were updated, so maybe the regression came from there. (I haven't
tested next-20140903 because it won't boot for unrelated reasons.)

Let me know if I should just file this in Bugzilla or what information I
can provide to help track this down, if it hasn't already been identified.

The here are the good (-) and bad (+) trees from Next/SHA1s at the
next-* tags mentioned earlier that I built from.

-net            38ab1fa981d543e1b00f4ffbce4ddb480cd2effe
+net            cc25f0cbe4409d6a573b1f3bf7020d5b04076ee9

-net-next       dace1b54726bffe1c009f7661e3cee6b762f30c8
+net-next       364a9e93243d1785f310c0964af0e24bf1adac03

-- 
Cheers,
-Ted