Re: [PATCH] net: Reduce skb_warn_bad_offload() noise.

[Tonghao Zhang]

Thanks, I send a patch, which will revert commit 6e7bc478c9a0 in net-next.

On Wed, Aug 9, 2017 at 1:46 AM, Willem de Bruijn
 wrote:
>>> @@ -2670,6 +2670,7 @@ static inline bool skb_needs_check(struct
>>> sk_buff *skb, bool tx_path)
>>>  {
>>> if (tx_path)
>>> return skb->ip_summed != CHECKSUM_PARTIAL &&
>>> +  skb->ip_summed != CHECKSUM_UNNECESSARY &&
>>>skb->ip_summed != CHECKSUM_NONE;
>>
>> Good catch. Only, the CHECKSUM_NONE case was added specifically to
>> work around this UFO issue on the tx path in commit 6e7bc478c9a0
>> ("net: skb_needs_check() accepts CHECKSUM_NONE for tx"). If we change
>> the value generated by UFO, we can remove that statement, so
>>
>> +  skb->ip_summed != CHECKSUM_UNNECESSARY;
>> -skb->ip_summed != CHECKSUM_NONE;
>>
>> Else the entire check becomes a NOOP. These are the only three valid
>> states on tx. With very few codepaths generating CHECKSUM_UNNECESSARY
>> to begin with, it arguably already is practically a NOOP. I need to
>> look more closely what the statement is intended to protect against,
>> before we relax it even further.
>
> On transmit, packets entering skb_gso_segment are expected to always
> have ip_summed CHECKSUM_PARTIAL. This check was added to track down
> unexpected exceptions in commit 67fd1a731ff1 ("net: Add debug info to
> track down GSO checksum bug").
>
> Only when called for the second time, after skb_mac_gso_segment, do we
> have to possibly handle the case where the GSO layer computes the
> checksum and changes ip_summed.
>
> Since this only goes into 4.11 to 4.13, making two separate
> skb_needs_check variants for these two call sites seems overkill. I
> will send the simple fix to convert CHECKSUM_NONE to
> CHECKSUM_UNNECESSARY.
>
> As a side effect of removing UFO in 4.14-rc1, we can also revert
> commit 6e7bc478c9a0 ("net: skb_needs_check() accepts CHECKSUM_NONE for
> tx") in net-next.

Re: [PATCH] net: Reduce skb_warn_bad_offload() noise.

[Willem de Bruijn]

>> @@ -2670,6 +2670,7 @@ static inline bool skb_needs_check(struct
>> sk_buff *skb, bool tx_path)
>>  {
>> if (tx_path)
>> return skb->ip_summed != CHECKSUM_PARTIAL &&
>> +  skb->ip_summed != CHECKSUM_UNNECESSARY &&
>>skb->ip_summed != CHECKSUM_NONE;
>
> Good catch. Only, the CHECKSUM_NONE case was added specifically to
> work around this UFO issue on the tx path in commit 6e7bc478c9a0
> ("net: skb_needs_check() accepts CHECKSUM_NONE for tx"). If we change
> the value generated by UFO, we can remove that statement, so
>
> +  skb->ip_summed != CHECKSUM_UNNECESSARY;
> -skb->ip_summed != CHECKSUM_NONE;
>
> Else the entire check becomes a NOOP. These are the only three valid
> states on tx. With very few codepaths generating CHECKSUM_UNNECESSARY
> to begin with, it arguably already is practically a NOOP. I need to
> look more closely what the statement is intended to protect against,
> before we relax it even further.

On transmit, packets entering skb_gso_segment are expected to always
have ip_summed CHECKSUM_PARTIAL. This check was added to track down
unexpected exceptions in commit 67fd1a731ff1 ("net: Add debug info to
track down GSO checksum bug").

Only when called for the second time, after skb_mac_gso_segment, do we
have to possibly handle the case where the GSO layer computes the
checksum and changes ip_summed.

Since this only goes into 4.11 to 4.13, making two separate
skb_needs_check variants for these two call sites seems overkill. I
will send the simple fix to convert CHECKSUM_NONE to
CHECKSUM_UNNECESSARY.

As a side effect of removing UFO in 4.14-rc1, we can also revert
commit 6e7bc478c9a0 ("net: skb_needs_check() accepts CHECKSUM_NONE for
tx") in net-next.

Re: [PATCH] net: Reduce skb_warn_bad_offload() noise.

[Willem de Bruijn]

> In a case, there is also warn info. The test topo is shown as below.

Thanks for testing the patch, Tonghao.

> The warn info is shown as below [1]. If we change the CHECKSUM_NONE to
> CHECKSUM_UNNECESSARY in the udp4_ufo_fragment().
> and we should add a check in skb_needs_check() when outputting a packet.
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 416137c..8fe12a7 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -2670,6 +2670,7 @@ static inline bool skb_needs_check(struct
> sk_buff *skb, bool tx_path)
>  {
> if (tx_path)
> return skb->ip_summed != CHECKSUM_PARTIAL &&
> +  skb->ip_summed != CHECKSUM_UNNECESSARY &&
>skb->ip_summed != CHECKSUM_NONE;

Good catch. Only, the CHECKSUM_NONE case was added specifically to
work around this UFO issue on the tx path in commit 6e7bc478c9a0
("net: skb_needs_check() accepts CHECKSUM_NONE for tx"). If we change
the value generated by UFO, we can remove that statement, so

+  skb->ip_summed != CHECKSUM_UNNECESSARY;
-skb->ip_summed != CHECKSUM_NONE;

Else the entire check becomes a NOOP. These are the only three valid
states on tx. With very few codepaths generating CHECKSUM_UNNECESSARY
to begin with, it arguably already is practically a NOOP. I need to
look more closely what the statement is intended to protect against,
before we relax it even further.

The patch will need the same UFO change in ipv6_ufo_fragment.

I also had to verify that it is indeed correct to use
CHECKSUM_UNNECESSARY for this case. It is not commonly used as alias
for CHECKSUM_NONE on the tx path. And indeed does not carry the exact
same meaning. It was defined as "no need to checksum" on tx for
protocols that skb_checksum_help does not support:

  http://patchwork.ozlabs.org/patch/146567/

Given that, it should be fine to use in this case.

Re: [PATCH] net: Reduce skb_warn_bad_offload() noise.

[Tonghao Zhang]

Hi Willem

In a case, there is also warn info. The test topo is shown as below.

VM01: veth1 and eth0 in the VM01 are inserted to ovs br0.
veth0(IP: 172.16.34.100/24) —— veth1--br0--eth0

iperf3  -c 172.168.100.13 -i 1 -P 10 -t 10 -u -b 1000M -l 10K



VM02
eth0(IP: 172.16.34.200/24)
iperf3  -s

The warn info is shown as below [1]. If we change the CHECKSUM_NONE to
CHECKSUM_UNNECESSARY in the udp4_ufo_fragment().
and we should add a check in skb_needs_check() when outputting a packet.

diff --git a/net/core/dev.c b/net/core/dev.c
index 416137c..8fe12a7 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2670,6 +2670,7 @@ static inline bool skb_needs_check(struct
sk_buff *skb, bool tx_path)
 {
if (tx_path)
return skb->ip_summed != CHECKSUM_PARTIAL &&
+  skb->ip_summed != CHECKSUM_UNNECESSARY &&
   skb->ip_summed != CHECKSUM_NONE;

return skb->ip_summed == CHECKSUM_NONE;
diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index 7812501..0932c85 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -235,7 +235,7 @@ static struct sk_buff *udp4_ufo_fragment(struct
sk_buff *skb,
if (uh->check == 0)
uh->check = CSUM_MANGLED_0;

-   skb->ip_summed = CHECKSUM_NONE;
+   skb->ip_summed = CHECKSUM_UNNECESSARY;

/* If there is no outer header we can fake a checksum offload
 * due to the fact that we have already done the checksum in


[1]:
[ 1291.596232] vmxnet3: caps=(0x006000214ba9, 0x)
len=10282 data_len=10240 gso_size=1480 gso_type=2 ip_summed=1
[ 1291.596239] [ cut here ]
[ 1291.596242] WARNING: CPU: 1 PID: 2203 at net/core/dev.c:2564
skb_warn_bad_offload+0xd3/0xde
[ 1291.596242] Modules linked in: veth udp_tunnel gre openvswitch
nf_conntrack_ipv6 nf_nat_ipv6 nf_conntrack_ipv4 nf_defrag_ipv4
nf_nat_ipv4 nf_defrag_ipv6 nf_nat nf_conntrack cfg80211 rfkill ext4
jbd2 mbcache sb_edac coretemp crct10dif_pclmul crc32_pclmul
ghash_clmulni_intel pcbc aesni_intel crypto_simd glue_helper ppdev
vmw_balloon cryptd vmw_vmci sg i2c_piix4 pcspkr parport_pc parport
shpchp ip_tables xfs libcrc32c sd_mod ata_generic pata_acpi vmwgfx
drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops ttm drm
crc32c_intel serio_raw vmxnet3 ata_piix mptspi libata
scsi_transport_spi mptscsih mptbase i2c_core floppy dm_mirror
dm_region_hash dm_log dm_mod
[ 1291.596280] CPU: 1 PID: 2203 Comm: iperf3 Tainted: GW
4.12.0+ #1
[ 1291.596280] Hardware name: VMware, Inc. VMware Virtual
Platform/440BX Desktop Reference Platform, BIOS 6.00 09/21/2015
[ 1291.596281] task: 8be36f5e9680 task.stack: b6c840d44000
[ 1291.596283] RIP: 0010:skb_warn_bad_offload+0xd3/0xde
[ 1291.596284] RSP: 0018:8be3796438c8 EFLAGS: 00010246
[ 1291.596285] RAX: 0074 RBX: 8be363ff9f00 RCX: 0006
[ 1291.596286] RDX:  RSI: 0086 RDI: 8be37964e0a0
[ 1291.596287] RBP: 8be3796438f0 R08:  R09: 0bf2
[ 1291.596287] R10: 0004 R11: 0bf1 R12: 8be3660f
[ 1291.596288] R13: 0001 R14:  R15: 8be3660f
[ 1291.596289] FS:  7f8d93bdb740() GS:8be37964()
knlGS:
[ 1291.596290] CS:  0010 DS:  ES:  CR0: 80050033
[ 1291.596291] CR2: 555f94a6e0c8 CR3: 00012b84a000 CR4: 001406e0
[ 1291.596296] Call Trace:
[ 1291.596297]  
[ 1291.596300]  __skb_gso_segment+0x15d/0x170
[ 1291.596301]  validate_xmit_skb+0x12d/0x2b0
[ 1291.596303]  validate_xmit_skb_list+0x42/0x70
[ 1291.596306]  sch_direct_xmit+0xd0/0x1b0
[ 1291.596308]  __dev_queue_xmit+0x42e/0x630
[ 1291.596310]  ? hrtimer_interrupt+0xd2/0x1a0
[ 1291.596312]  dev_queue_xmit+0x10/0x20
[ 1291.596315]  ovs_vport_send+0xc2/0x150 [openvswitch]
[ 1291.596318]  do_output+0x53/0xf0 [openvswitch]
[ 1291.596322]  do_execute_actions+0x9bc/0x9d0 [openvswitch]
[ 1291.596324]  ? __bpf_prog_run+0x385/0x1310
[ 1291.596327]  ovs_execute_actions+0x40/0x120 [openvswitch]
[ 1291.596330]  ovs_dp_process_packet+0x84/0x120 [openvswitch]
[ 1291.596333]  ? ovs_ct_update_key+0x9a/0xe0 [openvswitch]
[ 1291.596336]  ovs_vport_receive+0x73/0xd0 [openvswitch]
[ 1291.596339]  ? handle_irq_event_percpu+0x54/0x80
[ 1291.596340]  ? handle_irq_event+0x46/0x60
[ 1291.596342]  ? handle_edge_irq+0x8d/0x130
[ 1291.596344]  ? handle_irq+0xab/0x120
[ 1291.596346]  ? irq_exit+0x77/0xf0
[ 1291.596348]  ? do_IRQ+0x51/0xd0
[ 1291.596352]  netdev_frame_hook+0xd3/0x160 [openvswitch]
[ 1291.596355]  __netif_receive_skb_core+0x1da/0x9e0
[ 1291.596358]  ? vport_netdev_free+0x30/0x30 [openvswitch]
[ 1291.596360]  ? kfree_skbmem+0x5a/0x60
[ 1291.596361]  ? consume_skb+0x34/0x90
[ 1291.596363]  __netif_receive_skb+0x18/0x60
[ 1291.596365]  process_backlog+0x95/0x140
[ 1291.596367]  net_rx_action+0x26c/0x3b0
[ 1291.596369]  __do_softirq+0xc9/0x269
[ 1291.596371]  

Re: [PATCH] net: Reduce skb_warn_bad_offload() noise.

[Tonghao Zhang]

That is fine to me. I have tested it. Thanks.

On Mon, Aug 7, 2017 at 12:42 PM, Willem de Bruijn
 wrote:
>> The openvswitch kernel module calls the __skb_gso_segment()(and sets
>> tx_path = false) when passing packets to userspace. The UFO will set
>> the ip_summed to CHECKSUM_NONE. There are a lot of warn logs. The warn
>> log is shown as below. I guess we should revert the patch.
>
> Indeed, the software UFO code computes the checksum and
> sets ip_summed to CHECKSUM_NONE, as is correct on the
> egress path.
>
> Commit 6e7bc478c9a0 ("net: skb_needs_check() accepts
> CHECKSUM_NONE for tx") revised the tx_path case in
> skb_needs_check to avoid the warning exactly for the UFO case.
>
> We cannot just make an exception for CHECKSUM_NONE in the
> !tx_path case, as the entire statement then becomes false:
>
>return skb->ip_summed == CHECKSUM_NONE;
>
> Since on egress CHECKSUM_UNNECESSARY is equivalent to
> CHECKSUM_NONE, it should be fine to update the UFO code
> to set that, instead:
>
> @@ -235,7 +235,7 @@ static struct sk_buff *udp4_ufo_fragment(struct
> sk_buff *skb,
> if (uh->check == 0)
> uh->check = CSUM_MANGLED_0;
>
> -   skb->ip_summed = CHECKSUM_NONE;
> +   skb->ip_summed = CHECKSUM_UNNECESSARY;

Re: [PATCH] net: Reduce skb_warn_bad_offload() noise.

[Willem de Bruijn]

> The openvswitch kernel module calls the __skb_gso_segment()(and sets
> tx_path = false) when passing packets to userspace. The UFO will set
> the ip_summed to CHECKSUM_NONE. There are a lot of warn logs. The warn
> log is shown as below. I guess we should revert the patch.

Indeed, the software UFO code computes the checksum and
sets ip_summed to CHECKSUM_NONE, as is correct on the
egress path.

Commit 6e7bc478c9a0 ("net: skb_needs_check() accepts
CHECKSUM_NONE for tx") revised the tx_path case in
skb_needs_check to avoid the warning exactly for the UFO case.

We cannot just make an exception for CHECKSUM_NONE in the
!tx_path case, as the entire statement then becomes false:

   return skb->ip_summed == CHECKSUM_NONE;

Since on egress CHECKSUM_UNNECESSARY is equivalent to
CHECKSUM_NONE, it should be fine to update the UFO code
to set that, instead:

@@ -235,7 +235,7 @@ static struct sk_buff *udp4_ufo_fragment(struct
sk_buff *skb,
if (uh->check == 0)
uh->check = CSUM_MANGLED_0;

-   skb->ip_summed = CHECKSUM_NONE;
+   skb->ip_summed = CHECKSUM_UNNECESSARY;

Re: [PATCH] net: Reduce skb_warn_bad_offload() noise.

[Tonghao Zhang]

On Fri, Aug 4, 2017 at 9:29 PM, Eric Dumazet  wrote:
> On Fri, 2017-08-04 at 06:11 -0700, Tonghao Zhang wrote:
>> This patch will revert the b2504a5dbe "net: reduce
>> skb_warn_bad_offload() noise". The ovs will call the
>> __skb_gso_segment() with tx false. When segmenting UDP with UFO,
>> the __skb_gso_segment raises a warning as below [1], because the
>> ip_summed is CHECKSUM_NONE. While the net-next has removed the
>> UFO support, but the 4.11 and 4.12 kernel don't address that problem.
>>
>> In the kernel, only qdisc_pkt_len_init() (__dev_queue_xmit call it.)
>> uses the SKB_GSO_DODGY to do something. Other places just set it.
>> The warn described in b2504a5dbe is shown [2]. We may know that:
>> 1. the net_device don’t have qdisc.
>> 2. the skb->ip_summed was changed to CHECKSUM_NONE. it maybe
>> changed in skb_checksum_help() when calling validate_xmit_skb().
>> or other place.
>>
>> And we should not revert the  6e7bc478c9 "net: skb_needs_check() accepts
>> CHECKSUM_NONE for tx". The check is necessary.
>
> Why is it necessary ?
I am not familiar with __skb_gso_segment() when tx_path == true. I
should say sorry to you.

>
> If you revert b2504a5dbe, then we also need to revert 6e7bc478c9,
> unless you provide hard facts.
>
The openvswitch kernel module calls the __skb_gso_segment()(and sets
tx_path = false) when passing packets to userspace. The UFO will set
the ip_summed to CHECKSUM_NONE. There are a lot of warn logs. The warn
log is shown as below. I guess we should revert the patch.


>>
>> [1]
>> [321428.168903] WARNING: CPU: 0 PID: 2279 at net/core/dev.c:2562
>> skb_warn_bad_offload+0xc4/0x110
>> [321428.168906] san0: caps=(0x04009fbb58e9, 0x) len=6769
>> data_len=6727 gso_size=1480 gso_type=2 ip_summed=0
>>
>> [321428.168955] CPU: 0 PID: 2279 Comm: ruby-mri 4.11.12-200.fc25.x86_64 #1
>> [321428.168956] Hardware name: Supermicro SYS-1028U-TNRTP+/X10DRU-i+, BIOS 
>> 1.1 07/22/2015
>> [321428.168957] Call Trace:
>> [321428.168962]  dump_stack+0x63/0x86
>> [321428.168965]  __warn+0xcb/0xf0
>> [321428.168966]  warn_slowpath_fmt+0x5a/0x80
>> [321428.168968]  skb_warn_bad_offload+0xc4/0x110
>> [321428.168970]  __skb_gso_segment+0x190/0x1a0
>> [321428.168977]  queue_gso_packets+0x62/0x160 [openvswitch]
>> [321428.168992]  ovs_dp_upcall+0x31/0x60 [openvswitch]
>> [321428.168994]  ovs_dp_process_packet+0x10d/0x130 [openvswitch]
>> [321428.168997]  ovs_vport_receive+0x76/0xd0 [openvswitch]
>> [321428.169013]  internal_dev_xmit+0x28/0x60 [openvswitch]
>> [321428.169014]  dev_hard_start_xmit+0xa3/0x1f0
>> [321428.169016]  __dev_queue_xmit+0x592/0x650
>> [321428.169026]  dev_queue_xmit+0x10/0x20
>>
>> [2]
>> WARNING: CPU: 1 PID: 6768 at net/core/dev.c:2439 
>> skb_warn_bad_offload+0x2af/0x390 net/core/dev.c:2434
>> lo: caps=(0x00a2803b7c69, 0x) len=138 data_len=0 
>> gso_size=15883 gso_type=4 ip_summed=0
>> Kernel panic - not syncing: panic_on_warn set ...
>>
>> CPU: 1 PID: 6768 Comm: syz-executor1 Not tainted 4.9.0 #5
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS 
>> Google 01/01/2011
>> 8801c063ecd8 82346bdf 0001 1100380c7d2e
>> ed00380c7d26 41b58ab3 84b37e38 823468f1
>> 84820740 84f289c0 dc00 8801c063ee20
>> Call Trace:
>
> Why are you adding this trace that was part of the b2504a5dbef3
> changelog ?
I hope we can fix the bug with other solution.

>
>> [] __dump_stack lib/dump_stack.c:15 [inline]
>> [] dump_stack+0x2ee/0x3ef lib/dump_stack.c:51
>> [] panic+0x1fb/0x412 kernel/panic.c:179
>> [] __warn+0x1c4/0x1e0 kernel/panic.c:542
>> [] warn_slowpath_fmt+0xc5/0x100 kernel/panic.c:565
>> [] skb_warn_bad_offload+0x2af/0x390 net/core/dev.c:2434
>> [] __skb_gso_segment+0x482/0x780 net/core/dev.c:2706
>> [] skb_gso_segment include/linux/netdevice.h:3985 [inline]
>> [] validate_xmit_skb+0x5c9/0xc20 net/core/dev.c:2969
>> [] __dev_queue_xmit+0xe6b/0x1e70 net/core/dev.c:3383
>> [] dev_queue_xmit+0x17/0x20 net/core/dev.c:3424
>>
>> Cc: Eric Dumazet 
>> Cc: Willem de Bruijn 
>> Cc: Pravin B Shelar 
>> Signed-off-by: Tonghao Zhang 
>> ---
>
>
> IMO, this description is too confusing, I do not understand this patch.
sorry

Re: [PATCH] net: Reduce skb_warn_bad_offload() noise.

[Eric Dumazet]

On Fri, 2017-08-04 at 06:11 -0700, Tonghao Zhang wrote:
> This patch will revert the b2504a5dbe "net: reduce
> skb_warn_bad_offload() noise". The ovs will call the
> __skb_gso_segment() with tx false. When segmenting UDP with UFO,
> the __skb_gso_segment raises a warning as below [1], because the
> ip_summed is CHECKSUM_NONE. While the net-next has removed the
> UFO support, but the 4.11 and 4.12 kernel don't address that problem.
> 
> In the kernel, only qdisc_pkt_len_init() (__dev_queue_xmit call it.)
> uses the SKB_GSO_DODGY to do something. Other places just set it.
> The warn described in b2504a5dbe is shown [2]. We may know that:
> 1. the net_device don’t have qdisc.
> 2. the skb->ip_summed was changed to CHECKSUM_NONE. it maybe
> changed in skb_checksum_help() when calling validate_xmit_skb().
> or other place.
> 
> And we should not revert the  6e7bc478c9 "net: skb_needs_check() accepts
> CHECKSUM_NONE for tx". The check is necessary.

Why is it necessary ?

If you revert b2504a5dbe, then we also need to revert 6e7bc478c9,
unless you provide hard facts.

> 
> [1]
> [321428.168903] WARNING: CPU: 0 PID: 2279 at net/core/dev.c:2562
> skb_warn_bad_offload+0xc4/0x110
> [321428.168906] san0: caps=(0x04009fbb58e9, 0x) len=6769
> data_len=6727 gso_size=1480 gso_type=2 ip_summed=0
> 
> [321428.168955] CPU: 0 PID: 2279 Comm: ruby-mri 4.11.12-200.fc25.x86_64 #1
> [321428.168956] Hardware name: Supermicro SYS-1028U-TNRTP+/X10DRU-i+, BIOS 
> 1.1 07/22/2015
> [321428.168957] Call Trace:
> [321428.168962]  dump_stack+0x63/0x86
> [321428.168965]  __warn+0xcb/0xf0
> [321428.168966]  warn_slowpath_fmt+0x5a/0x80
> [321428.168968]  skb_warn_bad_offload+0xc4/0x110
> [321428.168970]  __skb_gso_segment+0x190/0x1a0
> [321428.168977]  queue_gso_packets+0x62/0x160 [openvswitch]
> [321428.168992]  ovs_dp_upcall+0x31/0x60 [openvswitch]
> [321428.168994]  ovs_dp_process_packet+0x10d/0x130 [openvswitch]
> [321428.168997]  ovs_vport_receive+0x76/0xd0 [openvswitch]
> [321428.169013]  internal_dev_xmit+0x28/0x60 [openvswitch]
> [321428.169014]  dev_hard_start_xmit+0xa3/0x1f0
> [321428.169016]  __dev_queue_xmit+0x592/0x650
> [321428.169026]  dev_queue_xmit+0x10/0x20
> 
> [2]
> WARNING: CPU: 1 PID: 6768 at net/core/dev.c:2439 
> skb_warn_bad_offload+0x2af/0x390 net/core/dev.c:2434
> lo: caps=(0x00a2803b7c69, 0x) len=138 data_len=0 
> gso_size=15883 gso_type=4 ip_summed=0
> Kernel panic - not syncing: panic_on_warn set ...
> 
> CPU: 1 PID: 6768 Comm: syz-executor1 Not tainted 4.9.0 #5
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS 
> Google 01/01/2011
> 8801c063ecd8 82346bdf 0001 1100380c7d2e
> ed00380c7d26 41b58ab3 84b37e38 823468f1
> 84820740 84f289c0 dc00 8801c063ee20
> Call Trace:

Why are you adding this trace that was part of the b2504a5dbef3
changelog ?

> [] __dump_stack lib/dump_stack.c:15 [inline]
> [] dump_stack+0x2ee/0x3ef lib/dump_stack.c:51
> [] panic+0x1fb/0x412 kernel/panic.c:179
> [] __warn+0x1c4/0x1e0 kernel/panic.c:542
> [] warn_slowpath_fmt+0xc5/0x100 kernel/panic.c:565
> [] skb_warn_bad_offload+0x2af/0x390 net/core/dev.c:2434
> [] __skb_gso_segment+0x482/0x780 net/core/dev.c:2706
> [] skb_gso_segment include/linux/netdevice.h:3985 [inline]
> [] validate_xmit_skb+0x5c9/0xc20 net/core/dev.c:2969
> [] __dev_queue_xmit+0xe6b/0x1e70 net/core/dev.c:3383
> [] dev_queue_xmit+0x17/0x20 net/core/dev.c:3424
> 
> Cc: Eric Dumazet 
> Cc: Willem de Bruijn 
> Cc: Pravin B Shelar 
> Signed-off-by: Tonghao Zhang 
> ---


IMO, this description is too confusing, I do not understand this patch.

[PATCH] net: Reduce skb_warn_bad_offload() noise.

[Tonghao Zhang]

This patch will revert the b2504a5dbe "net: reduce
skb_warn_bad_offload() noise". The ovs will call the
__skb_gso_segment() with tx false. When segmenting UDP with UFO,
the __skb_gso_segment raises a warning as below [1], because the
ip_summed is CHECKSUM_NONE. While the net-next has removed the
UFO support, but the 4.11 and 4.12 kernel don't address that problem.

In the kernel, only qdisc_pkt_len_init() (__dev_queue_xmit call it.)
uses the SKB_GSO_DODGY to do something. Other places just set it.
The warn described in b2504a5dbe is shown [2]. We may know that:
1. the net_device don’t have qdisc.
2. the skb->ip_summed was changed to CHECKSUM_NONE. it maybe
changed in skb_checksum_help() when calling validate_xmit_skb().
or other place.

And we should not revert the  6e7bc478c9 "net: skb_needs_check() accepts
CHECKSUM_NONE for tx". The check is necessary.

[1]
[321428.168903] WARNING: CPU: 0 PID: 2279 at net/core/dev.c:2562
skb_warn_bad_offload+0xc4/0x110
[321428.168906] san0: caps=(0x04009fbb58e9, 0x) len=6769
data_len=6727 gso_size=1480 gso_type=2 ip_summed=0

[321428.168955] CPU: 0 PID: 2279 Comm: ruby-mri 4.11.12-200.fc25.x86_64 #1
[321428.168956] Hardware name: Supermicro SYS-1028U-TNRTP+/X10DRU-i+, BIOS 1.1 
07/22/2015
[321428.168957] Call Trace:
[321428.168962]  dump_stack+0x63/0x86
[321428.168965]  __warn+0xcb/0xf0
[321428.168966]  warn_slowpath_fmt+0x5a/0x80
[321428.168968]  skb_warn_bad_offload+0xc4/0x110
[321428.168970]  __skb_gso_segment+0x190/0x1a0
[321428.168977]  queue_gso_packets+0x62/0x160 [openvswitch]
[321428.168992]  ovs_dp_upcall+0x31/0x60 [openvswitch]
[321428.168994]  ovs_dp_process_packet+0x10d/0x130 [openvswitch]
[321428.168997]  ovs_vport_receive+0x76/0xd0 [openvswitch]
[321428.169013]  internal_dev_xmit+0x28/0x60 [openvswitch]
[321428.169014]  dev_hard_start_xmit+0xa3/0x1f0
[321428.169016]  __dev_queue_xmit+0x592/0x650
[321428.169026]  dev_queue_xmit+0x10/0x20

[2]
WARNING: CPU: 1 PID: 6768 at net/core/dev.c:2439 
skb_warn_bad_offload+0x2af/0x390 net/core/dev.c:2434
lo: caps=(0x00a2803b7c69, 0x) len=138 data_len=0 
gso_size=15883 gso_type=4 ip_summed=0
Kernel panic - not syncing: panic_on_warn set ...

CPU: 1 PID: 6768 Comm: syz-executor1 Not tainted 4.9.0 #5
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 
01/01/2011
8801c063ecd8 82346bdf 0001 1100380c7d2e
ed00380c7d26 41b58ab3 84b37e38 823468f1
84820740 84f289c0 dc00 8801c063ee20
Call Trace:
[] __dump_stack lib/dump_stack.c:15 [inline]
[] dump_stack+0x2ee/0x3ef lib/dump_stack.c:51
[] panic+0x1fb/0x412 kernel/panic.c:179
[] __warn+0x1c4/0x1e0 kernel/panic.c:542
[] warn_slowpath_fmt+0xc5/0x100 kernel/panic.c:565
[] skb_warn_bad_offload+0x2af/0x390 net/core/dev.c:2434
[] __skb_gso_segment+0x482/0x780 net/core/dev.c:2706
[] skb_gso_segment include/linux/netdevice.h:3985 [inline]
[] validate_xmit_skb+0x5c9/0xc20 net/core/dev.c:2969
[] __dev_queue_xmit+0xe6b/0x1e70 net/core/dev.c:3383
[] dev_queue_xmit+0x17/0x20 net/core/dev.c:3424

Cc: Eric Dumazet 
Cc: Willem de Bruijn 
Cc: Pravin B Shelar 
Signed-off-by: Tonghao Zhang 
---
 net/core/dev.c | 11 +++
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 1d75499..97e6989 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2753,11 +2753,11 @@ static inline bool skb_needs_check(struct sk_buff *skb, 
bool tx_path)
 struct sk_buff *__skb_gso_segment(struct sk_buff *skb,
  netdev_features_t features, bool tx_path)
 {
-   struct sk_buff *segs;
-
if (unlikely(skb_needs_check(skb, tx_path))) {
int err;
 
+   skb_warn_bad_offload(skb);
+
/* We're going to init ->check field in TCP or UDP header */
err = skb_cow_head(skb, 0);
if (err < 0)
@@ -2786,12 +2786,7 @@ struct sk_buff *__skb_gso_segment(struct sk_buff *skb,
skb_reset_mac_header(skb);
skb_reset_mac_len(skb);
 
-   segs = skb_mac_gso_segment(skb, features);
-
-   if (unlikely(skb_needs_check(skb, tx_path)))
-   skb_warn_bad_offload(skb);
-
-   return segs;
+   return skb_mac_gso_segment(skb, features);
 }
 EXPORT_SYMBOL(__skb_gso_segment);
 
-- 
1.8.3.1