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Date: Mon, 7 Jun 2021 16:52:23 +0200
From: Oleksandr Natalenko <oleksandr@...alenko.name>
To: linux-kernel@...r.kernel.org
Cc: netdev@...r.kernel.org, linux-wireless@...r.kernel.org,
Jakub Kicinski <kuba@...nel.org>,
"David S. Miller" <davem@...emloft.net>,
Johannes Berg <johannes@...solutions.net>,
Felix Fietkau <nbd@....name>
Subject: Re: Divide error in minstrel_ht_get_tp_avg()
On Sat, May 29, 2021 at 06:57:29PM +0200, Oleksandr Natalenko wrote:
> The following woe happened on my home router running as a Wi-Fi AP with
> MT7612:
>
> ```
> [16592.157962] divide error: 0000 [#1] PREEMPT SMP PTI
> [16592.163169] CPU: 2 PID: 683 Comm: mt76-tx phy0 Tainted: G C 5.12.0-pf4 #1
> [16592.171745] Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./J3710-ITX, BIOS P1.50 04/16/2018
> [16592.182155] RIP: 0010:minstrel_ht_get_tp_avg+0xb1/0x100 [mac80211]
> [16592.188795] Code: 04 00 00 00 7f 1c 31 c9 81 fa f1 49 02 00 0f 9c c1 8d 0c cd 08 00 00 00 eb 08 8b 47 30 b9 01 00 00 00 69 c0 e8 03 00 00 31 d2 <f7> f1 ba 66 0e 00 00 39 d6 0f 4f f2 49 63 d1 48 8d 0c 52 48 8d 0c
> [16592.208796] RSP: 0018:ffffb6a601293be8 EFLAGS: 00010246
> [16592.214292] RAX: 000000000001a5e0 RBX: ffff9c658ee95170 RCX: 0000000000000000
> [16592.222054] RDX: 0000000000000000 RSI: 0000000000000585 RDI: ffff9c658ee94000
> [16592.229620] RBP: 0000000000000006 R08: 0000000000000006 R09: 0000000000000012
> [16592.237254] R10: 0000000000000007 R11: 0000000000000000 R12: ffff9c658ee94000
> [16592.244828] R13: 0000000000000012 R14: ffff9c658ee9534c R15: 0000000000000585
> [16592.252635] FS: 0000000000000000(0000) GS:ffff9c66f8500000(0000) knlGS:0000000000000000
> [16592.261086] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [16592.267326] CR2: 0000555d81abe7b8 CR3: 0000000116c10000 CR4: 00000000001006e0
> [16592.274904] Call Trace:
> [16592.277549] minstrel_ht_update_stats+0x1fe/0x1320 [mac80211]
> [16592.283730] minstrel_ht_tx_status+0x67f/0x710 [mac80211]
> [16592.289442] rate_control_tx_status+0x6e/0xb0 [mac80211]
> [16592.295267] ieee80211_tx_status_ext+0x22e/0xb00 [mac80211]
> [16592.311290] ieee80211_tx_status+0x7d/0xa0 [mac80211]
> [16592.316714] mt76_tx_status_unlock+0x83/0xa0 [mt76]
> [16592.321999] mt76x02_send_tx_status+0x1b7/0x400 [mt76x02_lib]
> [16592.334516] mt76x02_tx_worker+0x8f/0xd0 [mt76x02_lib]
> [16592.340091] __mt76_worker_fn+0x78/0xb0 [mt76]
> [16592.345067] kthread+0x183/0x1b0
> [16592.353378] ret_from_fork+0x22/0x30
> ```
>
> `faddr2line` says it is this:
>
> ```
> 430 int
> 431 minstrel_ht_get_tp_avg(struct minstrel_ht_sta *mi, int group, int rate,
> 432 int prob_avg)
> 433 {
> …
> 435 unsigned int ampdu_len = 1;
> …
> 441 if (minstrel_ht_is_legacy_group(group))
> …
> 443 else
> 444 ampdu_len = minstrel_ht_avg_ampdu_len(mi);
> …
> 446 nsecs = 1000 * overhead / ampdu_len;
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> ```
>
> So, it seems `minstrel_ht_avg_ampdu_len()` can return 0, which is not
> really legitimate.
>
> Looking at `minstrel_ht_avg_ampdu_len()`, I see the following:
>
> ```
> 16:#define MINSTREL_SCALE 12
> …
> 18:#define MINSTREL_TRUNC(val) ((val) >> MINSTREL_SCALE)
> ```
>
> ```
> 401 static unsigned int
> 402 minstrel_ht_avg_ampdu_len(struct minstrel_ht_sta *mi)
> 403 {
> …
> 406 if (mi->avg_ampdu_len)
> 407 return MINSTREL_TRUNC(mi->avg_ampdu_len);
> ```
>
> So, likely, `mi->avg_ampdu_len` is non-zero, but it's too small, hence
> right bitshift makes it zero.
>
> Should there be some protection against such a situation?
>
> Felix, could you maybe check this? Looks like you were the last one to
> overhaul that part of code.
Quick hack from me:
commit a23d3f77658de968c5bb852ba478402c93958f7f
Author: Oleksandr Natalenko <oleksandr@...alenko.name>
Date: Mon Jun 7 16:45:45 2021 +0200
mac80211: minstrel_ht: force ampdu_len to be > 0
This is a hack.
Work around the following crash:
```
divide error: 0000 [#1] PREEMPT SMP PTI
CPU: 2 PID: 683 Comm: mt76-tx phy0 Tainted: G C 5.12.0-pf4 #1
Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./J3710-ITX, BIOS P1.50 04/16/2018
RIP: 0010:minstrel_ht_get_tp_avg+0xb1/0x100 [mac80211]
…
Call Trace:
minstrel_ht_update_stats+0x1fe/0x1320 [mac80211]
minstrel_ht_tx_status+0x67f/0x710 [mac80211]
rate_control_tx_status+0x6e/0xb0 [mac80211]
ieee80211_tx_status_ext+0x22e/0xb00 [mac80211]
ieee80211_tx_status+0x7d/0xa0 [mac80211]
mt76_tx_status_unlock+0x83/0xa0 [mt76]
mt76x02_send_tx_status+0x1b7/0x400 [mt76x02_lib]
mt76x02_tx_worker+0x8f/0xd0 [mt76x02_lib]
__mt76_worker_fn+0x78/0xb0 [mt76]
kthread+0x183/0x1b0
ret_from_fork+0x22/0x30
```
Link: https://lore.kernel.org/lkml/20210529165728.bskaozwtmwxnvucx@spock.localdomain/
Signed-off-by: Oleksandr Natalenko <oleksandr@...alenko.name>
---
diff --git a/net/mac80211/rc80211_minstrel_ht.c b/net/mac80211/rc80211_minstrel_ht.c
index ecad9b10984f..6ad188c4101e 100644
--- a/net/mac80211/rc80211_minstrel_ht.c
+++ b/net/mac80211/rc80211_minstrel_ht.c
@@ -440,8 +440,13 @@ minstrel_ht_get_tp_avg(struct minstrel_ht_sta *mi, int group, int rate,
if (minstrel_ht_is_legacy_group(group))
overhead = mi->overhead_legacy;
- else
+ else {
ampdu_len = minstrel_ht_avg_ampdu_len(mi);
+ if (unlikely(!ampdu_len)) {
+ pr_err_once("minstrel_ht_get_tp_avg: ampdu_len == 0!");
+ ampdu_len = 1;
+ }
+ }
nsecs = 1000 * overhead / ampdu_len;
nsecs += minstrel_mcs_groups[group].duration[rate] <<
--
Oleksandr Natalenko (post-factum)
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