Re: Divide error in minstrel_ht_get_tp_avg()

[Oleksandr Natalenko <oleksandr@natalenko.name>]

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@natalenko.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@natalenko.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)