Re: NULL pointer dereference in msi_set_mask_bit

[Paul Menzel <pmenzel+linux-pci@molgen.mpg.de>]

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[I removed the folks from the unrelated patches.]

Dear Thomas,


On 07/18/18 17:39, Thomas Gleixner wrote:

> On Wed, 18 Jul 2018, Paul Menzel wrote:
>> On 07/18/18 17:02, Thomas Gleixner wrote:
>>>>> 93.885: [   23.020572] BUG: unable to handle kernel NULL pointer dereference at 000000000000003c
>>>>> 93.885: [   23.029011] PGD 0 P4D 0 
>>>>> 93.885: [   23.031670] Oops: 0000 [#1] SMP NOPTI
>>>>> 93.885: [   23.035455] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.18.0-rc5+ #1
>>>>> 93.885: [   23.042079] Hardware name: MSI MS-7A37/B350M MORTAR (MS-7A37), BIOS 1.G1 05/17/2018
>>>>> 93.886: [   23.049868] RIP: 0010:msi_set_mask_bit+0xe/0x70
>>>
>>>>> 93.913: [   23.049868] Code: 00 53 48 89 fb e8 12 f8 ff ff 48 89 df 5b e9 c9 fe ff ff 66 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 53 48 8b 47 10 48 8b 58 10 <f6> 43 3c 01 74 3c 8b 15 2e 85 21 01 31 c0 85 d2 75 25 8b 43 38 48
>>>
>>>   f6 43 3c 01          	testb  $0x1,0x3c(%rbx)
>>>
>>> That's:
>>>
>>> 	if (desc->msi_attrib.is_msix)
>>
>> Is there a tool to translate that?
> 
> That tool is called brain :)
> 
> Seriously, what you can do is run the 'Code: ...' line through
> scripts/decodecode and that will show you the disassembly. Now staring at
> msi_set_mask_bit() makes it pretty obvious which part it is and the offset of
> msi_attrib in msi_desc is 0x3c, which matches the BUG: line above. You can
> figure that out by counting or by using pahole.
> 
> Of course if you have the vmlinux around then scripts/faddr2line is what
> you want to use.

Thank you for the explanation. I’ll keep that in mind for the future.

>>>>> 93.957: [   23.049880] RSP: 0018:ffff9e8e5e803f78 EFLAGS: 00010046
>>>>> 93.957: [   23.049881] RAX: ffff9e8e45919000 RBX: 0000000000000000 RCX: 0000000000000000
>>>>> 93.958: [   23.049882] RDX: ffff9e8e45919000 RSI: 0000000000000001 RDI: ffff9e8e45919098
>>>>> 93.958: [   23.049882] RBP: ffff9e8e45919098 R08: 0000000000000000 R09: 0000000000000000
>>>>> 93.958: [   23.049882] R10: 0000000000000000 R11: 0000000000000000 R12: ffff9e8e45919000
>>>>> 93.958: [   23.049883] R13: 0000000000000027 R14: 0000000000000000 R15: 0000000000000000
>>>>> 93.959: [   23.049884] FS:  0000000000000000(0000) GS:ffff9e8e5e800000(0000) knlGS:0000000000000000
>>>>> 93.959: [   23.049884] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>>> 93.959: [   23.049885] CR2: 000000000000003c CR3: 00000003fc5a4000 CR4: 00000000003406f0
>>>>> 93.959: [   23.049885] Call Trace:
>>>>> 93.959: [   23.049887]  <IRQ>
>>>>> 93.960: [   23.049889]  __irq_move_irq+0x3c/0x70
>>>>> 93.960: [   23.049892]  apic_ack_irq+0x2b/0x30
>>>>> 93.960: [   23.049893]  handle_edge_irq+0x7d/0x1d0
>>>>> 93.960: [   23.049895]  handle_irq+0x1f/0x30
>>>>> 93.960: [   23.049898]  do_IRQ+0x41/0xc0
>>>>> 93.960: [   23.049899]  common_interrupt+0xf/0xf
>>>>> 93.961: [   23.049900]  </IRQ>
>>>
>>> and desc comes from irq_data->common->msi_desc
>>>
>>> I have no idea how that can happen for an MSI interrupt.
>>>
>>> Paul, is this reproducible?
>>
>> No, unfortunately not. I only hit this once, since I attached the serial
>> console.
> 
> Bah. Could you please enable GENERIC_IRQ_DEBUGFS and after a successful
> boot up provide me the content of all files in /sys/kernel/debug/irq/ and
> its subfolders?

Sure, please find them attached. `7zr x sys-kernel-debug-irq.7z` should
extract it.

```
$ sudo ls -R /sys/kernel/debug/irq/
/sys/kernel/debug/irq/:
domains  irqs

/sys/kernel/debug/irq/domains:
AMD-IR-0  AMD-IR-MSI-0-2  default  IO-APIC-IR-0  IO-APIC-IR-1  PCI-MSI-2  VECTOR

/sys/kernel/debug/irq/irqs:
0   11	14  24	27  3	32  35	38  40	43  46	49  51	54  6  9
1   12	15  25	28  30	33  36	39  41	44  47	5   52	55  7
10  13	2   26	29  31	34  37	4   42	45  48	50  53	56  8
```

> I assume you have irqbalanced running, right?

Yes, I do.

```
$ systemctl status irqbalance
● irqbalance.service - irqbalance daemon
   Loaded: loaded (/lib/systemd/system/irqbalance.service; enabled; vendor preset: enabled)
   Active: active (running) since Wed 2018-07-18 18:26:05 CEST; 5min ago
 Main PID: 491 (irqbalance)
    Tasks: 2 (limit: 4915)
   Memory: 1.4M
   CGroup: /system.slice/irqbalance.service
           └─491 /usr/sbin/irqbalance --foreground

Jul 18 18:26:05 tokeiihto systemd[1]: Started irqbalance daemon.
```


Kind regards,

Paul

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