linux-ext4 - Re: 4.5.1: UBSAN: Undefined behaviour in ./arch/x86/include/asm/atomic.h:156:9

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Message-ID: <2121831c-e066-bb83-3b66-d3d3907064bf@gmail.com>
Date:	Thu, 21 Apr 2016 13:17:19 +0200
From:	Martin MOKREJŠ <mmokrejs@...il.com>
To:	Greg KH <greg@...ah.com>
Cc:	USB list <linux-usb@...r.kernel.org>, linux-ext4@...r.kernel.org
Subject: Re: 4.5.1: UBSAN: Undefined behaviour in
 ./arch/x86/include/asm/atomic.h:156:9

Greg KH wrote:
> On Thu, Apr 21, 2016 at 11:18:16AM +0200, Martin MOKREJŠ wrote:
>> Hi Greg,
>>
>> thank you for your answer.
>>
>> Greg KH wrote:
>>> On Thu, Apr 21, 2016 at 12:22:49AM +0200, Martin MOKREJŠ wrote:
>>>> Hi,
>>>>   I am not certain to to forward this to, so I am trying linux-usb and ext4. Please add relevant people/lists, thank you.
>>>>
>>>> # dmesg | grep "UBSAN: Undefined behaviour"
>>>> [    2.638843] UBSAN: Undefined behaviour in drivers/usb/host/ehci-hub.c:873:47
>>>> [    8.553620] UBSAN: Undefined behaviour in fs/ext4/mballoc.c:2612:15
>>>> [   25.341048] UBSAN: Undefined behaviour in fs/ext4/mballoc.c:1274:11
>>>> [   41.460542] UBSAN: Undefined behaviour in ./arch/x86/include/asm/atomic.h:156:9
>>>> #
>>>>
>>>> Full dmesg is attached.
>>>>
>>>> Hope this helps,
>>>> Martin
>>>
>>>> [    0.000000] Linux version 4.5.1-default-pciehp (root@...tro) (gcc version 5.3.0 (Gentoo 5.3.0 p1.0, pie-0.6.5) ) #1 SMP Thu Apr 14 14:17:48 CEST 2016
>>>
>>> Please try 4.6-rc4, a number of UBSAN issues have been fixed there.
>>
>> They are still present:
>>
>> # dmesg | grep "UBSAN: Undefined behaviour"
>> [    3.113405] UBSAN: Undefined behaviour in drivers/usb/host/ehci-hub.c:877:47
>> [    5.678949] UBSAN: Undefined behaviour in fs/ext4/mballoc.c:2619:15
>> [   18.345599] UBSAN: Undefined behaviour in fs/ext4/mballoc.c:1281:11
>> [   34.544363] UBSAN: Undefined behaviour in ./arch/x86/include/asm/atomic.h:156:9
>
> Ah, sorry about that, turns out this is a bug in the tool, not in the
> code, see the thread from a few weeks ago in the linux-usb archives with
> the subject:
> 	Subject: function ehci_hub_control in ehci-hub.c
> for all of the details.

Thank you. I am not a programmer so I only infer from this thread I should disable
the UBSAN in .config. The kernel code is technically correct but triggers false alarm.
Maybe the UBSAN could have a whitelist of falsely matching locations?

https://www.mail-archive.com/linux-usb@vger.kernel.org/msg72438.html

Did anybody check the ext4 code or is it also just believed the UBSAN complaint is
just broken and code changes are not necessary?
Martin
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