linux-kernel - Re: PROBLEM: NULL pointer dereference in kernel 4.14.6

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Message-ID: <95d82ae1-fe4c-4eee-8e94-fa0df3e25532@incorrekt.com>
Date:   Mon, 18 Dec 2017 19:56:17 +0000
From:   Bronek Kozicki <brok@...orrekt.com>
To:     vcaputo@...garu.com, linux-kernel@...r.kernel.org, tj@...nel.org
Cc:     cgroups@...r.kernel.org
Subject: Re: PROBLEM: NULL pointer dereference in kernel 4.14.6

On 17/12/2017 23:24, vcaputo@...garu.com wrote:
> On Sun, Dec 17, 2017 at 05:49:44PM +0000, Bronek Kozicki wrote:
>> I just upgraded to 4.14.7 and tried to reproduce this error, this time under strace. As you can see this happens when systemctl tries to read a specific entry under /sys/fs . In case this matters, the entry is for a small virtual machine running under qemu/kvm and managed by libvirt.
>>
>> open("/sys/fs/cgroup/unified/machine.slice", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 5
>> fstat(5, {st_mode=S_IFDIR|0755, st_size=0, ...}) = 0
>> getdents(5, /* 12 entries */, 32768)    = 464
>> openat(AT_FDCWD, "/sys/fs/cgroup/unified/machine.slice/machine-qemu\\x2d1\\x2dkartuzy\\x2dspice.scope/cgroup.procs", O_RDONLY|O_CLOEXEC) = 8
>> fstat(8, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
>> read(8,  <unfinished ...>)              = ?
>> +++ killed by SIGKILL +++
>> [1]    12078 killed     strace -- systemctl status
>>
>>
> 
> This recently came through lkml, may be related:
> https://marc.info/?l=linux-kernel&m=151320108922415&w=2

thank you, it certainly seems related. Is there some debugging option I could enable, or patch I could apply, which would make the point of data corruption easier to find? I'm ok taking untested patches, if that helps finding the location of the bug.


B.