linux-kernel - Re: [Linux 5.2.7] powertop --auto-tune: BUG: kernel NULL pointer dereference, address: 0000000000000000

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Date:   Sat, 10 Aug 2019 20:28:37 +0200
From:   Paul Menzel <pmenzel@...gen.mpg.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     x86@...nel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Linux 5.2.7] powertop --auto-tune: BUG: kernel NULL pointer
 dereference, address: 0000000000000000

Dear Thomas,


On 10.08.19 19:31, Thomas Gleixner wrote:

> On Sat, 10 Aug 2019, Paul Menzel wrote:
>>
>> I have no idea, who to report this to, so I please refer me to the correct
>> list.
> 
> I have no idea yet either :)
> 
>> With Linux 5.2.7 from Debian Sid/unstable and PowerTOP 2.10, executing
>>
>>      sudo powertop --auto-tune
>>
>> causes a NULL pointer dereference, and the graphical session crashes due to an
>> effect on the i915 driver. It worked in the past with the 4.19 series from
>> Debian.
>>
>> Here is the trace, and please find all Linux kernel logs attached.
>>
>>> [ 2027.170589] BUG: kernel NULL pointer dereference, address:
>>> 0000000000000000
>>> [ 2027.170600] #PF: supervisor instruction fetch in kernel mode
>>> [ 2027.170604] #PF: error_code(0x0010) - not-present page
>>> [ 2027.170609] PGD 0 P4D 0 [ 2027.170619] Oops: 0010 [#1] SMP PTI
> ...
>>> [ 2027.170730]  do_dentry_open+0x13a/0x370
> 
> If you have compiled with debug info, please decode the line:
> 
>    linux/scripts/faddr2line vmlinux do_dentry_open+0x13a/0x370
> 
> That gives us the fops pointer which is NULL.

Hah, luckily it’s reproducible.

```
$ scripts/faddr2line /usr/lib/debug/boot/vmlinux-5.2.0-2-amd64 
do_dentry_open+0x13a/0x370
do_dentry_open+0x13a/0x370:
do_dentry_open at fs/open.c:799
```

>>> [ 2027.170745]  path_openat+0x2c6/0x1480
>>> [ 2027.170757]  ? terminate_walk+0xe6/0x100
>>> [ 2027.170767]  ? path_lookupat.isra.48+0xa3/0x220
>>> [ 2027.170779]  ? reuse_swap_page+0x105/0x320
>>> [ 2027.170791]  do_filp_open+0x93/0x100
>>> [ 2027.170804]  ? __check_object_size+0x15d/0x189
>>> [ 2027.170816]  do_sys_open+0x184/0x220
>>> [ 2027.170828]  do_syscall_64+0x53/0x130
>>> [ 2027.170837]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> That's an open crashing. We just don't know which file. Is the machine
> completely hosed after that or is it just the graphics stuff dying?

No, the graphical login manager showed up, and I could log back in, and 
continue using hte machine.

> If it's not completely dead then instead of running it from your graphical
> desktop you could switch to a VGA terminal Alt+Ctrl+F1 (or whatever
> function key your distro maps to) after boot and run powertop with strace
> from there:
> 
>    strace -f -o xxx.log powertop
> 
> With a bit of luck xxx.log should contain the information about the file it
> tries to open.

```
2157  access("/sys/class/drm/card0/power/rc6_residency_ms", R_OK) = 0
2157  openat(AT_FDCWD, 
"/sys/kernel/debug/tracing/events/power/cpu_idle/id", O_RDONLY) = ?
2157  +++ killed by SIGKILL +++
```

> Alternatively if you have a serial console you can enable the
> sys_enter_open* tracepoints:
> 
> # echo 1 >/sys/kernel/debug/tracing/events/syscalls/sys_enter_open
> # echo 1 >/sys/kernel/debug/tracing/events/syscalls/sys_enter_openat
> 
> Either add 'ftrace_dump_on_oops' to the kernel command line or enable it
> from the shell:
> 
> # echo 1 > /proc/sys/kernel/ftrace_dump_on_oops
> 
> Then run powertop. After the crash it will take some time to spill out the
> trace buffer over serial, but it will pinpoint the offending file.

I do not have serial console on this device.

> Once we know which file it is, we also know who needs to stare at it :)

Hopefully the information is enough.


Kind regards,

Paul

View attachment "powertop-strace.txt" of type "text/plain" (63198 bytes)