[Ovmsdev] Heap corruption alerts & heap tracing

[Michael Balzer]

Everyone,

from the crash reports 
(https://ovms.dexters-web.de/firmware/developer/), most remaining 
crashes seem to be caused by heap corruptions.

Not all heap corruptions are easily detectable from the backtrace 
analysis, and the component or action causing the corruption isn't 
detectable at all that way. So I've added a debug option to enable a 
regular heap integrity check ever 5 minutes, with the module sending an 
alert notification when a corruption has been detected. Example:

> Heap corruption detected; reboot advised ASAP!
> Please forward including task records and system log:
>
> CORRUPT HEAP: Bad tail at 0x3f8e44f0 owner 0x3ffea9bc. Expected 
> 0xbaad5678 got 0xbaad5600

I also added a final heap integrity check to our crash handler, so the 
crash debug records should now show exactly which crashes occurred with 
a corrupted heap.

In combination with the system log and the task log, that should give us 
some more opportunities to narrow down the cause(s).

I've also added task ownershop to the heap corruption report. Note, this 
needs my latest additions to our esp-idf fork, so take care to pull 
these before building.

Be aware, task ownership of corrupted blocks doesn't necessarily tell 
about the task doing the corruption. If the tail canary is compromised, 
and no other block located before that block is compromised, it *may* be 
that task doing the out of bounds write. But it may also be a use after 
free of some previous owner. So take task ownership with a grain of salt.

The corruptions are most probably caused by some unclean shutdown of a 
component or by an undetected race conditions within a shutdown 
procedure. The heap seems to be stable on modules with standard 
configurations and components not being started & shut down on a regular 
base. The heap corruptions are especially present now with Smart (SQ) 
vehicles -- as the Smart doesn't keep the 12V battery charged from the 
main battery, most Smart users probably use the power management to shut 
down Wifi and/or modem while parking.

So our main focus should be on analysing what happens before the 
corruption. Ask users reporting heap corruptions to provide their system 
logs, and possibly also their task logs. To encourage enabling these, 
I've added the config to the web UI (Config→Notifications).


Once you can reproduce (!) the corruption, heap tracing might provide 
some more insight as to where exactly the corruption occurs.

Heap tracing means recording all memory allocations and frees. This adds 
a recording layer on top of the heap functions, so comes with some cost, 
even when inactive. CPU overhead is low, but stack overhead may be an 
issue, so I think we should not enable heap tracing by default for now, 
but rather use a debug build specifically in cases we think heap tracing 
might help.

To enable heap tracing on some user device, I've reworked the ESP-IDF 
heap tracing to enable remote execution and to also include the task 
handles performing the allocations and deallocations.

To enable heap tracing for a build:

>   * Under|makemenuconfig|, navigate
>     to|Componentsettings|->|HeapMemoryDebugging|and
>     setCONFIG_HEAP_TRACING
>     <https://docs.espressif.com/projects/esp-idf/en/v3.3/api-reference/kconfig.html#config-heap-tracing>.
>

There's also an option to set the number of stack backtrace frames. 
Tracing with two frames is mostly useless in our (C++) context, as it 
will normally only show some inner frames of the allocator. I've tried 
raising that to 5, and got an immediate crash in the mdns component. I 
assume raising the depth will need raising some stack sizes. If you find 
a good compromise, please report.

Heap tracing will work best in a reduced configuration. In normal 
operation, my module fills a 500 records buffer within seconds. The 
buffer is a FIFO, so will always contain the latest n allocations, and 
the last entry in the dump is the newest one.

Reduced example dump:

> OVMS# mod trace dump
> Heap tracing started/resumed at ccount 0x1c70b453 = logtime 19415
> 1000 allocations trace (1000 entry buffer)
> 258 bytes (@ 0x3fffb3a8) allocated CPU 1 task 0x3ffc92e8 ccount 
> 0x89074e74 caller 0x4031d4f0:0x4031f553
> freed task 0x3ffc92e8 by 0x4031d514:0x4031f595
> 201 bytes (@ 0x3f8e92f4) allocated CPU 1 task 0x3fff2290 ccount 
> 0x89218bbc caller 0x40131b12:0x4014633c
> freed task 0x3fff2290 by 0x402c4634:0x402cb14d
> 112 bytes (@ 0x3ffebc80) allocated CPU 1 task 0x3ffee918 ccount 
> 0x8a25d47c caller 0x4029b455:0x4017a5dc
> freed task 0x3ffee918 by 0x4029b744:0x4017a5dc
> 112 bytes (@ 0x3ffebc80) allocated CPU 1 task 0x3ffee918 ccount 
> 0x8db89c40 caller 0x4029b455:0x4017a5dc
> freed task 0x3ffee918 by 0x4029b744:0x4017a5dc
> 12 bytes (@ 0x3f85d0d4) allocated CPU 1 task 0x3ffdfcd0 ccount 
> 0x8fccdfd8 caller 0x40131b12:0x4014633c
> freed task 0x3ffdfcd0 by 0x402c4634:0x401e0a04
> […]
> 12 bytes (@ 0x3f85d0d4) allocated CPU 1 task 0x3ffdfcd0 ccount 
> 0x3cf429f4 caller 0x40131b12:0x4014633c
> freed task 0x3ffdfcd0 by 0x402c4634:0x401e0a04
> 112 bytes (@ 0x3ffebc80) allocated CPU 1 task 0x3ffee918 ccount 
> 0x3fe1a0bc caller 0x4029b455:0x4017a5dc
> 229 bytes alive in trace (3/1000 allocations)
> total allocations 1207 total frees 3356
> (NB: Buffer has overflowed; so trace data is incomplete.)

"ccount" is the ESP32 CCOUNT register (CPU cycles), so provides an 
orientation on when the allocation occurred.


Some more basics on heap debugging are also covered here: 
https://docs.espressif.com/projects/esp-idf/en/v3.3/api-reference/system/heap_debug.html

Regards,
Michael

-- 
Michael Balzer * Am Rahmen 5 * D-58313 Herdecke
Fon 02330 9104094 * Handy 0176 20698926

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