Alignment fault in pvDataCPP

[Till Straumann]

We experienced a (reproduceable) crash of pvData on RTEMS/powerpc.

I tracked it down to the ByteBuffer (de-)serialization code which
does IMHO ugly things:

class ByteBuffer {
  char *position;

  ...

  template <typename T>
  inline void put(T value)
  {
    ...
    *((T*)position) = value;
  }
};

PLEASE: don't code like this.

The template was instantiated for a 'double' value and -- since this is a 'byte buffer' -- it is obvious that the 'position' pointer does not meet any alignment constraints.

The PowerPC (and possibly other machines, too) cannot store floating-point registers to unaligned addresses and the type cast invites the compiler to generate inappropriate code (claiming that 'position' is a valid 'double*' which in fact is not necessarily true).

During compilation I had also seen several 'breaking strict alias rule' warnings flying by (albeit not for this particular code, of course) -- but after seeing the above code I would recommend to study such warnings carefully.

[mdavidsaver (mdavidsaver)]

> ByteBuffer (de-)serialization code which does IMHO ugly things:

Agreed...

Can you provide a stack-trace?

[mdavidsaver (mdavidsaver)]

Also, a stack trace should clarify where bug report(s) should be opened. The ByteBuffer code, and most of the serialization code which calls it, lives in https://github.com/epics-base/pvDataCPP.

[mdavidsaver (mdavidsaver)]

https://github.com/epics-base/pvDataCPP/issues/54

[Till Straumann (strauman)]

On 03/11/2018 10:49 AM, mdavidsaver wrote:
>> ByteBuffer (de-)serialization code which does IMHO ugly things:
> Agreed...
>
> Can you provide a stack-trace?
>

The faulting instruction (powerpc) is:

  9c:   db e8 00 00     stfd    f31,0(r8)

in ByteBuffer::put().

The stack trace goes like this (no line information, sorry - had to
disassemble by hand)

epics::pvData::PVScalarValue<double>::serialize(epics::pvData::ByteBuffer*,
epics::pvData::SerializableControl*) const
epics::pvData::PVStructure::serialize(epics::pvData::ByteBuffer*,
epics::pvData::SerializableControl*) const
epics::pvData::PVStructure::serialize(epics::pvData::ByteBuffer*,
epics::pvData::SerializableControl*, epics::pvData::BitSet*) const
epics::pvAccess::ServerChannelGetRequesterImpl::send(epics::pvData::ByteBuffer*,
epics::pvAccess::TransportSendControl*)
epics::pvAccess::detail::AbstractCodec::processSender(std::tr1::shared_ptr<epics::pvAccess::TransportSender>
const&)
epics::pvAccess::detail::AbstractCodec::processSendQueue()
epics::pvAccess::detail::AbstractCodec::processWrite()
epics::pvAccess::detail::BlockingTCPTransportCodec::sendThread()
epics::pvData::detail::MethRunner<epics::pvAccess::detail::BlockingTCPTransportCodec>::run()
epicsThreadCallEntryPoint()
threadWrapper()
...

HTH
- Till