Alignment issues on some ARM implementations

[satta]

While building and testing pktanon on Debian, which is available for a large variety of architectures, we noticed that pktanon fails on ARM with a bus error:

-----------------------------------------------
pktanon --- profile-based traffic anonymization
-----------------------------------------------
initializing PktAnon,  configuration = 
/usr/share/doc/pktanon/examples/profiles/profile.xml
unknown element: pktanon-config: 37
unknown element: anonymizations: 102
istream: opened file 
/tmp/autopkgtest-lxc.tgw0nfyu/downtmp/build.ybV/src/profiles/sample.pcap
ostream: opened output file ./out.pcap
initialized
Bus error

See Debian issue ticket with more details here: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=999620

We have tracked the problem down to an unaligned access via hton32() on a field that is not 4-byte aligned, and this architecture requires alignment:

# gdb --args pktanon -c /usr/share/doc/pktanon/examples/profiles/profile.xml ./profiles/sample.pcap ./out.pcap
[...]
Reading symbols from pktanon...
Reading symbols from /usr/lib/debug/.build-id/af/1ac53f46ae133c8898358966960cba95ac7a70.debug...
(gdb) run
Starting program: /usr/bin/pktanon -c /usr/share/doc/pktanon/examples/profiles/profile.xml ./profiles/sample.pcap ./out.pcap
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/arm-linux-gnueabihf/libthread_db.so.1".
-----------------------------------------------
pktanon --- profile-based traffic anonymization
-----------------------------------------------
initializing PktAnon,  configuration = /usr/share/doc/pktanon/examples/profiles/profile.xml
unknown element: pktanon-config: 37
unknown element: anonymizations: 102
istream: opened file ./profiles/sample.pcap
ostream: opened output file ./out.pcap
initialized

Program received signal SIGBUS, Bus error.
pktanon::TcpPacketTransformation::transform (this=<optimized out>, source_buffer=<optimized out>, destination_buffer=0xfffef35a "\212y\262X\335\300l\221", max_packet_length=40) at transformations/TcpPacketTransformation.cpp:88
88	  hton32 (output_header->ack_num);
(gdb) bt
#0  pktanon::TcpPacketTransformation::transform (this=<optimized out>, 
    source_buffer=<optimized out>, 
    destination_buffer=0xfffef35a "\212y\262X\335\300l\221", 
    max_packet_length=40) at transformations/TcpPacketTransformation.cpp:88
#1  0x0040b77c in pktanon::IPv4PacketTransformation::transform (this=0x4b4eb0, 
    source_buffer=<optimized out>, destination_buffer=0xfffef346 "E", 
    max_packet_length=<optimized out>)
    at transformations/IPv4PacketTransformation.cpp:153
#2  0x0040af64 in pktanon::EthernetPacketTransformation::transform (
    this=0x4ad780, source_buffer=<optimized out>, 
    destination_buffer=0xfffef338 "\376\212\a\213\001\254\303\341\372DI\355\b", max_packet_length=74) at transformations/EthernetPacketTransformation.cpp:53
#3  0x00416862 in pktanon::transform_packet (stats=..., 
    packet_len=<optimized out>, 
    transformed_packet=0xfffef338 "\376\212\a\213\001\254\303\341\372DI\355\b", original_packet=0xfffef438 "", record_header=...) at Utils.h:26
#4  pktanon::IstreamInput::read_packets (this=0x4b3ce0)
    at IstreamRecordsHandler.cpp:121
#5  0x00415130 in pktanon::PktAnonRuntime::run () at PktAnonRuntime.cpp:37
#6  0x00405bfa in main (argc=<optimized out>, argv=<optimized out>)
    at src/Main.cpp:73
(gdb)