lttng-sequence-len-type-mwe

Minimal working example (MWE) reproducing a lttng-ust bug: decoding failure for trace generated on aarch64 with ctf_sequence with a _length_type of uint32_t

See: https://bugs.lttng.org/issues/1412

How to reproduce

1. Install
   • lttng-ust (version used: 2.13)
2. Compile

   $ git clone https://github.com/christophebedard/lttng-sequence-len-type-mwe
   $ cd lttng-sequence-len-type-mwe
   $ make

3. Trace on aarch64

   $ lttng create mwe
   $ lttng enable-event -u 'my_provider:my_tracepoint_does_not_work'
   $ lttng start
   $ ./app/myapp
   $ lttng stop
   $ lttng destroy

4. (Try to) read trace with babeltrace
   • There is a decoding error

   $ babeltrace ~/lttng-traces/mwe-*
   [16:10:28.980897046] (+?.?????????) host my_provider:my_tracepoint_does_not_work: { cpu_id = 7 }, { _arr_length = 0, arr = [ ] }
   [16:10:29.499535163] (+0.518638117) host my_provider:my_tracepoint_does_not_work: { cpu_id = 7 }, { _arr_length = 0, arr = [ ] }
   [16:10:29.499535163] (+0.000000000) host my_provider:my_tracepoint_does_not_work: { cpu_id = 7 }, { _arr_length = 0, arr = [ ] }
   [16:10:29.499535163] (+0.000000000) host my_provider:my_tracepoint_does_not_work: { cpu_id = 7 }, { _arr_length = 0, arr = [ ] }
   [16:10:29.499535163] (+0.000000000) host my_provider:my_tracepoint_does_not_work: { cpu_id = 7 }, { _arr_length = 0, arr = [ ] }
   [16:10:29.499535163] (+0.000000000) host my_provider:my_tracepoint_does_not_work: { cpu_id = 7 }, { _arr_length = 0, arr = [ ] }
   [16:10:29.499535163] (+0.000000000) host my_provider:my_tracepoint_does_not_work: { cpu_id = 7 }, { _arr_length = 0, arr = [ ] }
   [16:10:29.499535163] (+0.000000000) host my_provider:my_tracepoint_does_not_work: { cpu_id = 7 }, { _arr_length = 0, arr = [ ] }
   [16:10:29.499535163] (+0.000000000) host my_provider:my_tracepoint_does_not_work: { cpu_id = 7 }, { _arr_length = 0, arr = [ ] }
   [16:10:29.499535163] (+0.000000000) host my_provider:my_tracepoint_does_not_work: { cpu_id = 7 }, { _arr_length = 0, arr = [ ] }
   [error] Unexpected end of packet. Either the trace data stream is corrupted or metadata description does not match data layout.
   [error] Reading event failed.
   Error printing trace.
   

5. However, it works if you:
   1. Trace on x86_64; or
   2. Change (on aarch64) the _length_type from uint32_t to uint64_t (see lib/tp.h).

   $ babeltrace ~/lttng-traces/mwe-*
   [09:04:48.392369382] (+0.000088963) host my_provider:my_tracepoint_does_not_work: { cpu_id = 12 }, { _arr_length = 0, arr = [ ] }
   [09:04:48.392370809] (+0.000001427) host my_provider:my_tracepoint_does_not_work: { cpu_id = 12 }, { _arr_length = 0, arr = [ ] }
   [09:04:48.392371018] (+0.000000209) host my_provider:my_tracepoint_does_not_work: { cpu_id = 12 }, { _arr_length = 0, arr = [ ] }
   [09:04:48.392371186] (+0.000000168) host my_provider:my_tracepoint_does_not_work: { cpu_id = 12 }, { _arr_length = 0, arr = [ ] }
   [09:04:48.392371322] (+0.000000136) host my_provider:my_tracepoint_does_not_work: { cpu_id = 12 }, { _arr_length = 0, arr = [ ] }
   [09:04:48.392371453] (+0.000000131) host my_provider:my_tracepoint_does_not_work: { cpu_id = 12 }, { _arr_length = 0, arr = [ ] }
   [09:04:48.392371580] (+0.000000127) host my_provider:my_tracepoint_does_not_work: { cpu_id = 12 }, { _arr_length = 0, arr = [ ] }
   [09:04:48.392371729] (+0.000000149) host my_provider:my_tracepoint_does_not_work: { cpu_id = 12 }, { _arr_length = 0, arr = [ ] }
   [09:04:48.392371849] (+0.000000120) host my_provider:my_tracepoint_does_not_work: { cpu_id = 12 }, { _arr_length = 0, arr = [ ] }
   [09:04:48.392371971] (+0.000000122) host my_provider:my_tracepoint_does_not_work: { cpu_id = 12 }, { _arr_length = 0, arr = [ ] }