Some subsystems only have bare tracepoints (a tracepoint with no
associated trace event) to avoid the problem of trace events being an
ABI that can't be changed.

>From bpf presepective, bare tracepoints are what it calls
RAW_TRACEPOINT().

Since bpf assumed there's 1:1 mapping, it relied on hooking to
DEFINE_EVENT() macro to create bpf mapping of the tracepoints. Since
bare tracepoints use DECLARE_TRACE() to create the tracepoint, bpf had
no knowledge about their existence.

By teaching bpf_probe.h to parse DECLARE_TRACE() in a similar fashion to
DEFINE_EVENT(), bpf can find and attach to the new raw tracepoints.

Enabling that comes with the contract that changes to raw tracepoints
don't constitute a regression if they break existing bpf programs.
We need the ability to continue to morph and modify these raw
tracepoints without worrying about any ABI.

Update Documentation/bpf/bpf_design_QA.rst to document this contract.
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210119122237.2426878-2-qais.yousef@arm.com