This is a small header-only library that makes it simple to define the runtime required by (UndefinedBehaviorSanitizer)ubsan. The runtime implementations in LLVM's compiler-rt, even the “minimal” one, cannot fit into custom build situations such as kernel or embedded code, for a variety of reasons.
TODO(https://fxbug.dev/334165273): This may be upstreamed into LLVM in the future, removing the need for this separate header library.
The <lib/ubsan-custom/ubsan-report.h> header file describes the few functions that the embedder must define. These have to take care of printing messages via a printf-style API; and whatever it should look to prepare for a problem report or panic before printing details and to panic afterwards.