[llvm-dev] Where does the LLVM implement the Ubsan's instrumentations?

[Vedant Kumar via llvm-dev]

> On Jul 21, 2017, at 6:04 AM, Shi, Steven via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
>>> I think your best bet for controlling code bloat is to compile with
>>> -fsanitize=undefined -fsanitize-trap=undefined.
>> 
>> Also you may not need all of UBSan's checks at the same time -- so pick
>> and choose among its checks using the finer-grained flags.
>> 
>> If you're really stuck against a hard limit on code size, try applying
>> UBSan to a subset of files in your project at a time.
> 
> Hi John,
> Thank your suggestion. I like the trap-funcion way. With the compile options "-fsanitize=undefined -fsanitize-trap=undefined -ftrap-function=__my_trap_function", my firmware can save +40% code size. It is great!

You might try compiling with '-fsanitize=undefined -fno-sanitize-recover=undefined'. You should still see some code size savings with this option. You'll need to link in the ubsan runtime when compiling in no-recovery mode, but the diagnostics will be better.

Ubsan doesn't provide an option of using callbacks to implement its instrumentation. If the no-recovery mode won't work for you, it's pretty simply to write a custom ubsan runtime that fits in a single object file. That's what I ended up doing to sanitize our kernel (xnu), so I can offer help if you decide to go down that path.

best
vedant

> 
> 
> But I have another question about the trap-function. I hope to print the wrong code running IP address in my trap function (then, I can use llvm-symbolizer to know its code location), and I also hope to print undefined behavior specific type info, e.g. add_overflow, type_mismatch_v1, etc., which are defined in compiler-rt\lib\ubsan\ubsan_interface.inc. How should I correctly define the __my_sanitizer_trap() and let clang/llvm input relative info into my trap function?
> 
> Thanks
> Steven
> 
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