Add an option to build with UBSan.

Change-Id: I31d5660fa4792bbb1ef8a721bad4bdbdb0e56863
Reviewed-on: https://boringssl-review.googlesource.com/c/34445
Reviewed-by: Adam Langley <agl@google.com>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 64181f5..de92237 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -335,6 +335,22 @@
   set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -fsanitize=thread")
 endif()
 
+if(UBSAN)
+  if(NOT CLANG)
+    message(FATAL_ERROR "Cannot enable UBSAN unless using Clang")
+  endif()
+
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fsanitize=undefined")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsanitize=undefined")
+  set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -fsanitize=undefined")
+
+  if(NOT UBSAN_RECOVER)
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fno-sanitize-recover=undefined")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-sanitize-recover=undefined")
+    set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -fno-sanitize-recover=undefined")
+  endif()
+endif()
+
 if(GCOV)
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fprofile-arcs -ftest-coverage")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fprofile-arcs -ftest-coverage")