fuzz: Hide conflicts with cstdlib and use clang++ for linking.

This creates a build target called libec.a by setting the visibility
of functions that conflict with cstdlib to hidden. It then links
those symbols locally into one large object file that makes up libec.a

Fuzzing targets are linked against libec.a so that they can invoke ec
functionality while depending on outside libraries that need cstdlib.
When linking a particular object against cstdlib, to avoid conflicting
function declarations put the following before any includes from the
ec codebase:
   #define __stdlib_compat(...)

The fuzzing targets are now linked using clang++, so that c++ libraries
and objects can be used as part of the fuzzers.

BRANCH=none
BUG=chromium:876582
TEST=make -j buildfuzztests &&
  ./build/host/host_command_fuzz/host_command_fuzz.exe

Change-Id: Ifdfdc6a51c6ef23b4e192b013ca993bf48a4411b
Signed-off-by: Allen Webb <allenwebb@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1180401
Reviewed-by: Randall Spangler <rspangler@chromium.org>

1 files changed, 13 insertions, 1 deletions

diff --git a/fuzz/build.mk b/fuzz/build.mk
index dc3fb8eb29..7a45116fe2 100644
--- a/fuzz/build.mk
+++ b/fuzz/build.mk
@@ -8,4 +8,16 @@
 
 fuzz-test-list-host = host_command_fuzz
 
-host_command_fuzz-y = host_command_fuzz.o
+# For fuzzing targets libec.a is built from the ro objects and hides functions
+# that collide with stdlib. The rw only objects are then linked against libec.a
+# with stdlib support. Therefore fuzzing targets that need to call this internal
+# functions should be marked "-y" or "-ro", and fuzzing targets that need stdlib
+# should be marked "-rw". In other words:
+#
+# Does your object file need to link against the Cr50 implementations of stdlib
+# functions?
+#   Yes -> use <obj_name>-y
+# Does your object file need to link against cstdlib?
+#   Yes -> use <obj_name>-rw
+# Otherwise use <obj_name>-y
+host_command_fuzz-rw = host_command_fuzz.o