libbinder: Add log when FDs aren't supported in RpcSession

Give a hint when this error is hit and make it obvious that both sides
of the RpcSession need to agree on the FD support for the connection.

Test: none
Bug: none
Change-Id: If7a559721d5886df0b418923e879538179658d85

1 files changed, 5 insertions, 0 deletions

diff --git a/libs/binder/Parcel.cpp b/libs/binder/Parcel.cpp
index 77b32753de..f30b2aa3f4 100644
--- a/libs/binder/Parcel.cpp
+++ b/libs/binder/Parcel.cpp
@@ -1585,10 +1585,15 @@ status_t Parcel::writeFileDescriptor(int fd, bool takeOwnership) {
             fdVariant = borrowed_fd(fd);
         }
         if (!mAllowFds) {
+            ALOGE("FDs are not allowed in this parcel. Both the service and the client must set "
+                  "the FileDescriptorTransportMode and agree on the support.");
             return FDS_NOT_ALLOWED;
         }
         switch (rpcFields->mSession->getFileDescriptorTransportMode()) {
             case RpcSession::FileDescriptorTransportMode::NONE: {
+                ALOGE("FDs are not allowed in this RpcSession. Both the service and the client "
+                      "must set "
+                      "the FileDescriptorTransportMode and agree on the support.");
                 return FDS_NOT_ALLOWED;
             }
             case RpcSession::FileDescriptorTransportMode::UNIX: