linker: Enable debug logging via properties

This patch lets developers tune logging of dlopen/dlerror with
setting system property.

Note that for security purposes this option is disabled on user build
for non-debuggable apps.

For starters there are 3 debug options:
 dlerror - enables logging of all dlerrors
 dlopen - traces dlopen calls

To enable system-wide logging (works only for userdebug/eng builds)
use debug.ld.all property.

To enable logging for particular app use debug.ld.app.<appname> property.

Example: Running "adb shell setprop debug.ld.all dlerror,dlopen" will log all
dlerror message as well as trace all calls to dlopen.

Bug: http://b/29458203
Change-Id: I2392c80a795509e16fe5689d0500d18b99772a64
diff --git a/linker/linker_logger.h b/linker/linker_logger.h
new file mode 100644
index 0000000..6b06aff
--- /dev/null
+++ b/linker/linker_logger.h
@@ -0,0 +1,59 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#ifndef _LINKER_LOGGER_H_
+#define _LINKER_LOGGER_H_
+
+#include <stdlib.h>
+#include <limits.h>
+#include "private/bionic_macros.h"
+
+#define LD_LOG(type, x...) \
+  { \
+    g_linker_logger.Log(type, x); \
+  }
+
+constexpr const uint32_t kLogErrors = 1 << 0;
+constexpr const uint32_t kLogDlopen = 1 << 1;
+
+class LinkerLogger {
+ public:
+  LinkerLogger() : flags_(0) { }
+
+  void ResetState();
+  void Log(uint32_t type, const char* format, ...);
+ private:
+  uint32_t flags_;
+
+  DISALLOW_COPY_AND_ASSIGN(LinkerLogger);
+};
+
+extern LinkerLogger g_linker_logger;
+extern const char* g_argv0;
+
+#endif /* _LINKER_LOGGER_H_ */