Add variadic logging to libdebuggerd internal.

GWP-ASan's crash information retrieval services requires a Printf()
function (declared by the system/implementing allocator). In this
instance, because _LOG is called with additional arguments (the log_t),
this function must be wrapped to conform to printf_t defined by
GWP-ASan.

We can easily wrap the variadic version.

Bug: 135634846
Test: atest debuggerd_test
Change-Id: I17209cd2b7455ce889e2f8194969f606cac329eb

debuggerd/libdebuggerd/include/libdebuggerd/utility.h

@@ -20,6 +20,7 @@
 
 #include <inttypes.h>
 #include <signal.h>
+#include <stdarg.h>
 #include <stdbool.h>
 #include <sys/types.h>
 
@@ -71,6 +72,7 @@ typedef uint32_t word_t;
 
 // Log information onto the tombstone.
 void _LOG(log_t* log, logtype ltype, const char* fmt, ...) __attribute__((format(printf, 3, 4)));
+void _VLOG(log_t* log, logtype ltype, const char* fmt, va_list ap);
 
 namespace unwindstack {
 class Unwinder;

debuggerd/libdebuggerd/utility.cpp

@@ -67,6 +67,14 @@ static bool should_write_to_kmsg() {
 
 __attribute__((__weak__, visibility("default")))
 void _LOG(log_t* log, enum logtype ltype, const char* fmt, ...) {
+  va_list ap;
+  va_start(ap, fmt);
+  _VLOG(log, ltype, fmt, ap);
+  va_end(ap);
+}
+
+__attribute__((__weak__, visibility("default")))
+void _VLOG(log_t* log, enum logtype ltype, const char* fmt, va_list ap) {
   bool write_to_tombstone = (log->tfd != -1);
   bool write_to_logcat = is_allowed_in_logcat(ltype)
                       && log->crashed_tid != -1
@@ -75,10 +83,7 @@ void _LOG(log_t* log, enum logtype ltype, const char* fmt, ...) {
   static bool write_to_kmsg = should_write_to_kmsg();
 
   std::string msg;
-  va_list ap;
-  va_start(ap, fmt);
   android::base::StringAppendV(&msg, fmt, ap);
-  va_end(ap);
 
   if (msg.empty()) return;