[scudo] Only read urandom if getrandom syscall isn't available.

If the getrandom system call is available, but the call returns
an error, it could mean that the system doesn't have enough
randomness to respond yet. Trying to read /dev/urandom will likely
block and cause initialization to be stalled. Therefore, return
false in this case and use the backup random data.

Author: cferris1000

compiler-rt/lib/scudo/standalone/linux.cpp

@@ -192,6 +192,12 @@ bool getRandom(void *Buffer, uptr Length, UNUSED bool Blocking) {
       syscall(SYS_getrandom, Buffer, Length, Blocking ? 0 : GRND_NONBLOCK);
   if (ReadBytes == static_cast<ssize_t>(Length))
     return true;
+  // If this system call is not implemented in the kernel, then we will try
+  // and use /dev/urandom. Otherwise, if the syscall fails, return false
+  // assuming that trying to read /dev/urandom will cause a delay waiting for
+  // the random data to be usable.
+  if (errno != ENOSYS)
+    return false;
 #endif // defined(SYS_getrandom)
   // Up to 256 bytes, a read off /dev/urandom will not be interrupted.
   // Blocking is moot here, O_NONBLOCK has no effect when opening /dev/urandom.