I read lately about the signals mechanism, and I have a couple of question that I couldn't find the answer to them. it would be great if someone would answer.

1. when handling a signal before changing to user Mode, the kernel copies data to the user stack. what is the data that he saves in setup_frame()
2. what signals are blocked while in the signal handler could this mask be changed
3. when does a system call could be reentrant and when it cannot, when disturbed by a signal