Pending signals

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Introduction[edit]

A process can block a set of signals and all signals will wait in two kernel queues. One queue is shared between threads and the other is private for a thread. Signals in a queue are called pending signals. Each signal has a siginfo, it’s a small message. Several types of siginfo-s exist.

What do we need to dump pending signals?[edit]

We need to get the siginfo for each signal on dump, and then return it back on restore. It looks simple, doesn’t it? I thought so, before I started. The first problem is that the kernel doesn’t report complete siginfo-s in user-space. In a signal handler the kernel strips SI_CODE from siginfo. When a siginfo is received from signalfd, it has a different format with fixed sizes of fields. The interface of signalfd was extended. If a signalfd is created with the flag SFD_RAW, it returns siginfo in a raw format. We need to choose a queue, so two more flags SFD_GROUP and SFD_PRIVATE were added. Ok, signals can be dumped. Now we can think how to restore them. rt_sigqueueinfo looks suitable for that, but it can’t send siginfo with a positive si_code, because these codes are reserved for the kernel. In the real world each person has right to do anything with himself, so I think a process should able to send any siginfo to itself.