Implement advanced signal features (SIGCHLD, SA_RESTART, vDSO, RT signals)

[pbalduino]

Description

This is Part 5 of 5 for implementing UNIX signals (see #103). This issue covers advanced signal features for complete POSIX compatibility.

Scope

Implement advanced signal functionality:

• SIGCHLD for waitpid notification
• SA_RESTART flag for interrupted syscalls
• Stack-based signal trampoline (vDSO)
• Signal masks per thread (for multithreaded processes)
• Real-time signals (SIGRTMIN-SIGRTMAX)

Implementation Notes

SIGCHLD Support:

• Sent to parent when child exits or stops
• Parent can register handler to reap zombies asynchronously
• waitpid() can be called from handler
• Avoids polling for child status

SA_RESTART:

• Restarts interrupted syscalls automatically
• Syscalls like read(), write() resume after signal handler
• Some syscalls (accept(), sleep()) never restart

Signal Trampoline in vDSO:

• Current: trampoline code on stack (security issue)
• Better: trampoline in vDSO (virtual dynamic shared object)
• Maps read-only page into all processes
• Contains sigreturn stub

Per-Thread Signal Masks:

• Each thread has own sig_blocked mask
• pthread_sigmask() for thread-specific blocking
• Signals can target specific threads
• Process-wide signals delivered to any unblocked thread

Real-Time Signals:

• SIGRTMIN (34) to SIGRTMAX (64)
• Queued (not merged like standard signals)
• Delivered in order
• Can carry payload (sigqueue())

Acceptance Criteria

• [ ] SIGCHLD sent when child exits
• [ ] SA_RESTART restarts interrupted syscalls
• [ ] Signal trampoline in vDSO (more secure)
• [ ] Per-thread signal masks work correctly
• [ ] Real-time signals are queued and ordered
• [ ] Comprehensive test suite for all features
• [ ] Documentation covers all advanced features

Dependencies

Depends on:

• #213 - Shell signal integration (Part 4)
• #109 - pthread API (for per-thread masks)

Related Issues

• #103 - Parent issue for full signal implementation
• #210-#213 - Parts 1-4 of signal implementation
• #145 - waitpid implementation (needs SIGCHLD)

Known Future Work

• Job control signals (SIGTSTP, SIGCONT, SIGTTIN, SIGTTOU)
• Signal safety (async-signal-safe function list)
• signalfd() for event-driven signal handling
• sigwaitinfo(), sigtimedwait()

Estimated effort: 3-4 weeks Priority: Medium (nice to have, not blocking)