Kernel: Userspace signal handlers are now called one at a time

I added a syscall for telling the kernel when signal execution has
finished. We should send a random hash or id to the signal trampoline
that we would include in the syscall, so validity of signal exit can
be confirmed.

kernel/CMakeLists.txt

@@ -46,6 +46,7 @@ set(KERNEL_SOURCES
 	kernel/Scheduler.cpp
 	kernel/Semaphore.cpp
 	kernel/Serial.cpp
+	kernel/Signal.cpp
 	kernel/SpinLock.cpp
 	kernel/SSP.cpp
 	kernel/Storage/ATABus.cpp

kernel/arch/x86_64/Signal.S

@@ -24,6 +24,9 @@ signal_trampoline:
 	movq 120(%rsp), %rax
 	call *%rax
 
+	movq 128(%rsp), %rdi
+	call signal_done
+
 	popq %r15
 	popq %r14
 	popq %r13

kernel/include/kernel/Thread.h

@@ -38,6 +38,7 @@ namespace Kernel
 		void setup_exec();
 
 		bool has_signal_to_execute() const;
+		void set_signal_done(int signal);
 		void handle_next_signal();
 		void handle_signal(int signal, uintptr_t& return_rsp, uintptr_t& return_rip);
 

kernel/kernel/Signal.cpp

@@ -0,0 +1,8 @@
+#include <sys/syscall.h>
+#include <kernel/Syscall.h>
+
+extern "C" __attribute__((section(".userspace")))
+void signal_done(int signal)
+{
+	Kernel::syscall(SYS_SIGNAL_DONE, signal);
+}

kernel/kernel/Syscall.cpp

@@ -22,6 +22,12 @@ namespace Kernel
 	{
 		Thread::current().set_in_syscall(true);
 
+		if (syscall == SYS_SIGNAL_DONE)
+		{
+			Thread::current().set_signal_done((int)arg1);
+			return 0;
+		}
+
 		asm volatile("sti");
 
 		(void)arg1;
@@ -149,6 +155,9 @@ namespace Kernel
 		case SYS_SIGNAL:
 			ret = Process::current().sys_signal((int)arg1, (void (*)(int))arg2);
 			break;
+		case SYS_SIGNAL_DONE:
+			// Handled above
+			ASSERT_NOT_REACHED();
 		default:
 			dwarnln("Unknown syscall {}", syscall);
 			break;

kernel/kernel/Thread.cpp

@@ -150,6 +150,19 @@ namespace Kernel
 		return !m_signal_queue.empty() && !m_handling_signal;
 	}
 
+	void Thread::set_signal_done(int signal)
+	{
+		ASSERT(!interrupts_enabled());
+		if (!m_handling_signal)
+			derrorln("set_signal_done called while not handling singal");
+		else if (m_signal_queue.empty())
+			derrorln("set_signal_done called and there are no signals in queue");
+		else if (m_signal_queue.front() != signal)
+			derrorln("set_signal_done called with wrong signal");
+		else
+			m_signal_queue.pop();
+	}
+
 	void Thread::handle_next_signal()
 	{
 		ASSERT(!interrupts_enabled());
@@ -167,8 +180,6 @@ namespace Kernel
 
 		vaddr_t signal_handler = process().m_signal_handlers[signal];
 
-		m_handling_signal = true;
-
 		// Skip masked and ignored signals
 		if (m_signal_mask & (1ull << signal))
 			;
@@ -176,13 +187,15 @@ namespace Kernel
 			;
 		else if (signal_handler != (vaddr_t)SIG_DFL)
 		{
+			// call userspace signal handlers
+			// FIXME: signal trampoline should take a hash etc
+			//        to only allow marking signals done from it
+			m_handling_signal = true;
 			write_to_stack(return_rsp, return_rip);
 			write_to_stack(return_rsp, signal);
 			write_to_stack(return_rsp, signal_handler);
 			return_rip = (uintptr_t)signal_trampoline;
-
+			return;
-			// FIXME: we should only mark this signal as done when
-			//        handler returns
 		}
 		else
 		{
@@ -241,7 +254,6 @@ namespace Kernel
 		}
 
 		m_signal_queue.pop();
-		m_handling_signal = false;
 	}
 
 	void Thread::validate_stack() const

libc/include/sys/syscall.h

@@ -44,6 +44,7 @@ __BEGIN_DECLS
 #define SYS_RAISE 37
 #define SYS_KILL 38
 #define SYS_SIGNAL 39
+#define SYS_SIGNAL_DONE 40
 
 __END_DECLS
 

libc/unistd.cpp

@@ -286,6 +286,9 @@ long syscall(long syscall, ...)
 			ret = Kernel::syscall(SYS_SIGNAL, signal, (uintptr_t)handler);
 			break;
 		}
+		case SYS_SIGNAL_DONE:
+			// Should not be called by an user
+			// fall through
 		default:
 			puts("LibC: Unhandeled syscall");
 			ret = -ENOSYS;

userspace/Shell/main.cpp

@@ -281,6 +281,39 @@ int execute_command(BAN::Vector<BAN::String>& args)
 			return 1;
 		}
 	}
+	else if (args.front() == "signal-test"sv)
+	{
+		pid_t pid = fork();
+		if (pid == 0)
+		{
+			if (signal(SIGSEGV, [](int) { printf("SIGSEGV\n"); }) == SIG_ERR)
+			{
+				perror("signal");
+				exit(1);
+			}
+			printf("child\n");
+			for (;;);
+		}
+		if (pid == -1)
+		{
+			perror("fork");
+			return 1;
+		}
+
+		sleep(1);
+		if (kill(pid, SIGSEGV) == -1)
+		{
+			perror("kill");
+			return 1;
+		}
+
+		sleep(1);
+		if (kill(pid, SIGTERM) == -1)
+		{
+			perror("kill");
+			return 1;
+		}
+	}
 	else if (args.front() == "cd"sv)
 	{
 		if (args.size() > 2)