linux: improve exception-signal handling

First, we use an alternate stack for signal handling now. The stack is
shared among all threads of the component, which is okay as we only
handle exceptions with log output and pass on to the default handler
(that terminates the execution). The primary motivation for the
alternate stack is the detection of SIGSEGV due to stack overflows.

Also, hybrid components now handle exception signals by logging and the
support for multi-threaded applications was improved.

Fixes #1935

repos/base-linux/src/base/thread/thread_env.cc

@@ -47,7 +47,7 @@ extern "C" __attribute__((weak)) int stdout_write(char const *s)
 /**
  * Signal handler for exceptions like segmentation faults
  */
-static void exception_signal_handler(int signum)
+void exception_signal_handler(int signum)
 {
 	char const *reason = nullptr;
 
@@ -81,6 +81,15 @@ static void exception_signal_handler(int signum)
 }
 
 
+void lx_exception_signal_handlers()
+{
+	lx_sigaction(LX_SIGILL,  exception_signal_handler);
+	lx_sigaction(LX_SIGBUS,  exception_signal_handler);
+	lx_sigaction(LX_SIGFPE,  exception_signal_handler);
+	lx_sigaction(LX_SIGSEGV, exception_signal_handler);
+}
+
+
 /*****************************
  ** Startup library support **
  *****************************/
@@ -98,8 +107,5 @@ void prepare_init_main_thread()
 	 */
 	lx_environ = (char**)&__initial_sp[3];
 
-	lx_sigaction(LX_SIGILL,  exception_signal_handler);
-	lx_sigaction(LX_SIGBUS,  exception_signal_handler);
-	lx_sigaction(LX_SIGFPE,  exception_signal_handler);
-	lx_sigaction(LX_SIGSEGV, exception_signal_handler);
+	lx_exception_signal_handlers();
 }

repos/base-linux/src/base/thread/thread_linux.cc

@@ -45,8 +45,12 @@ static Lock &startup_lock()
 static void thread_exit_signal_handler(int) { lx_exit(0); }
 
 
+static char signal_stack[0x2000] __attribute__((aligned(0x1000)));
+
 void Thread_base::_thread_start()
 {
+	lx_sigaltstack(signal_stack, sizeof(signal_stack));
+
 	/*
 	 * Set signal handler such that canceled system calls get not
 	 * transparently retried after a signal gets received.

repos/base-linux/src/core/thread_linux.cc

@@ -26,9 +26,12 @@ using namespace Genode;
 
 static void empty_signal_handler(int) { }
 
+static char signal_stack[0x2000] __attribute__((aligned(0x1000)));
 
 void Thread_base::_thread_start()
 {
+	lx_sigaltstack(signal_stack, sizeof(signal_stack));
+
 	/*
 	 * Set signal handler such that canceled system calls get not transparently
 	 * retried after a signal gets received.

repos/base-linux/src/lib/lx_hybrid/lx_hybrid.cc

@@ -50,8 +50,12 @@ int genode___cxa_atexit(void (*func)(void*), void *arg, void *dso)
 extern char **environ;
 extern char **lx_environ;
 
+static char signal_stack[0x2000] __attribute__((aligned(0x1000)));
+
 static void empty_signal_handler(int) { }
 
+extern void lx_exception_signal_handlers();
+
 /*
  * This function must be called before any other static constructor in the Genode
  * application, so it gets the highest priority (lowest priority number >100)
@@ -60,6 +64,9 @@ __attribute__((constructor(101))) void lx_hybrid_init()
 {
 	lx_environ = environ;
 
+	lx_sigaltstack(signal_stack, sizeof(signal_stack));
+	lx_exception_signal_handlers();
+
 	/*
 	 * Set signal handler such that canceled system calls get not
 	 * transparently retried after a signal gets received.
@@ -348,6 +355,8 @@ Linux_cpu_session *cpu_session(Cpu_session * cpu_session)
 
 static void adopt_thread(Native_thread::Meta_data *meta_data)
 {
+	lx_sigaltstack(signal_stack, sizeof(signal_stack));
+
 	/*
 	 * Set signal handler such that canceled system calls get not
 	 * transparently retried after a signal gets received.

repos/base-linux/src/lib/syscall/linux_syscalls.h

@@ -258,10 +258,10 @@ inline int lx_sigaction(int signum, void (*handler)(int))
 	 * when leaving the signal handler and it should call the rt_sigreturn syscall.
 	 */
 	enum { SA_RESTORER = 0x04000000 };
-	act.flags    = SA_RESTORER;
+	act.flags    = SA_RESTORER | SA_ONSTACK;
 	act.restorer = lx_restore_rt;
 #else
-	act.flags    = 0;
+	act.flags    = SA_ONSTACK;
 	act.restorer = 0;
 #endif
 	lx_sigemptyset(&act.mask);
@@ -282,6 +282,17 @@ inline int lx_tgkill(int pid, int tid, int signal)
 }
 
 
+/**
+ * Alternate signal stack (handles also SIGSEGV in a safe way)
+ */
+inline int lx_sigaltstack(void *signal_stack, Genode::size_t stack_size)
+{
+	stack_t stack { signal_stack, 0, stack_size };
+
+	return lx_syscall(SYS_sigaltstack, &stack, nullptr);
+}
+
+
 inline int lx_create_thread(void (*entry)(), void *stack, void *arg)
 {
 	int flags = CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND