kernel: use per-thread signal stacks for guest stack faults

src/common/hostException.cpp

@@ -9,7 +9,9 @@
 #include <csignal>
 #include <sys/ucontext.h>
 #else
+#include <algorithm>
 #include <csignal>
+#include <cstdlib>
 #include <initializer_list>
 #include <ucontext.h> // IWYU pragma: keep
 #include <unistd.h>
@@ -164,6 +166,46 @@ static void SignalHandler(int sig, siginfo_t* si, void* uctx) {
 
 #else
 
+class ThreadSignalStack {
+public:
+	ThreadSignalStack() {
+		const auto page_size = static_cast<size_t>(::getpagesize());
+		const auto stack_size =
+		    (std::max<size_t>(64 * 1024, MINSIGSTKSZ) + page_size - 1) & ~(page_size - 1);
+		if (::posix_memalign(&m_memory, page_size, stack_size) != 0) {
+			return;
+		}
+
+		stack_t stack {};
+		stack.ss_sp   = m_memory;
+		stack.ss_size = stack_size;
+		if (::sigaltstack(&stack, &m_previous) != 0) {
+			std::free(m_memory);
+			m_memory = nullptr;
+		}
+	}
+
+	~ThreadSignalStack() {
+		if (m_memory != nullptr && ::sigaltstack(&m_previous, nullptr) == 0) {
+			std::free(m_memory);
+		}
+	}
+
+	[[nodiscard]] bool IsInitialized() const { return m_memory != nullptr; }
+
+	KYTY_CLASS_NO_COPY(ThreadSignalStack)
+
+private:
+	void*   m_memory = nullptr;
+	stack_t m_previous {};
+};
+
+bool InitializeThreadSignalStack() {
+	// Keep fault handling off guest stacks, which GPU tracking can make read-only.
+	thread_local ThreadSignalStack signal_stack;
+	return signal_stack.IsInitialized();
+}
+
 // x86-64 page-fault error bits.
 constexpr uint64_t PAGE_FAULT_ERROR_WRITE       = 0x02;
 constexpr uint64_t PAGE_FAULT_ERROR_INSTRUCTION = 0x10;
@@ -274,8 +316,7 @@ bool InstallHandler(Handler handler) {
 	struct sigaction action {};
 	action.sa_sigaction = SignalHandler;
 	sigemptyset(&action.sa_mask);
-	// Fault resolution needs the normal thread stack.
-	action.sa_flags = SA_SIGINFO | SA_RESTART;
+	action.sa_flags = SA_SIGINFO | SA_RESTART | SA_ONSTACK;
 
 	for (const int signal_number: {SIGSEGV, SIGBUS, SIGILL}) {
 		if (::sigaction(signal_number, &action, nullptr) != 0) {