arch: randomize initial user stack pointer

Inside the page randomization of initial user stack pointer is not only a part
of ASLR implementation but also a performance improvement that helps
eliminating aligned 64 kB data access.

Minimal user stack size is increased to 8 kB in order to ensure that regardless
of initial stack pointer value there is still enough space on stack.

headers/private/system/thread_defs.h

@@ -15,7 +15,7 @@
 #define USER_STACK_GUARD_SIZE		(4 * B_PAGE_SIZE)		// 16 kB
 #define USER_MAIN_THREAD_STACK_SIZE	(16 * 1024 * 1024)		// 16 MB
 #define USER_STACK_SIZE				(256 * 1024)			// 256 kB
-#define MIN_USER_STACK_SIZE			(4 * 1024)				// 4 KB
+#define MIN_USER_STACK_SIZE			(8 * 1024)				// 8 kB
 #define MAX_USER_STACK_SIZE			(16 * 1024 * 1024)		// 16 MB
 
 

src/system/kernel/arch/x86/32/thread.cpp

@@ -200,6 +200,14 @@ arch_thread_dump_info(void *info)
 }
 
 
+static addr_t
+arch_randomize_stack_pointer(addr_t value)
+{
+	value -= rand() & (B_PAGE_SIZE - 1);
+	return value & ~0xful;
+}
+
+
 /*!	Sets up initial thread context and enters user space
 */
 status_t
@@ -214,6 +222,8 @@ arch_thread_enter_userspace(Thread* thread, addr_t entry, void* args1,
 	TRACE(("arch_thread_enter_userspace: entry 0x%lx, args %p %p, "
 		"ustack_top 0x%lx\n", entry, args1, args2, stackTop));
 
+	stackTop = arch_randomize_stack_pointer(stackTop);
+
 	// copy the little stub that calls exit_thread() when the thread entry
 	// function returns, as well as the arguments of the entry function
 	stackTop -= codeSize;

src/system/kernel/arch/x86/64/thread.cpp

@@ -197,6 +197,14 @@ arch_thread_dump_info(void* info)
 }
 
 
+static addr_t
+arch_randomize_stack_pointer(addr_t value)
+{
+	value -= rand() & (B_PAGE_SIZE - 1);
+	return value & ~0xful;
+}
+
+
 /*!	Sets up initial thread context and enters user space
 */
 status_t
@@ -208,6 +216,8 @@ arch_thread_enter_userspace(Thread* thread, addr_t entry, void* args1,
 	TRACE("arch_thread_enter_userspace: entry %#lx, args %p %p, "
 		"stackTop %#lx\n", entry, args1, args2, stackTop);
 
+	stackTop = arch_randomize_stack_pointer(stackTop);
+
 	// Copy the little stub that calls exit_thread() when the thread entry
 	// function returns.
 	// TODO: This will become a problem later if we want to support execute