Fix stack pointer alignment for x86_64 platforms

The x86_64 ABI requires the stack pointer to be 16-byte aligned before the
call of a function and decreased by 8 at the function entrypoint (after
the return address has been pushed to the stack).

Currently, when a new Genode thread gets created, the initial stack
pointer is aligned to 16 byte. On Genode/Linux, the thread entry function
is entered by a 'call' instruction, so the stack pointer alignment at the
function entrypoint is correct. On Fiasco.OC and NOVA, however, the thread
entry function gets executed without a return address being pushed to the
stack, so at the function entrypoint the stack pointer is still aligned to
16 byte, which can cause problems with compiler-generated SSE
instructions.

With this patch, the stack pointer given to a new thread gets aligned to
16 bytes and decreased by 8 by default, since most of the currently
supported base platforms execute the thread entry function without pushing
a return address to the stack. For base-linux, the stack pointer gets
realigned to 16 bytes before the thread entry function gets called.

Fixes #1043.

base-foc/src/base/thread/thread.cc

@@ -153,6 +153,12 @@ Thread_base::Context *Thread_base::_alloc_context(size_t stack_size)
 	context->stack_base  = ds_addr;
 	context->ds_cap      = ds_cap;
 
+	/*
+	 * The value at the top of the stack might get interpreted as return
+	 * address of the thread start function by GDB, so we set it to 0.
+	 */
+	*(addr_t*)context->stack_top() = 0;
+
 	return context;
 }
 

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

@@ -122,9 +122,7 @@ void Thread_base::start()
 		threadlib_initialized = true;
 	}
 
-	/* align initial stack to 16 byte boundary */
-	void *thread_sp = (void *)((addr_t)(stack_top()) & ~0xf);
-	_tid.tid = lx_create_thread(Thread_base::_thread_start, thread_sp, this);
+	_tid.tid = lx_create_thread(Thread_base::_thread_start, stack_top(), this);
 	_tid.pid = lx_getpid();
 
 	/* wait until the 'thread_start' function got entered */

base-linux/src/platform/x86_32/lx_clone.S

@@ -83,8 +83,7 @@ L(thread_start):
     .cfi_startproc;
     /* Clearing frame pointer is insufficient, use CFI.  */
     .cfi_undefined %eip;
-    /* Note: %esi is zero.  */
-    movl    %esi,%ebp   /* terminate the stack frame */
+    xorl    %ebp,%ebp   /* terminate the stack frame */
     call    *%ebx
 #ifdef PIC
     call    L(here)

base-linux/src/platform/x86_64/lx_clone.S

@@ -17,6 +17,9 @@
 lx_clone:
 	.cfi_startproc
 
+	/* Align the new stack pointer to 16 bytes. */
+	andq    $0xfffffffffffffff0, %rsi
+
 	/* Insert the argument onto the new stack.  */
 	subq	$16,%rsi
 	movq	%rcx,8(%rsi)
@@ -51,7 +54,7 @@ L(thread_start):
 
 	/* Clear the frame pointer.  The ABI suggests this be done, to mark 
 	   the outermost frame obviously.  */
-	xorl	%ebp, %ebp
+	xorq	%rbp, %rbp
 
 	/* Set up arguments for the function call.  */
 	popq	%rax	/* Function to call.  */

base/include/base/thread.h

@@ -100,11 +100,16 @@ namespace Genode {
 				public:
 
 				/**
-				 * Top of stack aligned to 16 byte
+				 * Top of stack
 				 *
-				 * The alignment is also sufficient for the AMD64 ABI.
+				 * The alignment matches an initial stack frame, which is
+				 * sufficient for the AMD64 ABI (stack top + 8 is 16-byte
+				 * aligned).
 				 */
-				addr_t stack_top() const { return (addr_t)_stack & ~0xf; }
+				addr_t stack_top() const
+				{
+					return ((addr_t)_stack & ~0xf) - sizeof(addr_t);
+				}
 
 				/**
 				 * Virtual address of the start of the stack

base/src/base/thread/thread.cc

@@ -153,6 +153,12 @@ Thread_base::Context *Thread_base::_alloc_context(size_t stack_size)
 	context->stack_base  = ds_addr;
 	context->ds_cap      = ds_cap;
 
+	/*
+	 * The value at the top of the stack might get interpreted as return
+	 * address of the thread start function by GDB, so we set it to 0.
+	 */
+	*(addr_t*)context->stack_top() = 0;
+
 	return context;
 }