Changes to make sure after a fork a child process can safely return.

This copies the parent kernel stack to child only for the part where
the previous context is saved. Then the child registers are modified
so that it would begin execution from returning of the system call.

src/arch/arm/vectors.S

@@ -144,11 +144,14 @@ BEGIN_PROC(arm_swi_exception)
 	mrs	r0, spsr_fc	@ psr also need saving in case this context is interrupted.
 	stmfd	sp!, {r0}
 	enable_irqs r0
-	add	r0, sp, #4	@ Pass sp address + 4 as a pointer to saved regs.
-	ktcb_ref_saved_regs r0, r1, r2 @ Save regs pointer in ktcb
+	mov	r0, sp		@ Current SP has pointer to all saved context.
+	ktcb_ref_saved_regs r0, r1, r2 @ Save syscall context pointer in ktcb
 	mov	r1, lr		@ Pass swi instruction address in LR as arg1
 	mov 	lr, pc
 	ldr 	pc, =syscall
+
+.global return_from_syscall;    @ Newly created threads use this path to return,
+return_from_syscall:		@ if they duplicated another thread's address space.
 	disable_irqs r1		@ Not disabling irqs at this point causes the SP_USR and spsr
 				@ to get corrupt causing havoc.
 	ldmfd	sp!, {r1}