during the context switch, we have to update esp0 in TSS

arch/x86/kernel/entry.asm

@@ -199,6 +199,7 @@ isrstub_pseudo_error 9
 extern irq_handler
 extern get_current_stack
 extern finish_task_switch
+extern set_kernel_stack
 
 global switch_context
 ALIGN 4
@@ -249,6 +250,9 @@ common_switch:
     or eax, 8
     mov cr0, eax
 
+    ; set esp0 in the task state segment
+    call set_kernel_stack
+
     ; call cleanup code
     call finish_task_switch
 

arch/x86/kernel/gdt.c

@@ -35,7 +35,7 @@
 #include <asm/gdt.h>
 #include <asm/tss.h>
 
-gdt_ptr_t				gp;
+gdt_ptr_t					gp;
 static tss_t				task_state_segment __attribute__ ((aligned (PAGE_SIZE)));
 // currently, our kernel has full access to the ioports
 static gdt_entry_t			gdt[GDT_ENTRIES] = {[0 ... GDT_ENTRIES-1] = {0, 0, 0, 0, 0, 0}};
@@ -46,6 +46,13 @@ static gdt_entry_t			gdt[GDT_ENTRIES] = {[0 ... GDT_ENTRIES-1] = {0, 0, 0, 0, 0,
  */
 extern void gdt_flush(void);
 
+void set_kernel_stack(void)
+{
+	task_t* curr_task = current_task;
+
+	task_state_segment.esp0 = (size_t) curr_task->stack + KERNEL_STACK_SIZE-16;
+}
+
 /* Setup a descriptor in the Global Descriptor Table */
 void gdt_set_gate(int num, unsigned long base, unsigned long limit,
 			  unsigned char access, unsigned char gran)
@@ -122,13 +129,15 @@ void gdt_install(void)
 	 * Create data segement for userspace applications (ring 3)
 	 */
 	gdt_set_gate(4, 0, limit,
-                GDT_FLAG_RING3 | GDT_FLAG_SEGMENT | GDT_FLAG_DATASEG | GDT_FLAG_PRESENT,
-                GDT_FLAG_4K_GRAN | mode);
+				GDT_FLAG_RING3 | GDT_FLAG_SEGMENT | GDT_FLAG_DATASEG | GDT_FLAG_PRESENT,
+				GDT_FLAG_4K_GRAN | mode);
 
 	/* set default values */
 	task_state_segment.eflags = 0x1202;
-	task_state_segment.ss0 = 0x10;		// data segment
+	task_state_segment.ss0 = 0x10;			// data segment
 	task_state_segment.esp0 = 0xDEADBEEF;	// invalid pseudo address
+	task_state_segment.cs = 0x0b;
+	task_state_segment.ss = task_state_segment.ds = task_state_segment.es = task_state_segment.fs = task_state_segment.gs = 0x13;
 	gdt_set_gate(5, (unsigned long) (&task_state_segment), sizeof(tss_t)-1,
 			GDT_FLAG_PRESENT | GDT_FLAG_TSS | GDT_FLAG_RING0, mode);