path: root/kernel/traps.s

;
; traps.s provides entry points to the exception handlers
; as well as the system call interface.
;

global check_signals
global syscall_init
global traps_init
extern call_table
extern ctask
extern idt
extern panic
extern printk
extern register_trap
extern reschedule
extern sighandler_default
extern ticked

; here we define offsets into task_struct.
; if task_struct is modified, this must be
; too.

%define ts_signal       6
%define ts_sig_handlers 18
%define ts_tss_esp0     150

%macro SAVE 0
  pusha
  ; save the data segment selectors
  mov ax, ds
  push ax
  ; switch to kernel data segments
  mov ax, 0x10
  mov ds, ax
  mov es, ax
  mov fs, ax
  mov gs, ax
  ; restore EAX (needed for syscalls)
  mov eax, [esp+30]
%endmacro

%macro SAVE_ERR 0
  pusha
  ; fetch the error code that was pushed
  mov ebx, [esp+32]
  ; save the data segment selectors
  mov ax, ds
  push ax
  ; switch to kernel data segments
  mov ax, 0x10
  mov ds, ax
  mov es, ax
  mov fs, ax
  mov gs, ax
%endmacro

%macro RESTORE 0
  ; restore the data segment selectors
  pop ax
  mov ds, ax
  mov es, ax
  mov fs, ax
  mov gs, ax
  popa
  iret
%endmacro

%macro RESTORE_ERR 0
  ; restore the data segment selectors
  pop ax
  mov ds, ax
  mov es, ax
  mov fs, ax
  mov gs, ax
  popa
  add esp, 4
  iret
%endmacro

traps:
  dd exc_div
  dd exc_debug
  dd exc_nmi
  dd exc_brk
  dd exc_ovf
  dd exc_bounds
  dd exc_invop
  dd exc_nomath
  dd exc_dbf
  dd exc_coseg
  dd exc_invtss
  dd exc_segnotpres
  dd exc_stackf
  dd exc_gprot
  dd exc_pagef
  dd exc_reserved
  dd exc_mathf
  dd exc_alignchk
  dd exc_machinechk
  dd exc_simdfp
  dd exc_virt
  dd exc_reserved
  dd exc_reserved
  dd exc_reserved
  dd exc_reserved
  dd exc_reserved
  dd exc_reserved
  dd exc_reserved
  dd exc_reserved
  dd exc_reserved
  dd exc_reserved
  dd exc_reserved

traps_init:
  push ebp
  mov ebp, esp
  mov ecx, 0
  mov edx, idt
.loop:
  mov eax, ecx
  shl eax, 2
  add eax, traps
  mov eax, [eax]
  mov [edx], ax
  shr eax, 16
  mov [edx+6], ax
  mov word [edx+2], 0x08
  mov byte [edx+4], 0x00
  mov byte [edx+5], 0x8E
  inc ecx
  cmp ecx, 32
  je .end
  add edx, 8
  jmp .loop
.end:
  pop ebp
  ret

exc_div:
  SAVE
  RESTORE

exc_debug:
  SAVE
  RESTORE

exc_nmi:
  SAVE
  RESTORE

exc_brk:
  SAVE
  RESTORE

exc_ovf:
  SAVE
  RESTORE

exc_bounds:
  SAVE
  RESTORE

exc_invop:
  SAVE
  RESTORE

exc_nomath:
  SAVE
  RESTORE

exc_dbf:
  SAVE_ERR
  RESTORE_ERR

exc_coseg:
  SAVE
  RESTORE

exc_invtss:
  SAVE_ERR
  RESTORE_ERR

exc_segnotpres:
  SAVE_ERR
  RESTORE_ERR

exc_stackf:
  SAVE_ERR
  RESTORE_ERR

exc_gprot:
  SAVE_ERR

  push ebx
  push .fmt
  call printk
  add esp, 8
  call panic

  RESTORE_ERR
.fmt:
  db "General Protection Fault (0x%08X)", 10, 0

exc_pagef:
  SAVE_ERR

  ; here we check if the page fault was
  ; caused by a process trying to return
  ; from a signal handler (this should cause
  ; a page fault for accessing page
  ; 0xFFFFE000 during the handler's
  ; execution).

  ; check if the offending address is 0xFFFFE000
  mov eax, cr2
  cmp eax, 0xFFFFE000
  jne .skip
  ; check if we're actually handling a signal
  mov eax, [ctask]
  mov eax, [eax+ts_tss_esp0]
  cmp eax, 0xFFFFFFFF
  je .skip
  ; final checks that a user-initiated read caused the fault
  mov eax, ebx
  and eax, 6
  cmp eax, 4
  jne .skip
  jmp sigret
.skip:

  ; if the above conditions are false, a
  ; signal handler was not trying to return,
  ; so we handle the page fault normally

  push ebx
  mov ebx, cr2
  push ebx
  push .fmt
  call printk
  add esp, 12
  call panic

  RESTORE_ERR
.fmt:
  db "Page Fault (illegal memory access: 0x%08X:0x%02X)", 10, 0

exc_mathf:
  SAVE
  RESTORE

exc_alignchk:
  SAVE
  RESTORE

exc_machinechk:
  SAVE
  RESTORE

exc_simdfp:
  SAVE
  RESTORE

exc_virt:
  SAVE
  RESTORE

exc_reserved:
  SAVE
  RESTORE

syscall_handler:
  SAVE

  mov dword [ticked], 0
  ; push the arguments onto the stack, then
  ; calculate the offset for the appropriate
  ; handler function and call it
  push edx
  push ecx
  push ebx
  and eax, 0xFF
  shl eax, 2
  add eax, call_table
  call [eax]
  add esp, 12
  ; preserve the syscall's return value
  mov [esp+30], eax
  ; check if the timer fired while we were in kernel mode
  mov eax, [ticked]
  jz .skip
  call reschedule
.skip:
  ; check pending signals and handle them (if any)
  mov eax, esp
  push eax
  call check_signals
  add esp, 4

  RESTORE

; void check_signals(void *saved_state)
check_signals:
  push ebp
  mov ebp, esp
  push ebx
  push esi
  ; abort if a signal is already being handled
  mov eax, [ctask]
  mov ebx, [eax+ts_tss_esp0]
  cmp ebx, 0xFFFFFFFF
  jne .end
  ; check if there are any pending signals
  ; if so, we'll resume at the signal
  ; handler if a custom handler has been
  ; defined, otherwise, we run the default
  ; handler.
  mov edx, [eax+ts_signal]
  cmp edx, 0
  je .end
  ; find which signal has been triggered (then clear it's flag)
  bsf ecx, edx
  mov ebx, 1
  shl ebx, cl
  xor ebx, 0xFFFFFFFF
  and dword [eax+ts_signal], ebx
  inc ecx
  mov ebx, ecx
  ; calculate offset to the handler function
  shl ecx, 2
  add ecx, eax
  add ecx, ts_sig_handlers
  cmp dword [ecx], 0
  jne .custom_handler
  ; call the default handler
  push ebx
  call sighandler_default
  add esp, 4
  jmp .end
.custom_handler:
  ; save our stack pointer
  mov esi, [ebp+8]
  mov [eax+ts_tss_esp0], esi
  ; here, we push the return address onto
  ; the user's stack. we give a bogus return
  ; address of 0xFFFFE000 (the second-last
  ; page) to deliberately trigger a page
  ; fault that will alert us when the signal
  ; handler tried to return.
  mov ebx, [ebp+8]
  mov esi, ebx
  add esi, 46
  mov esi, [esi]
  sub esi, 4
  mov dword [esi], 0xFFFFE000
  ; push a new state onto our stack
  push dword [ebx+50]   ; SS
  push dword esi        ; ESP
  push dword [ebx+42]   ; EFLAGS
  push dword [ebx+38]   ; CS
  push dword [ecx]      ; EIP
  push dword 0          ; EAX
  push dword 0          ; ECX
  push dword 0          ; EDX
  push dword 0          ; EBX
  push dword 0          ; ESP
  push dword [ebx+10]   ; EBP
  push dword 0          ; ESI
  push dword 0          ; EDI
  push word [ebx]       ; DS
  RESTORE
.end:
  pop esi
  pop ebx
  pop ebp
  ret

sigret:
  mov ebx, esp
  push ebx
  call check_signals
  add esp, 4
  ; restore our original stack frame and reset the TSS's ESP0
  mov esp, [eax+ts_tss_esp0]
  mov dword [eax+ts_tss_esp0], 0xFFFFFFFF
  ; finally, perform the normal syscall return
  RESTORE

syscall_init:
  push ebp
  mov ebp, esp
  push syscall_handler
  push dword 3
  push dword 0x80
  call register_trap
  add esp, 12
  pop ebp
  ret