Luigi Capogrosso
83 lines
3.1 KiB
NASM
83 lines
3.1 KiB
NASM
; MentOS, The Mentoring Operating system project
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; @file user.asm
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; @brief
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; @copyright (c) 2019 This file is distributed under the MIT License.
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; See LICENSE.md for details.
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; Enter userspace (ring3) (from Ring 0, namely Kernel)
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; Usage: enter_userspace(uintptr_t location, uintptr_t stack);
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; On stack
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; | stack |
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; | location |
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; | return address |
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; | EBP | EBP
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; | SS |
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; | ESP |
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; | EFLAGS |
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; | CS |
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; | EIP |
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global enter_userspace ; Allows the C code to call enter_userspace(...).
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enter_userspace:
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push ebp ; Save current ebp
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mov ebp, esp ; open a new stack frame
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;==== Segment selector =====================================================
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mov ax, 0x23
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mov ds, ax
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mov es, ax
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mov fs, ax
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mov gs, ax
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; we don't need to worry about SS. it's handled by iret
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;---------------------------------------------------------------------------
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; We have to prepare the following stack before executing iret
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; SS --> Segment selector
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; ESP --> Stack address
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; EFLAGS --> CPU state flgas
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; CS --> Code segment
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; EIP --> Entry point
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;
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;==== User data segmenet with bottom 2 bits set for ring3 ?=================
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push 0x23 ; push SS on Kernel's stack
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;---------------------------------------------------------------------------
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;==== (ESP) Stack address ==================================================
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mov eax, [ebp + 0xC] ; get uintptr_t stack
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push eax ; push process's stack address on Kernel's stack
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;---------------------------------------------------------------------------
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;==== (EFLAGS) =============================================================
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pushf ; push EFLAGS into Kernel's stack
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pop eax ; pop EFLAGS into eax
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or eax, 0x200 ; enable interrupt ?request ring3
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push eax ; push new EFLAGS on Kernel's stack
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;---------------------------------------------------------------------------
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;==== (CS) Code Segment ====================================================
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push 0x1B ;
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;---------------------------------------------------------------------------
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;==== (EIP) Entry point ====================================================
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mov eax, [ebp + 0x8] ; get uintptr_t location
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push eax ; push uintptr_t location on Kernel's stack
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;---------------------------------------------------------------------------
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iret ; interrupt return
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; WE SHOULD NOT STILL BE HERE! :(
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;==== Reset segment selector ===============================================
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mov ax, 0x10
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mov ds, ax
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mov es, ax
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mov fs, ax
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mov gs, ax
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;---------------------------------------------------------------------------
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add esp, 0x14 ; reset stack pointer (20 bytes)
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pop ebp ; reset value of ebp
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ret ; return to kernel code
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