god damn CPU_LOOP 1(done)

最新推荐文章于 2021-09-10 13:59:51 发布

原创最新推荐文章于 2021-09-10 13:59:51 发布 · 1k 阅读

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本文详细分析了在内核重建中断描述符表（IDT）模块时，出现CPU_LOOP1异常的原因，并提供了解决方法。通过修改内存写操作的循环次数，从8改为16，成功避免了异常发生。

updated 2014-02-18:

CPU_LOOP 1在这里的含义为：写内存异常，因为ds指向了cs属性的代码段，当然，执行如下代码的时候会出现一个异常！

 mov dword ptr ds:[edx], eax

需要将下面：

mov cx, 0x08

改成

mov cx, 0x10

------- history --------

在调试内核重建idt模块的时候，就是一个简单的内存write操作，会出现一个CPU_LOOP 1的异常，目前暂未找到合理的解释和解决办法：

<bochs:14> 
Next at t=5544195
(0) [0x000067d5] 0008:000067d5 (unk. ctxt): mov dword ptr ds:[edx], eax ; 3e8902
<bochs:15> info cpu
eax:0x000867f0, ebx:0x0000000f, ecx:0x000067f0, edx:0x00005093
ebp:0x00009ea8, esp:0x00009ea8, esi:0x00000416, edi:0x0000cf92
eip:0x000067d5, eflags:0x00000006, inhibit_mask:0
cs:s=0x0008, dl=0x0000ffff, dh=0x004f9800, valid=1
ss:s=0x0010, dl=0x00000fff, dh=0x00c09300, valid=7
ds:s=0x0008, dl=0x00000fff, dh=0x00c09a00, valid=1
es:s=0x0010, dl=0x00000fff, dh=0x00c09300, valid=1
fs:s=0x0010, dl=0x00000fff, dh=0x00c09300, valid=1
gs:s=0x0010, dl=0x00000fff, dh=0x00c09300, valid=1
ldtr:s=0x0000, dl=0x00000000, dh=0x00000000, valid=0
tr:s=0x0000, dl=0x00000000, dh=0x00000000, valid=0
gdtr:base=0x00005893, limit=0x7ff
idtr:base=0x00005093, limit=0x7ff
dr0:0x00000000, dr1:0x00000000, dr2:0x00000000
dr3:0x00000000, dr6:0xffff0ff0, dr7:0x00000400
cr0:0x80000011, cr1:0x00000000, cr2:0x00000000
cr3:0x00000000, cr4:0x00000000
done
<bochs:16> n
CPU_LOOP 1
CPU_LOOP 1
CPU_LOOP 1
CPU_LOOP 1
CPU_LOOP 1

<bochs:15> info gdt
Global Descriptor Table (0x00005893):
GDT[0x00]=??? descriptor hi=00000000, lo=00000000
GDT[0x01]=Code segment, linearaddr=00000000, len=00fff * 4Kbytes, Execute/Read, Accessed, 32-bit addrs
GDT[0x02]=Data segment, linearaddr=00000000, len=00fff * 4Kbytes, Read/Write, Accessed
GDT[0x03]=??? descriptor hi=00000000, lo=00000000
GDT[0x04]=??? descriptor hi=00000000, lo=00000000


<bochs:16> 	info idt
Interrupt Descriptor Table (0x00005093):
IDT[0x00]=32-Bit Interrupt Gate target=0x0008:0x000000aa, DPL=0
IDT[0x01]=32-Bit Interrupt Gate target=0x0008:0x000000aa, DPL=0
IDT[0x02]=32-Bit Interrupt Gate target=0x0008:0x000000aa, DPL=0
IDT[0x03]=32-Bit Interrupt Gate target=0x0008:0x000000aa, DPL=0
IDT[0x04]=32-Bit Interrupt Gate target=0x0008:0x000000aa, DPL=0
IDT[0x05]=32-Bit Interrupt Gate target=0x0008:0x000000aa, DPL=0
IDT[0x06]=32-Bit Interrupt Gate target=0x0008:0x000000aa, DPL=0
IDT[0x07]=32-Bit Interrupt Gate target=0x0008:0x000000aa, DPL=0

附上代码snapshot：

;Function:set_trap_gate(eax,bx,edx)
;input:回调入口地址entry_addr(eax)，门类型type(bl)，dpl(bh)，待写入的gate表项的绝对地址gate_addr(edx)
;output: null
;action:将给定的回调入口addr，门类型，dpl拼装成8bytes的gate描述符表项，写入到指定gate_addr
;att:后面的参数先入栈, func protype:	set_gate(entry_addr, TRAP_TYPE, dpl, gate_addr);
;调用栈关系如下
; +-----+
;gate_addr	4bytes edx
;   dpl		4bytes 
;   type	4bytes ebx
; entry_addr	4bytes eax
;  ret_val	4bytes
; old_ebp	4bytes
;          <------------esp
set_gate:
	push ebp
	mov ebp, esp
	mov edx, [esp+20]	;idt gate_addr
	mov ebx, [esp+12]	;type	fixme: 2014-02-15, 4bytes对齐
	mov eax, [esp+8]	;entry_addr
	mov cx, 0x08
	mov ds, cx
	;0.将type放在bl，将dpl放在bh
	mov bh, [esp+16]
	
	;1.eax - 门描述符低4bytes
	mov ecx, eax
	;先掩掉eax的高2bytes，后续存放段选择符01-000
	and eax, 0x0000ffff
	;然后或上对应的段选择符
	or eax, 0x00080000	;fixme: 2014-02-15, 段选择子应该在高2bytes上!Dude
	;首先将门描述符的低4bytes写入到内存中
	mov dword [ds:edx], eax	;eax可以解放出来了, TODO_2014-02-15 写内存出现中断异常?