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中断(一)——中断描述符表的定义和初始化 (基于3.16-rc4)

1.中断描述符表的定义(arch/x86/kernel/traps.c)

1 gate_desc debug_idt_table[NR_VECTORS] __page_aligned_bss;

定义的描述符表为一个结构体数组,数组元素类型为gate_desc,大小为8B。NR_VECTORS宏为256,即描述符表大小为256*8B。

 

2.idt_descr变量的定义(arch/x86/kernel/head_32.S)

1 idt_descr:2     .word IDT_ENTRIES*8-1        # idt contains 256 entries3     .long idt_table4 5 # boot GDT descriptor (later on used by CPU#0):6     .word 0                # 32 bit align gdt_desc.address

这是内核定义的一个全局变量,存放有中断描述符表的大小和首地址。该变量将存放在idtr寄存器中。

 

3.中断描述符初步的初始化(arch/x86/kernel/head_32.S)

 1 __INIT 2 setup_once: 3     /* 4      * Set up a idt with 256 entries pointing to ignore_int, 5      * interrupt gates. It doesn‘t actually load idt - that needs 6      * to be done on each CPU. Interrupts are enabled elsewhere, 7      * when we can be relatively sure everything is ok. 8      */ 9 10     movl $idt_table,%edi11     movl $early_idt_handlers,%eax12     movl $NUM_EXCEPTION_VECTORS,%ecx13 1:14     movl %eax,(%edi)15     movl %eax,4(%edi)16     /* interrupt gate, dpl=0, present */17     movl $(0x8E000000 + __KERNEL_CS),2(%edi)18     addl $9,%eax19     addl $8,%edi20     loop 1b21 22     movl $256 - NUM_EXCEPTION_VECTORS,%ecx23     movl $ignore_int,%edx24     movl $(__KERNEL_CS << 16),%eax25     movw %dx,%ax        /* selector = 0x0010 = cs */26     movw $0x8E00,%dx    /* interrupt gate - dpl=0, present */27 2:28     movl %eax,(%edi)29     movl %edx,4(%edi)30     addl $8,%edi31     loop 2b32         ...33         ...

这段代码是对中断描述符表的初步初始化,14-20行是对前32个中断描述符进行初始化,让所有描述符指向early_idt_handlers处理函数。22-31行是对后256-32=224个中断描述符进行初始化,使之指向ignore_int处理函数。省略号以后是对GDT描述符表的初始化,这里不予讨论。

 

4.中断描述符表最终的初始化(arch/x86/kernel/traps.c)

 1 void __init trap_init(void) 2 { 3     int i; 4  5 #ifdef CONFIG_EISA 6     void __iomem *p = early_ioremap(0x0FFFD9, 4); 7  8     if (readl(p) == E + (I<<8) + (S<<16) + (A<<24)) 9         EISA_bus = 1;10     early_iounmap(p, 4);11 #endif12 13     set_intr_gate(X86_TRAP_DE, divide_error);14     set_intr_gate_ist(X86_TRAP_NMI, &nmi, NMI_STACK);15     /* int4 can be called from all */16     set_system_intr_gate(X86_TRAP_OF, &overflow);17     set_intr_gate(X86_TRAP_BR, bounds);18     set_intr_gate(X86_TRAP_UD, invalid_op);19     set_intr_gate(X86_TRAP_NM, device_not_available);20 #ifdef CONFIG_X86_3221     set_task_gate(X86_TRAP_DF, GDT_ENTRY_DOUBLEFAULT_TSS);22 #else23     set_intr_gate_ist(X86_TRAP_DF, &double_fault, DOUBLEFAULT_STACK);24 #endif25     set_intr_gate(X86_TRAP_OLD_MF, coprocessor_segment_overrun);26     set_intr_gate(X86_TRAP_TS, invalid_TSS);27     set_intr_gate(X86_TRAP_NP, segment_not_present);28     set_intr_gate_ist(X86_TRAP_SS, &stack_segment, STACKFAULT_STACK);29     set_intr_gate(X86_TRAP_GP, general_protection);30     set_intr_gate(X86_TRAP_SPURIOUS, spurious_interrupt_bug);31     set_intr_gate(X86_TRAP_MF, coprocessor_error);32     set_intr_gate(X86_TRAP_AC, alignment_check);33 #ifdef CONFIG_X86_MCE34     set_intr_gate_ist(X86_TRAP_MC, &machine_check, MCE_STACK);35 #endif36     set_intr_gate(X86_TRAP_XF, simd_coprocessor_error);37 38     /* Reserve all the builtin and the syscall vector: */39     for (i = 0; i < FIRST_EXTERNAL_VECTOR; i++)40         set_bit(i, used_vectors);41 42 #ifdef CONFIG_IA32_EMULATION43     set_system_intr_gate(IA32_SYSCALL_VECTOR, ia32_syscall);44     set_bit(IA32_SYSCALL_VECTOR, used_vectors);45 #endif46 47 #ifdef CONFIG_X86_3248     set_system_trap_gate(SYSCALL_VECTOR, &system_call);FIRST_EXTERNAL_VECTOR49     set_bit(SYSCALL_VECTOR, used_vectors);50 #endif51 52     /*53      * Set the IDT descriptor to a fixed read-only location, so that the54      * "sidt" instruction will not leak the location of the kernel, and55      * to defend the IDT against arbitrary memory write vulnerabilities.56      * It will be reloaded in cpu_init() */57     __set_fixmap(FIX_RO_IDT, __pa_symbol(idt_table), PAGE_KERNEL_RO);58     idt_descr.address = fix_to_virt(FIX_RO_IDT);59 60     /*61      * Should be a barrier for any external CPU state:62      */63     cpu_init();64 65     x86_init.irqs.trap_init();66 67 #ifdef CONFIG_X86_6468     memcpy(&debug_idt_table, &idt_table, IDT_ENTRIES * 16);69     set_nmi_gate(X86_TRAP_DB, &debug);70     set_nmi_gate(X86_TRAP_BP, &int3);71 #endif72 }

该函数对中断描述表的进行了部分初始化,13-36行对系统已分配的异常和非屏蔽中断进行初始化,中断向量号为0-19。接着,39-40行在中断位图表中对已初始化的中断所对应的位进行标记。接着,43和48行又出始化了两个中断,一个是系统中断门,中断向量号为0x80,一个是系统陷阱门,中断向量号为2。

在该函数中,大家可以看出,对中断进行初始化的函数有如下几个:

1 set_intr_gate()2 set_system_intr_gate()3 set_system_trap_gate()
4 set_task_gate()

这几个函数也在arch/x86/kernel/traps.c中定义。分别是对中断门,系统中断门,系统陷阱门,任务门描述符的初始化。进一步深入可发现,这几个函数都调用了如下的函数:

 1 static inline void _set_gate(int gate, unsigned type, void *addr, 2                  unsigned dpl, unsigned ist, unsigned seg) 3 { 4     gate_desc s; 5  6     pack_gate(&s, type, (unsigned long)addr, dpl, ist, seg); 7     /* 8      * does not need to be atomic because it is only done once at 9      * setup time10      */11     write_idt_entry(idt_table, gate, &s);12     write_trace_idt_entry(gate, &s);13 }

该函数定义在arch/x86/include/asm/desc.h文件中。在该函数中定义了一个gate_desc类型变量s,并将s的指针传递给pack_gate函数,把要初始化的描述符各个字段的值临时存放在s中。下边分析下pack_gate函数,在分析该函数之前,我们先看下gate_desc结构体。

 1 struct desc_struct { 2     union { 3         struct { 4             unsigned int a; 5             unsigned int b; 6         }; 7         struct { 8             u16 limit0; 9             u16 base0;10             unsigned base1: 8, type: 4, s: 1, dpl: 2, p: 1;11             unsigned limit: 4, avl: 1, l: 1, d: 1, g: 1, base2: 8;12         };13     };14 } __attribute__((packed));

typedef struct desc_struct gate_desc

该结构体定义位于arch/x86/include/asm/desc_defs.h中。该结构体中包含了一个共用体,共用体中又包含了两个结构体。我们知道,共用体在分配内存单元时,并不为每个成员都分配,而是为最大的成员来分配。可以看出该共用体的两个结构体成员大小相等,都是8B,因此整个gate_desc结构体大小就为8B。我们可以使用共用体中的任意一个结构体成员来为这个gate_desc赋值,也就是说我们既可以将gate_desc看成是struct { unsigned int a;  unsigned int b; };也可以看成是structu16 limit0; u16 base0; .... };下面在分析pack_gate函数过程中将看到赋值过程,我们将gate_desc看作是struct { unsigned int a;  unsigned int b; };

1 static inline void pack_gate(gate_desc *gate, unsigned char type,2                  unsigned long base, unsigned dpl, unsigned flags,3                  unsigned short seg)4 {5     gate->a = (seg << 16) | (base & 0xffff);6     gate->b = (base & 0xffff0000) | (((0x80 | type | (dpl << 5)) & 0xff) << 8);7 }

该函数也定义在arch/x86/include/asm/desc.h文件中。在该函数中为gate所指向的gate_desc描述符进行初始化。gate->a是描述符的0-31位,gate->b是描述符的32-63位。描述符的如下所示:

接着,我们分析_set_gate()中的11行,write_idt_entry()调用。

1 static inline void native_write_idt_entry(gate_desc *idt, int entry, const gate_desc *gate)2 {3     memcpy(&idt[entry], gate, sizeof(*gate));4 }

#define write_idt_entry() native_write_idt_entry()   //粗略的写了下,大家能明白就行

该函数定义在arch/x86/include/asm/desc.h中。在该函数中,使用memcpy()函数将gate中的字段复制到&idt[entry]所指向的各个字段中。很显然,idt[]数组就是内核中定义的中断描述符表,我们在文章开头给大家看过该定义。gate就是我们在_set_gate()中定义的临时变量s,在这里我们将s中的字段值赋给idt[]数组的对应元素,至此一个描述符的初始化工作就全部完成了,s变量的用途也就结束了,另外,entry变量中存放的是要初始化的中断向量号,用该号来定位idt数组的元素。

 

最后,再补充说明一点东西,回头看下第4点中的trap_init()函数,在该函数中对中断描述符表进行初始化,使用了很多初始化函数比如set_intr_gate()或set_system_intr_gate()等等,我们拿第一个初始化函数set_intr_gate(X86_TRAP_DE, divide_error)来做说明。X86_TRAP_DE是枚举类型参数,代表的是中断向量号,定义在arch/x86/include/asm/traps.h文件中。这种枚举类型其实有很多。

 1 /* Interrupts/Exceptions */ 2 enum { 3     X86_TRAP_DE = 0,    /*  0, Divide-by-zero */ 4     X86_TRAP_DB,        /*  1, Debug */ 5     X86_TRAP_NMI,        /*  2, Non-maskable Interrupt */ 6     X86_TRAP_BP,        /*  3, Breakpoint */ 7     X86_TRAP_OF,        /*  4, Overflow */ 8     X86_TRAP_BR,        /*  5, Bound Range Exceeded */ 9     X86_TRAP_UD,        /*  6, Invalid Opcode */10     X86_TRAP_NM,        /*  7, Device Not Available */11     X86_TRAP_DF,        /*  8, Double Fault */12     X86_TRAP_OLD_MF,    /*  9, Coprocessor Segment Overrun */13     X86_TRAP_TS,        /* 10, Invalid TSS */14     X86_TRAP_NP,        /* 11, Segment Not Present */15     X86_TRAP_SS,        /* 12, Stack Segment Fault */16     X86_TRAP_GP,        /* 13, General Protection Fault */17     X86_TRAP_PF,        /* 14, Page Fault */18     X86_TRAP_SPURIOUS,    /* 15, Spurious Interrupt */19     X86_TRAP_MF,        /* 16, x87 Floating-Point Exception */20     X86_TRAP_AC,        /* 17, Alignment Check */21     X86_TRAP_MC,        /* 18, Machine Check */22     X86_TRAP_XF,        /* 19, SIMD Floating-Point Exception */23     X86_TRAP_IRET = 32,    /* 32, IRET Exception */24 };

第二个参数,是汇编函数的函数名(在这里作为函数指针来使用),该函数为内核原先就定义好的中断或异常处理程序。这种类型的函数有很多,都定义在arch/x86/kernel/entry_32.S文件中,下边我们列举几个给大家看看,有兴趣自己去查。

 

 1 ENTRY(segment_not_present) 2     RING0_EC_FRAME 3     ASM_CLAC 4     pushl_cfi $do_segment_not_present 5     jmp error_code 6     CFI_ENDPROC 7 END(segment_not_present) 8  9 ENTRY(stack_segment)10     RING0_EC_FRAME11     ASM_CLAC12     pushl_cfi $do_stack_segment13     jmp error_code14     CFI_ENDPROC15 END(stack_segment)16 17 ENTRY(alignment_check)18     RING0_EC_FRAME19     ASM_CLAC20     pushl_cfi $do_alignment_check21     jmp error_code22     CFI_ENDPROC23 END(alignment_check)24 25 ENTRY(divide_error)26     RING0_INT_FRAME27     ASM_CLAC28     pushl_cfi $0            # no error code29     pushl_cfi $do_divide_error30     jmp error_code31     CFI_ENDPROC32 END(divide_error)

 

这些汇编代码只是异常处理程序的开头一部分,可以看到每一个汇编段中,都有一条pushl_cfi $do_***的指令,该$do_***才是真正的异常处理程序(函数名,也是函数指针),现将该函数名压入栈中,然后通过jmp error_code指令跳转到$do_***函数中。error_code其实也是一段汇编代码,在下篇博文中断(二)中,我们将分析该段代码。

 

 

至此,中断描述符的初始化工作就告一段落。文中有问题的地方希望大家指正。qq:1193533825

中断(一)——中断描述符表的定义和初始化 (基于3.16-rc4)