Linux进程内核栈

进程创建的时候Linux内核会创建内核栈（arm手册也要求内核态有单独的栈），如应用进程在用户态通过系统调用陷入内核态的时候，上下文信息（如cpu寄存器）需要有个地方保存，如此，从内核态切换回用户态时候，能继续从系统调用之后的代码开始执行，这个保存的地方就是进程的内核栈，本文主要描述arm32下内核栈的生成过程和结构。

1.内核栈数据结构

正如进程在用户态执行函数跳转有一个栈，在内核态执行的时候同样有一个内核态的栈，分成两个栈也是处于安全的考虑，如果都使用用户态的栈，那么内核的数据可以被应用态访问不安全。我们不禁要问如下几个问题：

• 内核栈大小/结构/创建过程
• 怎么找到内核栈（哪些数据结构和API可以索引到）

标识进程的核心数据结构task_struct中有一个void *stack成员指向进程内核栈：

  struct task_struct {
      #ifdef CONFIG_THREAD_INFO_IN_TASK
      /*
       * For reasons of header soup (see current_thread_info()), this
       * must be the first element of task_struct.
       */
      struct thread_info      thread_info;
      #endif
      void * stack;
	  ...
  }

目前平台没有配置 CONFIG_THREAD_INFO_IN_TASK，所以thread_info放在了stack指向的内存中，thread_info中存储了体系结构相关的信息，arm32 内核栈大小8KB:

//ARM架构 , 8K
#define THREAD_SIZE_ORDER	1
#define THREAD_SIZE		(PAGE_SIZE << THREAD_SIZE_ORDER)
#define THREAD_START_SP		(THREAD_SIZE - 8)

 2.内核栈相关的API和数据结构

• task_stack_page

static inline void *task_stack_page(const struct task_struct *task)
{
	return task->stack;
}

• task_pt_regs

#define task_pt_regs(p) \                                                                                                                                                
    ((struct pt_regs *)(THREAD_START_SP + task_stack_page(p)) - 1)

• pt_regs 

struct pt_regs {                                                                                                                                           
    unsigned long uregs[18];
};
 
#define ARM_cpsr    uregs[16]
#define ARM_pc      uregs[15]
#define ARM_lr      uregs[14]
#define ARM_sp      uregs[13]
#define ARM_ip      uregs[12]
#define ARM_fp      uregs[11]
#define ARM_r10     uregs[10]
#define ARM_r9      uregs[9]
#define ARM_r8      uregs[8]
#define ARM_r7      uregs[7]
#define ARM_r6      uregs[6]
#define ARM_r5      uregs[5]
#define ARM_r4      uregs[4]
#define ARM_r3      uregs[3]
#define ARM_r2      uregs[2]
#define ARM_r1      uregs[1]
#define ARM_r0      uregs[0]
#define ARM_ORIG_r0 uregs[17]

 进程从用户态陷入内核态时候，用户态的上下文信息保存在pt_regs数据结构中。

• struct thread_info 

/*
 * low level task data that entry.S needs immediate access to.
 * __switch_to() assumes cpu_context follows immediately after cpu_domain.
 */
struct thread_info {
    unsigned long       flags;      /* low level flags */
    int         preempt_count;  /* 0 => preemptable, <0 => bug */
    mm_segment_t        addr_limit; /* address limit */
    struct task_struct  *task;      /* main task structure */
    __u32           cpu;        /* cpu */
    __u32           cpu_domain; /* cpu domain */
    struct cpu_context_save cpu_context;    /* cpu context */
    __u32           syscall;    /* syscall number */
    __u8            used_cp[16];    /* thread used copro */
    unsigned long       tp_value[2];    /* TLS registers */
#ifdef CONFIG_CRUNCH
    struct crunch_state crunchstate;
#endif
    union fp_state      fpstate __attribute__((aligned(8)));
    union vfp_state     vfpstate;
#ifdef CONFIG_ARM_THUMBEE
    unsigned long       thumbee_state;  /* ThumbEE Handler Base register */
#endif
    void            *regs_on_excp;  /* aee */
    int         cpu_excp;   /* aee */
};
 
struct cpu_context_save {
    __u32   r4;
    __u32   r5;
    __u32   r6;
    __u32   r7;
    __u32   r8;
    __u32   r9;
    __u32   sl;
    __u32   fp;
    __u32   sp;
    __u32   pc;
    __u32   extra[2];       /* Xscale 'acc' register, etc */
};

 3.内核态SP寄存器

我们知道进程在内核态执行的时候，sp寄存器指向了内核栈，为什么内核的sp寄存器指向进程内核栈？这是什么时候设置的？

答案：进程上下文切换的时候（switch_to汇编）

首先进程创建的时候，在copy_thread会创建内核栈，并将内核栈地址保存在thread_info->cpu_context中，代码如下：

//参数p时指新建进程的task_struct
int
copy_thread(unsigned long clone_flags, unsigned long stack_start,
{       
    struct thread_info *thread = task_thread_info(p);
    struct pt_regs *childregs = task_pt_regs(p);
    
    memset(&thread->cpu_context, 0, sizeof(struct cpu_context_save));
    
#ifdef CONFIG_CPU_USE_DOMAINS
    /*
     * Copy the initial value of the domain access control register
     * from the current thread: thread->addr_limit will have been
     * copied from the current thread via setup_thread_stack() in
     * kernel/fork.c
     */
    thread->cpu_domain = get_domain();
#endif
 
    if (likely(!(p->flags & PF_KTHREAD))) {
        *childregs = *current_pt_regs();
        childregs->ARM_r0 = 0;
        if (stack_start)
            childregs->ARM_sp = stack_start;
    } else {
        memset(childregs, 0, sizeof(struct pt_regs));
        thread->cpu_context.r4 = stk_sz;
        thread->cpu_context.r5 = stack_start;
        childregs->ARM_cpsr = SVC_MODE;
    }   
    thread->cpu_context.pc = (unsigned long)ret_from_fork;
    thread->cpu_context.sp = (unsigned long)childregs;
    
    clear_ptrace_hw_breakpoint(p);
    
    if (clone_flags & CLONE_SETTLS)
        thread->tp_value[0] = childregs->ARM_r3;
    thread->tp_value[1] = get_tpuser();
    
    thread_notify(THREAD_NOTIFY_COPY, thread);
    
    return 0;
} 

thread->cpu_context.pc = (unsigned long) ret_from_fork设置新建进程的执行入口时ret_from_frok函数。

thread->cpu_context.sp = (unsigned long)childregs；thread_info成员cpu_context的sp成员指向了内核栈的pt_regs数据结构，pt_regs保存了用户态的通用寄存器。

上下文切换switch_to函数会将thread->cpu_context.sp设置到cpu的寄存器中，那么其中的sp就设置了内核态的sp寄存器中：

/*
 * Register switch for ARMv3 and ARMv4 processors
 * r0 = previous task_struct, r1 = previous thread_info, r2 = next thread_info
 * previous and next are guaranteed not to be the same.
 */
ENTRY(__switch_to)
 UNWIND(.fnstart    )
 UNWIND(.cantunwind )
    add ip, r1, #TI_CPU_SAVE    @ip指向被换出进程的thread_info->cpu_context
 ARM(   stmia   ip!, {r4 - sl, fp, sp, lr} )    @ Store most regs on stack，即保存到cpu_context中
 THUMB( stmia   ip!, {r4 - sl, fp}     )    @ Store most regs on stack
 THUMB( str sp, [ip], #4           )
 THUMB( str lr, [ip], #4           )
    ldr r4, [r2, #TI_TP_VALUE]
    ldr r5, [r2, #TI_TP_VALUE + 4]
#ifdef CONFIG_CPU_USE_DOMAINS
    mrc p15, 0, r6, c3, c0, 0       @ Get domain register
    str r6, [r1, #TI_CPU_DOMAIN]    @ Save old domain register
    ldr r6, [r2, #TI_CPU_DOMAIN]
#endif
    switch_tls r1, r4, r5, r3, r7
#if defined(CONFIG_CC_STACKPROTECTOR) && !defined(CONFIG_SMP)
    ldr r7, [r2, #TI_TASK]
    ldr r8, =__stack_chk_guard
    .if (TSK_STACK_CANARY > IMM12_MASK)
    add r7, r7, #TSK_STACK_CANARY & ~IMM12_MASK
    .endif
    ldr r7, [r7, #TSK_STACK_CANARY & IMM12_MASK]
#endif
#ifdef CONFIG_CPU_USE_DOMAINS
    mcr p15, 0, r6, c3, c0, 0       @ Set domain register
#endif
    mov r5, r0
    add r4, r2, #TI_CPU_SAVE        @r4指向换入进程的cpu_context
    ldr r0, =thread_notify_head
    mov r1, #THREAD_NOTIFY_SWITCH
    bl  atomic_notifier_call_chain
#if defined(CONFIG_CC_STACKPROTECTOR) && !defined(CONFIG_SMP)
    str r7, [r8]
#endif
 THUMB( mov ip, r4             )
    mov r0, r5
 ARM(   ldmia   r4, {r4 - sl, fp, sp, pc}  )    @ Load all regs saved previously,即将cpu_context中值加载到cpu寄存器中
 THUMB( ldmia   ip!, {r4 - sl, fp}     )    @ Load all regs saved previously
 THUMB( ldr sp, [ip], #4           )
 THUMB( ldr pc, [ip]           )
 UNWIND(.fnend      )
ENDPROC(__switch_to)

ARM(   ldmia   r4, {r4 - sl, fp, sp, pc}  )会将进程thread_info->cpu_context中的值加载到cpu寄存器执行，上面分析我们知道进程创建的时候，thread->cpu_context.sp = (unsigned long)childregs，这样childregs值会加载到cpu sp寄存器，即内核态下sp指向了内核栈（更具体的说是内核栈中的pt_regs)