kprobes 机制

Kprobe机制是内核提供的一种调试机制，它提供了一种方法，能够在不修改现有代码的基础上，灵活的跟踪内核函数的执行。它的基本工作原理是：用户指定一个探测点，并把一个用户定义的处理函数关联到该探测点，当内核执行到该探测点时，相应的关联函数被执行，然后继续执行正常的代码路径。

Kprobe提供了三种形式的探测点，一种是最基本的kprobe，能够在指定代码执行前、执行后进行探测，但此时不能访问被探测函数内的相关变量信息；一种是jprobe，用于探测某一函数的入口，并且能够访问对应的函数参数；一种是kretprobe，用于完成指定函数返回值的探测功能。其中最基本的就是kprobe机制，jprobe以及kretprobe的实现都依赖于kprobe，但其代码的实现都很巧妙，强烈建议每一个内核爱好者阅读。

好了，闲话少叙，开始上代码：

首先是struct kprobe结构，每一个探测点的基本结构。

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struct kprobe {
/*用于保存kprobe的全局hash表，以被探测的addr为key*/
struct hlist_node hlist;

/* list of kprobes for multi-handler support */
/*当对同一个探测点存在多个探测函数时，所有的函数挂在这条链上*/
struct list_head list;

/*count the number of times this probe was temporarily disarmed */
 unsigned long nmissed;

/* location of the probe point */
/*被探测的目标地址*/
 kprobe_opcode_t *addr;

/* Allow user to indicate symbol name of the probe point */
/*symblo_name的存在，允许用户指定函数名而非确定的地址*/
constchar*symbol_name;

/* Offset into the symbol */
/*如果被探测点为函数内部某个指令，需要使用addr + offset的方式*/
 unsigned int offset;

/* Called before addr is executed. */
/*探测函数，在目标探测点执行之前调用*/
 kprobe_pre_handler_t pre_handler;

/* Called after addr is executed, unless... */
/*探测函数，在目标探测点执行之后调用*/
 kprobe_post_handler_t post_handler;

/*
 * ... called if executing addr causes a fault (eg. page fault).
 * Return 1 if it handled fault, otherwise kernel will see it.
 */
 kprobe_fault_handler_t fault_handler;

/*
 * ... called if breakpoint trap occurs in probe handler.
 * Return 1 if it handled break, otherwise kernel will see it.
 */
 kprobe_break_handler_t break_handler;

/*opcode 以及 ainsn 用于保存被替换的指令码*/

/* Saved opcode (which has been replaced with breakpoint) */
 kprobe_opcode_t opcode;

/* copy of the original instruction */
struct arch_specific_insn ainsn;

/*
 * Indicates various status flags.
 * Protected by kprobe_mutex after this kprobe is registered.
 */
 u32 flags;
};

对于kprobe功能的实现主要利用了内核中的两个功能特性：异常（尤其是int 3），单步执行（EFLAGS中的TF标志）。

大概的流程：

1）在注册探测点的时候，对被探测函数的指令码进行替换，替换为int 3的指令码；

2）在执行int 3的异常执行中，通过通知链的方式调用kprobe的异常处理函数；

3）在kprobe的异常出来函数中，判断是否存在pre_handler钩子，存在则执行；

4）执行完后，准备进入单步调试，通过设置EFLAGS中的TF标志位，并且把异常返回的地址修改为保存的原指令码；

5）代码返回，执行原有指令，执行结束后触发单步异常；

6）在单步异常的处理中，清除单步标志，执行post_handler流程，并最终返回；

下面又进入代码时间，首先看一下kprobe模块的初始化代码，初始化代码主要做了两件事：标记出哪些代码是不能被探测的，这些代码属于kprobe实现的关键代码；注册通知链到die_notifier，用于接收异常通知。

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初始化代码位于kernel/kprobes.c中
staticint __init init_kprobes(void)
{
int i, err = 0;
....

/*kprobe_blacklist中保存的是kprobe实现的关键代码路径，这些函数不应该被kprobe探测*/
/*
 * Lookup and populate the kprobe_blacklist.
 *
 * Unlike the kretprobe blacklist, we'll need to determine
 * the range of addresses that belong to the said functions,
 * since a kprobe need not necessarily be at the beginning
 * of a function.
 */
for(kb = kprobe_blacklist; kb->name!=NULL; kb++){
 kprobe_lookup_name(kb->name, addr);
if(!addr)
continue;

 kb->start_addr =(unsigned long)addr;
 symbol_name = kallsyms_lookup(kb->start_addr,
&size,&offset,&modname, namebuf);
if(!symbol_name)
 kb->range = 0;
else
 kb->range =size;
}
....
if(!err)
/*注册通知链到die_notifier，用于接收int 3的异常信息*/
 err = register_die_notifier(&kprobe_exceptions_nb);
....
}
其中的通知链：
staticstruct notifier_block kprobe_exceptions_nb ={
.notifier_call = kprobe_exceptions_notify,
/*优先级最高，保证最先执行*/
.priority = 0x7fffffff /* we need to be notified first */
};

kprobe的注册流程register_kprobe。

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int __kprobes register_kprobe(struct kprobe *p)
{
int ret = 0;
struct kprobe *old_p;
struct module *probed_mod;
 kprobe_opcode_t *addr;

/*获取被探测点的地址，指定了symbol_name，则从kallsyms中获取；指定了offset，则返回addr + offset*/
 addr = kprobe_addr(p);
if(!addr)
return-EINVAL;
 p->addr = addr;

/*判断同一个kprobe是否被重复注册*/
 ret = check_kprobe_rereg(p);
if(ret)
return ret;

 jump_label_lock();
 preempt_disable();
/*判断被注册的函数是否位于内核的代码段内，或位于不能探测的kprobe实现路径中*/
if(!kernel_text_address((unsigned long) p->addr)||
 in_kprobes_functions((unsigned long) p->addr)||
 ftrace_text_reserved(p->addr, p->addr)||
 jump_label_text_reserved(p->addr, p->addr))
goto fail_with_jump_label;

/* User can pass only KPROBE_FLAG_DISABLED to register_kprobe */
 p->flags&= KPROBE_FLAG_DISABLED;

/*
 * Check if are we probing a module.
 */
/*判断被探测的地址是否属于某一个模块，并且位于模块的text section内*/
 probed_mod = __module_text_address((unsigned long) p->addr);
if(probed_mod){
/*如果被探测的为模块地址，首先要增加模块的引用计数*/
/*
 * We must hold a refcount of the probed module while updating
 * its code to prohibit unexpected unloading.
 */
if(unlikely(!try_module_get(probed_mod)))
goto fail_with_jump_label;

/*
 * If the module freed .init.text, we couldn't insert
 * kprobes in there.
 */
/*如果被探测的地址位于模块的init地址段内，但该段代码区间已被释放，则直接退出*/
if(within_module_init((unsigned long)p->addr, probed_mod)&&
 probed_mod->state!= MODULE_STATE_COMING){
 module_put(probed_mod);
goto fail_with_jump_label;
}
}
 preempt_enable();
 jump_label_unlock();

 p->nmissed = 0;
 INIT_LIST_HEAD(&p->list);
 mutex_lock(&kprobe_mutex);

 jump_label_lock();/* needed to call jump_label_text_reserved() */

 get_online_cpus();/* For avoiding text_mutex deadlock. */
 mutex_lock(&text_mutex);

/*判断在同一个探测点是否已经注册了其他的探测函数*/
 old_p = get_kprobe(p->addr);
if(old_p){
/* Since this may unoptimize old_p, locking text_mutex. */
/*如果已经存在注册过的kprobe，则将探测点的函数修改为aggr_pre_handler，并将所有的handler挂载到其链表上，由其负责所有handler函数的执行*/
 ret = register_aggr_kprobe(old_p, p);
goto out;
}

/* 分配特定的内存地址用于保存原有的指令
 * 按照内核注释，被分配的地址必须must be on special executable page on x86.
 * 该地址被保存在kprobe->ainsn.insn
 */
 ret = arch_prepare_kprobe(p);
if(ret)
goto out;

/*将kprobe加入到相应的hash表内*/
 INIT_HLIST_NODE(&p->hlist);
 hlist_add_head_rcu(&p->hlist,
&kprobe_table[hash_ptr(p->addr, KPROBE_HASH_BITS)]);

if(!kprobes_all_disarmed &&!kprobe_disabled(p))
/*将探测点的指令码修改为int 3指令*/
 __arm_kprobe(p);

/* Try to optimize kprobe */
 try_to_optimize_kprobe(p);

out:
 mutex_unlock(&text_mutex);
 put_online_cpus();
 jump_label_unlock();
 mutex_unlock(&kprobe_mutex);

if(probed_mod)
 module_put(probed_mod);

return ret;

fail_with_jump_label:
 preempt_enable();
 jump_label_unlock();
return-EINVAL;

注册完毕，就开始kprobe的执行流程了。对于该探测点，由于其起始指令已经被修改为int3，因此在执行到该地址时，必然会触发3号中断向量的处理流程do_int3.

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/* May run on IST stack. */
dotraplinkage void __kprobes do_int3(struct pt_regs *regs,long error_code)
{
#ifdef CONFIG_KGDB_LOW_LEVEL_TRAP
if(kgdb_ll_trap(DIE_INT3,"int3", regs, error_code, 3, SIGTRAP)
== NOTIFY_STOP)
return;
#endif /* CONFIG_KGDB_LOW_LEVEL_TRAP */
#ifdef CONFIG_KPROBES
 /*在这里以DIE_INT3，通知kprobe注册的通知链*/
if(notify_die(DIE_INT3,"int3", regs, error_code, 3, SIGTRAP)
== NOTIFY_STOP)
return;
#else
if(notify_die(DIE_TRAP,"int3", regs, error_code, 3, SIGTRAP)
== NOTIFY_STOP)
return;
#endif

 preempt_conditional_sti(regs);
 do_trap(3, SIGTRAP,"int3", regs, error_code,NULL);
 preempt_conditional_cli(regs);
}

在do_int3中触发kprobe注册的通知链函数，kprobe_exceptions_notify。由于kprobe以及jprobe等机制的处理核心都在此函数内，这里只针对kprobe的流程进行分析：进入函数的原因是DIE_INT3,并且是第一次进入该函数。

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int __kprobes kprobe_exceptions_notify(struct notifier_block *self,
 unsigned long val,void*data)
{
struct die_args *args = data;
int ret = NOTIFY_DONE;

if(args->regs && user_mode_vm(args->regs))
return ret;

switch(val){
case DIE_INT3:
/*对于kprobe，进入kprobe_handle*/
if(kprobe_handler(args->regs))
 ret = NOTIFY_STOP;
break;
case DIE_DEBUG:
if(post_kprobe_handler(args->regs)){
/*
 * Reset the BS bit in dr6 (pointed by args->err) to
 * denote completion of processing
 */
(*(unsigned long*)ERR_PTR(args->err))&=~DR_STEP;
 ret = NOTIFY_STOP;
}
break;
case DIE_GPF:
/*
 * To be potentially processing a kprobe fault and to
 * trust the result from kprobe_running(), we have
 * be non-preemptible.
 */
if(!preemptible()&& kprobe_running()&&
 kprobe_fault_handler(args->regs, args->trapnr))
 ret = NOTIFY_STOP;
break;
default:
break;
}
return ret;
}

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staticint __kprobes kprobe_handler(struct pt_regs *regs)
{
 kprobe_opcode_t *addr;
struct kprobe *p;
struct kprobe_ctlblk *kcb;

 /*对于int 3中断，其被Intel定义为Trap，那么异常发生时EIP寄存器内指向的为异常指令的后一条指令*/
 addr =(kprobe_opcode_t *)(regs->ip - sizeof(kprobe_opcode_t));
/*
 * We don't want to be preempted for the entire
 * duration of kprobe processing. We conditionally
 * re-enable preemption at the end of this function,
 * and also in reenter_kprobe() and setup_singlestep().
 */
 preempt_disable();

 kcb = get_kprobe_ctlblk();
 /*获取addr对应的kprobe*/
 p = get_kprobe(addr);

if(p){
/*如果异常的进入是由kprobe导致，则进入reenter_kprobe(jprobe需要，到时候分析)*/
if(kprobe_running()){
if(reenter_kprobe(p, regs, kcb))
return 1;
}else{
 set_current_kprobe(p, regs, kcb);
 kcb->kprobe_status = KPROBE_HIT_ACTIVE;

/*
 * If we have no pre-handler or it returned 0, we
 * continue with normal processing. If we have a
 * pre-handler and it returned non-zero, it prepped
 * for calling the break_handler below on re-entry
 * for jprobe processing, so get out doing nothing
 * more here.
 */
/*执行在此地址上挂载的pre_handle函数*/
if(!p->pre_handler ||!p->pre_handler(p, regs))
/*设置单步调试模式，为post_handle函数的执行做准备*/
 setup_singlestep(p, regs, kcb, 0);
return 1;
}
}elseif(*addr != BREAKPOINT_INSTRUCTION){
/*
 * The breakpoint instruction was removed right
 * after we hit it. Another cpu has removed
 * either a probepoint or a debugger breakpoint
 * at this address. In either case, no further
 * handling of this interrupt is appropriate.
 * Back up over the (now missing) int3 and run
 * the original instruction.
 */
 regs->ip =(unsigned long)addr;
 preempt_enable_no_resched();
return 1;
}elseif(kprobe_running()){
 p = __this_cpu_read(current_kprobe);
if(p->break_handler && p->break_handler(p, regs)){
 setup_singlestep(p, regs, kcb, 0);
return 1;
}
}/* else: not a kprobe fault; let the kernel handle it */

 preempt_enable_no_resched();
return 0;
}

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staticvoid __kprobes setup_singlestep(struct kprobe *p,struct pt_regs *regs,
struct kprobe_ctlblk *kcb,int reenter)
{
if(setup_detour_execution(p, regs, reenter))
return;

#if!defined(CONFIG_PREEMPT)
if(p->ainsn.boostable == 1 &&!p->post_handler){
/* Boost up -- we can execute copied instructions directly */
if(!reenter)
 reset_current_kprobe();
/*
 * Reentering boosted probe doesn't reset current_kprobe,
 * nor set current_kprobe, because it doesn't use single
 * stepping.
 */
 regs->ip =(unsigned long)p->ainsn.insn;
 preempt_enable_no_resched();
return;
}
#endif
/*jprobe*/
if(reenter){
 save_previous_kprobe(kcb);
 set_current_kprobe(p, regs, kcb);
 kcb->kprobe_status = KPROBE_REENTER;
}else
 kcb->kprobe_status = KPROBE_HIT_SS;
/* Prepare real single stepping */
/*准备单步模式，设置EFLAGS的TF标志位，清楚IF标志位(禁止中断)*/
 clear_btf();
 regs->flags|= X86_EFLAGS_TF;
 regs->flags&=~X86_EFLAGS_IF;
/* single step inline if the instruction is an int3 */
if(p->opcode == BREAKPOINT_INSTRUCTION)
 regs->ip =(unsigned long)p->addr;
else
/*设置异常返回的指令为保存的被探测点的指令*/
 regs->ip =(unsigned long)p->ainsn.insn;
}

对应kprobe,pre_handle的执行就结束了，按照代码，程序开始执行保存的被探测点的指令，由于开启了单步调试模式，执行完指令后会继续触发异常，这次的是do_debug异常处理流程。

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dotraplinkage void __kprobes do_debug(struct pt_regs *regs,long error_code)
{
....

 /*在do_debug中，以DIE_DEBUG再一次触发kprobe的通知链*/
if(notify_die(DIE_DEBUG,"debug", regs, PTR_ERR(&dr6), error_code,
 SIGTRAP)== NOTIFY_STOP)
return;

 ....
return;
}

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/*对于kprobe_exceptions_notify，其DIE_DEBUG处理流程*/
case DIE_DEBUG:
if(post_kprobe_handler(args->regs)){
/*
 * Reset the BS bit in dr6 (pointed by args->err) to
 * denote completion of processing
 */
(*(unsigned long*)ERR_PTR(args->err))&=~DR_STEP;
 ret = NOTIFY_STOP;
}
break;

staticint __kprobes post_kprobe_handler(struct pt_regs *regs)
{
struct kprobe *cur = kprobe_running();
struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();

if(!cur)
return 0;

/*设置异常返回的EIP为下一条需要执行的指令*/
 resume_execution(cur, regs, kcb);
/*恢复异常执行前的EFLAGS*/
 regs->flags|= kcb->kprobe_saved_flags;

/*执行post_handler函数*/
if((kcb->kprobe_status != KPROBE_REENTER)&& cur->post_handler){
 kcb->kprobe_status = KPROBE_HIT_SSDONE;
 cur->post_handler(cur, regs, 0);
}

/* Restore back the original saved kprobes variables and continue. */
if(kcb->kprobe_status == KPROBE_REENTER){
 restore_previous_kprobe(kcb);
goto out;
}
 reset_current_kprobe();
out:
 preempt_enable_no_resched();

/*
 * if somebody else is singlestepping across a probe point, flags
 * will have TF set, in which case, continue the remaining processing
 * of do_debug, as if this is not a probe hit.
 */
if(regs->flags& X86_EFLAGS_TF)
return 0;

return 1;
}

至此，一个典型的kprobe的流程已经执行完毕了。