深入探究Linux Kprobe机制

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发布于 2020-12-14 09:46:39

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发布于 2020-12-14 09:46:39

来自：liuhangtiant

概述

kprobe机制用于在内核中动态添加一些探测点，可以满足一些调试需求。本文主要探寻kprobe的执行路径，也就是说如何trap到kprobe，以及如何回到原路径继续执行。

实例

先通过一个实例来感受下kprobe，linux中有一个现成的实例： samples/kprobes/kprobe_example.c 由于当前验证环境是基于qemu+arm64，我删除了其他架构的代码，并稍稍做了一下改动：

/*
 * NOTE: This example is works on x86 and powerpc.
 * Here's a sample kernel module showing the use of kprobes to dump a
 * stack trace and selected registers when _do_fork() is called.
 *
 * For more information on theory of operation of kprobes, see
 * Documentation/kprobes.txt
 *
 * You will see the trace data in /var/log/messages and on the console
 * whenever _do_fork() is invoked to create a new process.
 */

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/kprobes.h>

#define MAX_SYMBOL_LEN  64
static char symbol[MAX_SYMBOL_LEN] = "_do_fork";
module_param_string(symbol, symbol, sizeof(symbol), 0644);

/* For each probe you need to allocate a kprobe structure */
static struct kprobe kp = {
  .symbol_name  = symbol,
};

/* kprobe pre_handler: called just before the probed instruction is executed */
static int handler_pre(struct kprobe *p, struct pt_regs *regs)
{
  pr_info("<%s> pre_handler: p->addr = 0x%p, pc = 0x%lx,"
      " pstate = 0x%lx\n",
    p->symbol_name, p->addr, (long)regs->pc, (long)regs->pstate);

  dump_stack();
  /* A dump_stack() here will give a stack backtrace */
  return 0;
}

/* kprobe post_handler: called after the probed instruction is executed */
static void handler_post(struct kprobe *p, struct pt_regs *regs,
        unsigned long flags)
{
  pr_info("<%s> post_handler: p->addr = 0x%p, pstate = 0x%lx\n",
    p->symbol_name, p->addr, (long)regs->pstate);
  dump_stack();
}

/*
 * fault_handler: this is called if an exception is generated for any
 * instruction within the pre- or post-handler, or when Kprobes
 * single-steps the probed instruction.
 */
static int handler_fault(struct kprobe *p, struct pt_regs *regs, int trapnr)
{
  pr_info("fault_handler: p->addr = 0x%p, trap #%dn", p->addr, trapnr);
  /* Return 0 because we don't handle the fault. */
  return 0;
}

static int __init kprobe_init(void)
{
  int ret;
  kp.pre_handler = handler_pre;
  kp.post_handler = handler_post;
  kp.fault_handler = handler_fault;

  ret = register_kprobe(&kp);
  if (ret < 0) {
    pr_err("register_kprobe failed, returned %d\n", ret);
    return ret;
  }
  pr_info("Planted kprobe at %p\n", kp.addr);
  return 0;
}

static void __exit kprobe_exit(void)
{
  unregister_kprobe(&kp);
  pr_info("kprobe at %p unregistered\n", kp.addr);
}

module_init(kprobe_init)
module_exit(kprobe_exit)
MODULE_LICENSE("GPL");

这段代码很简单，默认情况下，kprobe做了3个钩子，分别在_do_fork对应位置的指令执行之前，执行之后，以及出异常的时候。插入该内核模块之后，随便输入一条命令，可看到下面的打印：

[   19.882832] kprobe_example: loading out-of-tree module taints kernel.
[   19.900442] Planted kprobe at (____ptrval____)
[   19.908571] <_do_fork> pre_handler: p->addr = 0x(____ptrval____), pc = 0xffff0000080d2c98, pstate = 0x80000005
[   19.913657] CPU: 0 PID: 1358 Comm: udevd Tainted: G           O      4.18.0 #7
[   19.916239] Hardware name: linux,dummy-virt (DT)
[   19.918400] Call trace:
[   19.919373]  dump_backtrace+0x0/0x180
[   19.920681]  show_stack+0x14/0x20
[   19.921817]  dump_stack+0x90/0xb4
[   19.923678]  handler_pre+0x24/0x68 [kprobe_example]
[   19.926357]  kprobe_breakpoint_handler+0xbc/0x160
[   19.926627]  brk_handler+0x70/0x88
[   19.926802]  do_debug_exception+0x94/0x160
[   19.927102]  el1_dbg+0x18/0x78
[   19.927299]  _do_fork+0x0/0x358
[   19.927465]  el0_svc_naked+0x30/0x34
[   19.928973] <_do_fork> post_handler: p->addr = 0x(____ptrval____), pstate = 0x80000005
[   19.929361] CPU: 0 PID: 1358 Comm: udevd Tainted: G           O      4.18.0 #7
[   19.929693] Hardware name: linux,dummy-virt (DT)
[   19.929962] Call trace:
[   19.930102]  dump_backtrace+0x0/0x180
[   19.930289]  show_stack+0x14/0x20
[   19.930461]  dump_stack+0x90/0xb4
[   19.934684]  handler_post+0x24/0x30 [kprobe_example]
[   19.934968]  post_kprobe_handler+0x54/0x98
[   19.935234]  kprobe_single_step_handler+0x74/0xa8
[   19.935389]  single_step_handler+0x3c/0xb0
[   19.935516]  do_debug_exception+0x94/0x160
[   19.935642]  el1_dbg+0x18/0x78
[   19.935965]  0xffff000000ac8004
[   19.936067]  el0_svc_naked+0x30/0x34

probe和post钩子得到执行，这对查看内核的调用栈非常有帮助。

深入探究

是否只能基于symbol_name做kprobe？

显然不太可能，struct kprobe中有一个addr成员，很明显是可以直接基于地址做kprobe的。把这段代码：

#define MAX_SYMBOL_LEN  64
static char symbol[MAX_SYMBOL_LEN] = "_do_fork";
module_param_string(symbol, symbol, sizeof(symbol), 0644);

/* For each probe you need to allocate a kprobe structure */
static struct kprobe kp = {
  .symbol_name  = symbol,
};

修改为：

/* For each probe you need to allocate a kprobe structure */
static struct kprobe kp = {
  .addr= (kprobe_opcode_t *)0xffff0000080d2c98,
};

效果是一样的。

kprobe是如何动态添加探针的？

这个肯定要分析代码了，好在代码相当简单：

register_kprobe
   |------arm_kprobe
   |   |------__arm_kprobe
   |   |   |------arch_arm_kprobe

/* arm kprobe: install breakpoint in text */
void __kprobes arch_arm_kprobe(struct kprobe *p)
{
  patch_text(p->addr, BRK64_OPCODE_KPROBES);
}

从注释就可以很明显看出来，是把addr对应位置的指令修改为brk指令，当然这里说的是ARM64架构。那么一旦CPU执行到addr，就会触发异常，trap到kprobe注册的钩子上。

post钩子为什么会用到single step

从上面的调用栈可以看到，post钩子实际上是通过单步断点trap过来的？为什么需要用到单步断点呢？这个其实很好解释。我们先来理一下kprobe的过程：

把addr位置的指令修改为brk指令
CPU执行到addr处trap到pre执行
pre执行完毕后需要把addr处的指令恢复
CPU继续执行addr处的指令
CPU执行post 那么CPU如何才能执行到post，很简单，使能单步执行就可以了。肯定有人会说，可以把addr+4的指令也替换成brk，这个肯定是不行的，因为ARM64可能是32位/16位指令混编的，即便是固定32位指令，CPU下一条要执行的指令也不一定是addr+4，比如当前addr是一条跳转指令。

fault_handler 钩子什么时候会用到

通过分析代码可知，当发生page fault的时候，会调用当前正在running的kprobe的fault_handler钩子，所以这里发生page fault的代码并不一定是addr处的指令，也可能是pre或者post中的指令。我在pre中注入一段访问0地址的逻辑：

static void * g_addr=0;
static int handler_pre(struct kprobe *p, struct pt_regs *regs) __attribute__((optimize("O0")));
static int handler_pre(struct kprobe *p, struct pt_regs *regs)
{
  pr_info("<%s> pre_handler: p->addr = 0x%p, pc = 0x%lx,"
      " pstate = 0x%lx\n",
    p->symbol_name, p->addr, (long)regs->pc, (long)regs->pstate);

    printk("%d\n", *(char *)g_addr);
  /* A dump_stack() here will give a stack backtrace */
  return 0;
}

经验证确实调用到了fault_handler钩子：

[   17.272594] kprobe_example: loading out-of-tree module taints kernel.
[   17.294266] Planted kprobe at (____ptrval____)
# 
# ls
[   19.072586] <(null)> pre_handler: p->addr = 0x(____ptrval____), pc = 0xffff0000080d2c98, pstate = 0x80000005
[   19.073189] fault_handler: p->addr = 0x(____ptrval____), trap #-1778384890n
[   19.073568] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
[   19.074271] Mem abort info:
[   19.074393]   ESR = 0x96000006
[   19.074641]   Exception class = DABT (current EL), IL = 32 bits
[   19.074887]   SET = 0, FnV = 0
[   19.075014]   EA = 0, S1PTW = 0
[   19.075174] Data abort info:
[   19.075324]   ISV = 0, ISS = 0x00000006
[   19.075455]   CM = 0, WnR = 0
[   19.075774] user pgtable: 4k pages, 48-bit VAs, pgdp = (____ptrval____)
[   19.076005] [0000000000000000] pgd=00000000485c6003, pud=00000000bb2f4003, pmd=0000000000000000
[   19.076596] Internal error: Oops: 96000006 [#1] PREEMPT SMP
[   19.076924] Modules linked in: kprobe_example(O)
[   19.077693] CPU: 0 PID: 1387 Comm: sh Tainted: G           O      4.18.0 #7
[   19.077927] Hardware name: linux,dummy-virt (DT)
[   19.078298] pstate: 400003c5 (nZcv DAIF -PAN -UAO)
[   19.078962] pc : handler_pre+0x50/0x70 [kprobe_example]
[   19.079149] lr : handler_pre+0x44/0x70 [kprobe_example]
[   19.079359] sp : ffff00000ac63c00
[   19.079565] x29: ffff00000ac63c00 x28: ffff80007a3c9a80 
[   19.079821] x27: ffff000008ac1000 x26: 00000000000000dc 
[   19.080047] x25: ffff80007dfb7788 x24: 0000000000000000 
[   19.080363] x23: ffff0000080d2c98 x22: ffff00000ac63d70 
[   19.080621] x21: ffff000000ac2000 x20: 0000800074f02000 
[   19.080863] x19: ffff0000090b5788 x18: ffffffffffffffff 
[   19.081197] x17: 0000000000000000 x16: 0000000000000000 
[   19.081501] x15: ffff0000090d96c8 x14: 3030303030666666 
[   19.081720] x13: 667830203d206370 x12: ffff0000090d9940 
[   19.081933] x11: ffff0000085dd8d8 x10: 5f287830203d2072 
[   19.082189] x9 : 0000000000000017 x8 : 2065746174737020 
[   19.082455] x7 : 2c38396332643038 x6 : ffff80007dfb8240 
[   19.082660] x5 : ffff80007dfb8240 x4 : 0000000000000000 
[   19.082871] x3 : ffff80007dfbf030 x2 : 793b575e486def00 
[   19.083068] x1 : 0000000000000000 x0 : 0000000000000000 
[   19.083390] Process sh (pid: 1387, stack limit = 0x(____ptrval____))
[   19.083783] Call trace:
[   19.084020]  handler_pre+0x50/0x70 [kprobe_example]
[   19.084470]  kprobe_breakpoint_handler+0xbc/0x160
[   19.084693]  brk_handler+0x70/0x88
[   19.084839]  do_debug_exception+0x94/0x160
[   19.085132]  el1_dbg+0x18/0x78
[   19.085259]  _do_fork+0x0/0x358
[   19.085443]  el0_svc_naked+0x30/0x34
[   19.085939] Code: 95d9a53f b0000000 9101c000 f9400000 (39400000) 
[   19.086713] ---[ end trace 3bb11c402bc37363 ]---

但由于fault_handler中没有对该异常做处理，所以依然挂死了。 fault_handler可以用于报错或者纠错，报错可以自定义一些错误信息给用户，以便分析错误；纠错用于修改错误，那么针对当前这个错误应该怎么做纠错呢？在fault_handler中为g_addr分配空间？，这显然不行，g_addr肯定已经被载入寄存器了，此时修改已经太迟。唯一的方法就是修改寄存器的值，而寄存器此时肯定已经入栈了，所以必须修改寄存器在栈里面的内容。下面我们来fixup这个挂死问题：

根据挂死信息

[   19.084020]  handler_pre+0x50/0x70 [kprobe_example]

是在handler_pre+0x50这个位置出异常的，通过反汇编得知这个位置对应的指令是：

  50:   39400000        ldrb    w0, [x0]

x0的内容是0，所以这里是读0地址，很明显，g_addr被载入到了x0中，所以只要修改x0就可以了。

fixup实现修改fault_handler函数：

static int g_addr1=0x5a;
static int handler_fault(struct kprobe *p, struct pt_regs *regs, int trapnr)
{
  pr_info("fault_handler: p->addr = 0x%p, trap #%dn", p->addr, trapnr);
  regs->regs[0] = (unsigned long)&g_addr1;
  /* Return 0 because we don't handle the fault. */
  return 1;
}

验证

[   58.882059] <(null)> pre_handler: p->addr = 0x(____ptrval____), pc = 0xffff0000080d2c98, pstate = 0x80000005
[   58.882393] fault_handler: p->addr = 0x(____ptrval____), trap #-1778384890n
[   58.882411] 90
[   58.882658] <(null)> post_handler: p->addr = 0x(____ptrval____), pstate = 0x80000005
[   58.882960] CPU: 1 PID: 1388 Comm: sh Tainted: G           O      4.18.0 #7

fault_handler之后，pre_handler打印了g_addr对应地址的内容是90，也就是0x5a。大功告成，我们成功的让内核访问了0地址，并且返回了0x5a。

当然，这一切都是假的！

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