Question

我正在学习一些内核模块和线程的基础知识。所以我试着制作一个示例模块并对其进行测试。现在，它成功加载。

模块代码：

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/kthread.h>
#include <linux/delay.h>
#include <linux/version.h>


static struct task_struct *thread_st;

// Function called by thread
static int thread_fun(void *unused)
{
    allow_signal(SIGKILL);
    while(!kthread_should_stop())
    {
        printk(KERN_INFO "Thread Running\n");
        ssleep(5);

        if(signal_pending(current))
            break;
    }
    printk(KERN_INFO "Thread Stopping\n");
    do_exit(0);
    return 0;
}



// Module initialisation
static int __init init_thread(void)
{
    printk(KERN_INFO "Creating Thread\n");

    thread_st = kthread_run(thread_fun, NULL, "mythread");
    if(thread_st)
        printk(KERN_INFO "Thread created successfully\n");
    else
        printk(KERN_INFO "Thread creation failed\n");
    return 0;

}




// Module exit
static void __exit cleanup_thread(void)
{
    printk(KERN_INFO "Cleaning up\n");
    if(thread_st)
    {
        kthread_stop(current);
        printk(KERN_INFO "Thread Stopped\n");
    }
}

module_init(init_thread);
module_exit(cleanup_thread);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Pinkesh Badjatiya");
MODULE_DESCRIPTION("Simple Kernel Module");

现在，一旦模块加载，我按照以下步骤卸载它，

发送SIGKILL信号， sudo kill -9 [PID]
等待dmesg显示'Thread Stopping'，这意味着kthread_should_stop()已返回true。
删除模块 sudo rmmod [MODULE_NAME]

dmesg 日志：

[  492.979030] Creating Thread
[  492.979753] Thread created successfully
[  492.979776] Thread Running
[  497.985420] Thread Running
[  502.992223] Thread Running
[  507.999007] Thread Running
[  513.005837] Thread Running
[  518.012585] Thread Running
[  523.019354] Thread Running
[  528.026195] Thread Running
[  533.032919] Thread Running
[  538.039795] Thread Running
[  543.046588] Thread Running
[  548.053383] Thread Stopping
[  556.317200] Cleaning up
[  556.317212] Thread Stopped

现在当我使用原来使用的结构指针 thread_st 更改变量 current ，然后加载模块并按照上面相同的步骤删除模块，内核给出恐慌（OOPS）并填写dmesg日志。

我还在Ubuntu上获得了Report Error弹出窗口。

dmesg 日志：

[ 1269.832922] Creating Thread
[ 1269.833888] Thread created successfully
[ 1269.834217] Thread Running
[ 1274.839425] Thread Running
[ 1279.846211] Thread Running
[ 1284.853017] Thread Running
[ 1289.859819] Thread Running
[ 1294.866589] Thread Running
[ 1299.873353] Thread Stopping
[ 1305.758783] Cleaning up
[ 1305.758853] BUG: unable to handle kernel NULL pointer dereference at           (null)
[ 1305.762603] IP: [<ffffffff81096d6b>] exit_creds+0x1b/0x70
[ 1305.766266] PGD 0 
[ 1305.769967] Oops: 0000 [#3] SMP 
[ 1305.774675] Modules linked in: kernel_thread_example(OE-) vmnet(OE) vmw_vsock_vmci_transport vsock vmw_vmci vmmon(OE) cmac rmd160 crypto_null camellia_generic camellia_x86_64 cast6_avx_x86_64 cast6_generic cast5_avx_x86_64 cast5_generic cast_common deflate cts ctr gcm ccm serpent_avx_x86_64 serpent_sse2_x86_64 serpent_generic blowfish_generic blowfish_x86_64 blowfish_common twofish_generic twofish_avx_x86_64 twofish_x86_64_3way xts twofish_x86_64 twofish_common xcbc sha256_ssse3 sha512_ssse3 des_generic aes_x86_64 lrw gf128mul glue_helper ablk_helper xfrm_user ah6 ah4 esp6 esp4 xfrm4_mode_beet xfrm4_tunnel tunnel4 xfrm4_mode_tunnel xfrm4_mode_transport xfrm6_mode_transport xfrm6_mode_ro xfrm6_mode_beet xfrm6_mode_tunnel ipcomp ipcomp6 xfrm6_tunnel tunnel6 xfrm_ipcomp af_key xfrm_algo bnep rfcomm bluetooth 6lowpan_iphc uvcvideo videobuf2_vmalloc videobuf2_memops videobuf2_core v4l2_common videodev media snd_hda_codec_hdmi snd_hda_codec_conexant snd_hda_codec_generic snd_hda_intel snd_hda_controller snd_hda_codec snd_hwdep snd_pcm snd_seq_midi snd_seq_midi_event snd_rawmidi arc4 snd_seq intel_rapl x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel kvm ath9k ath9k_common ath9k_hw crct10dif_pclmul snd_seq_device crc32_pclmul snd_timer ath ghash_clmulni_intel cryptd mac80211 joydev serio_raw snd cfg80211 i915 lpc_ich shpchp soundcore drm_kms_helper drm mei_me mei i2c_algo_bit mac_hid video wmi parport_pc ppdev lp parport hid_generic usbhid hid psmouse ahci libahci atl1c [last unloaded: kernel_thread_example]
[ 1305.817666] CPU: 3 PID: 4038 Comm: rmmod Tainted: G      D    OE 3.16.0-50-generic #66~14.04.1-Ubuntu
[ 1305.822078] Hardware name: HCL Infosystems Limited HCL ME LAPTOP/HCL Infosystems Limited, BIOS 203.T01 03/19/2011
[ 1305.826447] task: ffff8800a6221e90 ti: ffff880119700000 task.ti: ffff880119700000
[ 1305.830740] RIP: 0010:[<ffffffff81096d6b>]  [<ffffffff81096d6b>] exit_creds+0x1b/0x70
[ 1305.834968] RSP: 0018:ffff880119703e90  EFLAGS: 00010246
[ 1305.839081] RAX: 0000000000000000 RBX: ffff8800b6e065e0 RCX: 0000000000000000
[ 1305.843133] RDX: ffffffff81c8ea00 RSI: ffff8800b6e065e0 RDI: 0000000000000000
[ 1305.847062] RBP: ffff880119703e98 R08: 0000000000000086 R09: 0000000000000431
[ 1305.850897] R10: 0000000000000000 R11: ffff880119703c0e R12: ffff8800b6e065e0
[ 1305.854697] R13: 0000000000000000 R14: 0000000000000000 R15: 00007f0325bb6240
[ 1305.858456] FS:  00007f0325595740(0000) GS:ffff88011fa60000(0000) knlGS:0000000000000000
[ 1305.862225] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1305.866197] CR2: 0000000000000000 CR3: 00000000b6e23000 CR4: 00000000000407e0
[ 1305.866199] Stack:
[ 1305.866206]  ffff8800b6e065e0 ffff880119703eb8 ffffffff8106abf2 0000000000000000
[ 1305.866211]  ffff8800b6e065e0 ffff880119703ee0 ffffffff81091868 0000000000000000
[ 1305.866216]  ffffffffc0a61000 0000000000000800 ffff880119703ef0 ffffffffc0a5f086
[ 1305.866217] Call Trace:
[ 1305.866232]  [<ffffffff8106abf2>] __put_task_struct+0x52/0x140
[ 1305.866241]  [<ffffffff81091868>] kthread_stop+0xd8/0xe0
[ 1305.866249]  [<ffffffffc0a5f086>] cleanup_thread+0x23/0xf9d [kernel_thread_example]
[ 1305.866259]  [<ffffffff810ebbb2>] SyS_delete_module+0x162/0x200
[ 1305.866268]  [<ffffffff8176edcd>] system_call_fastpath+0x1a/0x1f
[ 1305.866318] Code: ff ff 85 c0 0f 84 33 fe ff ff e9 0c fe ff ff 90 66 66 66 66 90 55 48 89 e5 53 48 8b 87 c0 05 00 00 48 89 fb 48 8b bf b8 05 00 00 <8b> 00 48 c7 83 b8 05 00 00 00 00 00 00 f0 ff 0f 74 23 48 8b bb 
[ 1305.866324] RIP  [<ffffffff81096d6b>] exit_creds+0x1b/0x70
[ 1305.866326]  RSP <ffff880119703e90>
[ 1305.866328] CR2: 0000000000000000
[ 1305.866378] ---[ end trace 0bd516c6629996c7 ]---

我无法弄清楚为什么会发生这种情况我在网上搜索但找不到任何理由。

此外，变量当前是否已在上述任何标题中声明，使用上面创建的 thread_st 有什么问题？

Answer 1

来自kthread_stop函数的描述：

如果threadfn（）可以调用do_exit（）本身，则调用者必须确保task_struct不会消失。

这意味着如果它被其他地方的kthread_stop()终止，则不能简单地退出kthread。您应该只在找到kthread_should_stop()为真时退出，或者在退出之前 grub引用task_struct （以某种方式）。

等待dmesg显示“Thread Stopping”，这意味着kthread_should_stop（）返回true。

如果是signal_pending(current)，则在没有allow_signal()次调用的情况下为真。 kthread_should_stop()仅在有人为给定线程调用kthread_stop()时才为真。如果信号由用户空间明确发送（因为allow_signal()），signal_pending(current)不会反映kthread_should_stop()州。

所以，你的两个实现都是不正确的，因为它们会在从使用空间明确发送信号的情况下退出线程。

此外，在kthread函数中使用thread_st会引入竞争条件：线程函数可能会在kthread_run()返回之前开始（并且其结果将分配给thread_st）。

<强>更新：

您可以等到“线程停止”后立即调用kthreas_stop（）：

static int thread_fun(void *unused)
{
    allow_signal(SIGKILL);
    while(!kthread_should_stop())
    {
        printk(KERN_INFO "Thread Running\n");
        ssleep(5);

        if(signal_pending(current))
            break;
    }
    printk(KERN_INFO "Thread Stopping\n");

    // Wait until kthread will be actually stopped.
    while(!kthread_should_stop())
    {
        /* 
         * Flush any pending signal.
         *
         * Otherwise interruptible wait will not wait actually.
         */
        flush_signals(current);
        /* Stopping thread is some sort of interrupt. That's why we need interruptible wait. */        
        set_current_state(TASK_INTERRUPTIBLE);
        if(!kthread_should_stop()) schedule();
        set_current_state(TASK_RUNNING);
    }

    return 0;
}

Answer 2

current始终指向当前正在运行的任务，并通过一些内核头文件包含在内。所以我们需要仔细使用它。因此，在下面编写的函数中，您试图停止调用cleanup_thread（）即rmmod进程的任务，因为cleanup_thread（）是一个模块退出函数
```
static void __exit cleanup_thread(void)
{
    printk(KERN_INFO "Cleaning up\n");
    if(thread_st)
    {
        kthread_stop(current);
        printk(KERN_INFO "Thread Stopped\n");
    }
}
```
问题的可能原因是首先你用kill -9杀死线程。这会导致线程死亡并且task_struct被释放。但由于 thread_st 不是零，它是一个悬空指针，即它指向已经释放的位置。

然后在cleanup_exit（）中如果你调用kthread_stop（thread_st），那么实际上你传递的是无效的内存位置，因此内核崩溃了。

在执行 do_exit（）之前尝试取消 thread_st

“无法处理虚拟地址处的内核NULL指针取消引用。” - 发信号通知内核模块| OOPS

2 个答案: