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创建一个餐饮公司的模板,杭州网站推广优化公司,宁夏网页制作公司,wordpress4.8版权修改文章目录 1. 前言2. 背景3. Linux SysRq3.1 SysRq 简介3.1.1 SysRq 初始化 3.2 通过 procfs 发起 SysRq 请求3.2.1 修改内核日志等级3.2.1.1 触发3.2.1.2 实现简析 3.2.2 手动触发内核 panic3.2.2.1 触发3.2.2.2 实现简析3.2.2.3 应用场景 3.2.3 其它 SysRq 请求 3.3 通过特殊…

文章目录

1. 前言
2. 背景
3. Linux SysRq
- 3.1 SysRq 简介
- - 3.1.1 SysRq 初始化
- 3.2 通过 procfs 发起 SysRq 请求
- - 3.2.1 修改内核日志等级
  - - 3.2.1.1 触发
    - 3.2.1.2 实现简析
  - 3.2.2 手动触发内核 panic
  - - 3.2.2.1 触发
    - 3.2.2.2 实现简析
    - 3.2.2.3 应用场景
  - 3.2.3 其它 SysRq 请求
- 3.3 通过特殊按键发起 SysRq 请求
4. 参考资料

1. 前言

限于作者能力水平，本文可能存在谬误，因此而给读者带来的损失，作者不做任何承诺。

2. 背景

本文基于 ARM32 + Linux 4.14 内核源码进行分析。

3. Linux SysRq

3.1 SysRq 简介

Linux SysRq，是内核允许通过 特殊按键 和 procfs 文件节点 向系统发起一些特殊请求，用于查看系统状态和调试目的。

3.1.1 SysRq 初始化

使用 SysRq 功能，需要开启内核配置项 CONFIG_MAGIC_SYSRQ 。来看一下 SysRq 的初始化：

/* drivers/tty/sysrq.c */...static struct input_handler sysrq_handler = {.filter  = sysrq_filter,.connect = sysrq_connect,.disconnect = sysrq_disconnect,.name  = "sysrq",.id_table = sysrq_ids,
};static bool sysrq_handler_registered;static inline void sysrq_register_handler(void)
{int error;/* 注册 以 【特殊按键】 方式发起 SysRq 的按键处理接口 */error = input_register_handler(&sysrq_handler);if (error)pr_err("Failed to register input handler, error %d", error);elsesysrq_handler_registered = true;
}...static const struct file_operations proc_sysrq_trigger_operations = {.write  = write_sysrq_trigger,.llseek  = noop_llseek,
};static void sysrq_init_procfs(void)
{/* 建立 /proc/sysrq-trigger 节点 */if (!proc_create("sysrq-trigger", S_IWUSR, NULL,&proc_sysrq_trigger_operations))pr_err("Failed to register proc interface\n");
}static int __init sysrq_init(void)
{sysrq_init_procfs();if (sysrq_on())sysrq_register_handler();return 0;
}
device_initcall(sysrq_init);

3.2 通过 procfs 发起 SysRq 请求

本小节讨论以 procfs 文件节点 发起 SysRq 请求的方式，它们都是以向 /proc/sysrq-trigger 写入预定义字符发起：

# echo X > /proc/sysrq-trigger

操作需要特权用户权限。下面讲述几个常见的 SysRq 请求操作，并对它们的实现做简单分析。所有支持的 SysRq 请求列举在数据表格 sysrq_key_table[] 中：

static struct sysrq_key_op *sysrq_key_table[36] = {/* 修改内核日志等级 */&sysrq_loglevel_op,  /* 0 */&sysrq_loglevel_op,  /* 1 */&sysrq_loglevel_op,  /* 2 */&sysrq_loglevel_op,  /* 3 */&sysrq_loglevel_op,  /* 4 */&sysrq_loglevel_op,  /* 5 */&sysrq_loglevel_op,  /* 6 */&sysrq_loglevel_op,  /* 7 */&sysrq_loglevel_op,  /* 8 */&sysrq_loglevel_op,  /* 9 */.../* 触发系统重启，不会同步或者卸载磁盘 */&sysrq_reboot_op,  /* b *//* 触发内核 crash */&sysrq_crash_op,  /* c *//* 显示所有持有的锁(需开启 CONFIG_LOCKDEP) */&sysrq_showlocks_op,  /* d *//* 向除 init 外的所有进程发送 SIGTERM 信号 */&sysrq_term_op,   /* e *//* 触发 OOM 回收 */&sysrq_moom_op,   /* f */.../* 向除 init 外的所有进程发送 SIGKILL 信号 */&sysrq_kill_op,   /* i *//* 文件系统解冻操作 */
#ifdef CONFIG_BLOCK&sysrq_thaw_op,   /* j */
#elseNULL,    /* j */
#endif&sysrq_SAK_op,   /* k *//* 显示所有活动 cpu 的栈回溯 */
#ifdef CONFIG_SMP&sysrq_showallcpus_op,  /* l */
#elseNULL,    /* l */
#endif/* 显示系统内存信息, 如同 cat /proc/meminfo */&sysrq_showmem_op,  /* m *//* 用于将所有实时任务变成普通任务 */&sysrq_unrt_op,   /* n *//* o: This will often be registered as 'Off' at init time */NULL,    /* o *//* 显示 CPU 当前寄存器和标志位 */&sysrq_showregs_op,  /* p *//* 显示每个 CPU 上的高精度定时器 */&sysrq_show_timers_op,  /* q *//* 关闭键盘 RAW 模式 */&sysrq_unraw_op,  /* r *//* 尝试同步所有的已挂载文件系统 */&sysrq_sync_op,   /* s *//* 导出当前所有任务列表和它们的信息 */&sysrq_showstate_op,  /* t *//* 尝试重新挂载已挂载文件系统为只读 */&sysrq_mountro_op,  /* u *//* v: May be registered for frame buffer console restore */NULL,    /* v *//* 导出处于不可中断状态（阻塞）的任务 */&sysrq_showstate_blocked_op, /* w *//* x: May be registered on mips for TLB dump *//* x: May be registered on ppc/powerpc for xmon *//* x: May be registered on sparc64 for global PMU dump */NULL,    /* x *//* y: May be registered on sparc64 for global register dump */NULL,    /* y *//* 导出 ftrace 缓存信息 */&sysrq_ftrace_dump_op,  /* z */
};

对 /proc/sysrq-trigger 写入触发调用序列：

write()...write_sysrq_trigger()char c;get_user(c, buf);__handle_sysrq(c, false)void __handle_sysrq(int key, bool check_mask)
{struct sysrq_key_op *op_p;int orig_log_level;int i;rcu_sysrq_start();rcu_read_lock();/** Raise the apparent loglevel to maximum so that the sysrq header* is shown to provide the user with positive feedback.  We do not* simply emit this at KERN_EMERG as that would change message* routing in the consumers of /proc/kmsg.*/orig_log_level = console_loglevel;console_loglevel = CONSOLE_LOGLEVEL_DEFAULT;pr_info("SysRq : ");op_p = __sysrq_get_key_op(key); /* 从 sysrq_key_table[] 查找 c 对应的 SysRq 操作接口 */if (op_p) {/** Should we check for enabled operations (/proc/sysrq-trigger* should not) and is the invoked operation enabled?*/if (!check_mask || sysrq_on_mask(op_p->enable_mask)) {pr_cont("%s\n", op_p->action_msg);console_loglevel = orig_log_level;op_p->handler(key); /* 调用 SysRq 操作接口：sysrq_handle_crash(), ... */} else {...}} else {...}rcu_read_unlock();rcu_sysrq_end();
}

3.2.1 修改内核日志等级

3.2.1.1 触发

# echo 3 > /proc/sysrq-trigger 
[ 6956.852664] sysrq: SysRq : Changing Loglevel
[ 6956.856987] sysrq: Loglevel set to 3

3.2.1.2 实现简析

write()...write_sysrq_trigger()sysrq_handle_loglevel()

/* drivers/tty/sysrq.c */static void sysrq_handle_loglevel(int key)
{int i;i = key - '0';console_loglevel = CONSOLE_LOGLEVEL_DEFAULT;pr_info("Loglevel set to %d\n", i);console_loglevel = i;
}

3.2.2 手动触发内核 panic

3.2.2.1 触发

# echo c > /proc/sysrq-trigger
[  856.968802] sysrq: SysRq : Trigger a crash
[  856.973059] Unable to handle kernel NULL pointer dereference at virtual address 00000000
[  856.981194] pgd = 6e7ca3d4
[  856.984471] [00000000] *pgd=8a84c831, *pte=00000000, *ppte=00000000
[  856.990789] Internal error: Oops: 817 [#1] PREEMPT ARM
[  856.995946] Modules linked in:
[  856.999020] CPU: 0 PID: 123 Comm: sh Not tainted 4.19.94-g1194fe2-dirty #102
[  857.006095] Hardware name: Generic AM33XX (Flattened Device Tree)
[  857.012236] PC is at sysrq_handle_crash+0x2c/0x34
[  857.016958] LR is at sysrq_handle_crash+0x28/0x34
[  857.021678] pc : [<c04fd9bc>]    lr : [<c04fd9b8>]    psr: 60080013
[  857.027969] sp : ca8bbe38  ip : ca8bbe38  fp : ca8bbe4c
[  857.033212] r10: 00000004  r9 : ca8bbf60  r8 : c0e2c544
[  857.038455] r7 : 00000000  r6 : 00000063  r5 : 00000007  r4 : 00000001
[  857.045007] r3 : 00000000  r2 : 00000000  r1 : 00000000  r0 : c0e14618
[  857.051561] Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment none
[  857.058723] Control: 10c5387d  Table: 8a868019  DAC: 00000051
[  857.064493] Process sh (pid: 123, stack limit = 0xce84bd3c)
[  857.070087] Stack: (0xca8bbe38 to 0xca8bc000)
[  857.074462] be20:                                                       c0e0ff48 00000007
[  857.082676] be40: ca8bbe7c ca8bbe50 c04fdf6c c04fd99c 00000055 00000002 c04fe474 00000000
[  857.090890] be60: ca8bbf60 00000002 ca8bbf60 00000004 ca8bbe94 ca8bbe80 c04fe4d0 c04fdec8
[  857.099104] be80: cf26a980 c04fe474 ca8bbeb4 ca8bbe98 c029e968 c04fe480 c0e03048 ca87e240
[  857.107317] bea0: c029e908 ca8bbf60 ca8bbf2c ca8bbeb8 c02371f8 c029e914 00000000 00000000
[  857.115532] bec0: fffffff6 c0e03048 ca8bbf34 ca8bbed8 c012e2ac c01467e4 c0112fdc 00000004
[  857.123745] bee0: 00000007 00000000 00000000 00000000 00000000 00000000 ca873000 c012bc68
[  857.131960] bf00: 00000100 42749327 00000002 ca87e240 000c4b38 ca8bbf60 00000002 ca8ba000
[  857.140174] bf20: ca8bbf5c ca8bbf30 c02374b4 c02371c4 c012e42c c012e228 ca8bbf5c ca87e240
[  857.148388] bf40: c0e03048 ca87e240 000c4b38 00000002 ca8bbf94 ca8bbf60 c023773c c0237414
[  857.156602] bf60: 00000000 00000000 ca8bbf94 42749327 c0257618 000c2d5c 00000001 000c4b38
[  857.164816] bf80: 00000004 c0101204 ca8bbfa4 ca8bbf98 c02377c0 c02376dc 00000000 ca8bbfa8
[  857.173030] bfa0: c0101000 c02377bc 000c2d5c 00000001 00000001 000c4b38 00000002 00000000
[  857.181243] bfc0: 000c2d5c 00000001 000c4b38 00000004 00000001 00000020 00000000 00091144
[  857.189458] bfe0: 00000000 bec9a4bc 0001a908 b6f53556 60080030 00000001 00000000 00000000
[  857.197664] Backtrace: 
[  857.200127] [<c04fd990>] (sysrq_handle_crash) from [<c04fdf6c>] (__handle_sysrq+0xb0/0x180)
[  857.208513]  r5:00000007 r4:c0e0ff48
[  857.212106] [<c04fdebc>] (__handle_sysrq) from [<c04fe4d0>] (write_sysrq_trigger+0x5c/0x6c)
[  857.220494]  r10:00000004 r9:ca8bbf60 r8:00000002 r7:ca8bbf60 r6:00000000 r5:c04fe474
[  857.228354]  r4:00000002 r3:00000055
[  857.231948] [<c04fe474>] (write_sysrq_trigger) from [<c029e968>] (proc_reg_write+0x60/0x90)
[  857.240332]  r5:c04fe474 r4:cf26a980
[  857.243927] [<c029e908>] (proc_reg_write) from [<c02371f8>] (__vfs_write+0x40/0x164)
[  857.251703]  r7:ca8bbf60 r6:c029e908 r5:ca87e240 r4:c0e03048
[  857.257387] [<c02371b8>] (__vfs_write) from [<c02374b4>] (vfs_write+0xac/0x188)
[  857.264729]  r9:ca8ba000 r8:00000002 r7:ca8bbf60 r6:000c4b38 r5:ca87e240 r4:00000002
[  857.272506] [<c0237408>] (vfs_write) from [<c023773c>] (ksys_write+0x6c/0xe0)
[  857.279672]  r8:00000002 r7:000c4b38 r6:ca87e240 r5:c0e03048 r4:ca87e240
[  857.286402] [<c02376d0>] (ksys_write) from [<c02377c0>] (sys_write+0x10/0x14)
[  857.293568]  r8:c0101204 r7:00000004 r6:000c4b38 r5:00000001 r4:000c2d5c
[  857.300300] [<c02377b0>] (sys_write) from [<c0101000>] (ret_fast_syscall+0x0/0x54)
[  857.307899] Exception stack(0xca8bbfa8 to 0xca8bbff0)
[  857.312971] bfa0:                   000c2d5c 00000001 00000001 000c4b38 00000002 00000000
[  857.321185] bfc0: 000c2d5c 00000001 000c4b38 00000004 00000001 00000020 00000000 00091144
[  857.329397] bfe0: 00000000 bec9a4bc 0001a908 b6f53556
[  857.334472] Code: e5834000 f57ff04e ebf05e88 e3a03000 (e5c34000) 
[  857.343688] ---[ end trace 0caa0a25d6458889 ]---
[  857.348334] Kernel panic - not syncing: Fatal exception
[  857.353587] ---[ end Kernel panic - not syncing: Fatal exception ]---

3.2.2.2 实现简析

write()...write_sysrq_trigger()sysrq_handle_crash()

/* drivers/tty/sysrq.c */static void sysrq_handle_crash(int key)
{char *killer = NULL;/* we need to release the RCU read lock here,* otherwise we get an annoying* 'BUG: sleeping function called from invalid context'* complaint from the kernel before the panic.*/rcu_read_unlock();/* 强制 oops 导致内核 panic */panic_on_oops = 1; /* force panic */wmb();*killer = 1; /* 写空指针导致 页表访问异常 */
}
static struct sysrq_key_op sysrq_crash_op = {.handler = sysrq_handle_crash,.help_msg = "crash(c)",.action_msg = "Trigger a crash",.enable_mask = SYSRQ_ENABLE_DUMP,
};static struct sysrq_key_op *sysrq_key_table[36] = {...&sysrq_crash_op,  /* c */...
};

假定使用 AMR32 3级分页：

/* arch/arm/mm/fsr-3level.c */static struct fsr_info fsr_info[] = {...{ do_translation_fault, SIGSEGV, SEGV_MAPERR, "level 1 translation fault" },{ do_translation_fault, SIGSEGV, SEGV_MAPERR, "level 2 translation fault" },{ do_page_fault, SIGSEGV, SEGV_MAPERR, "level 3 translation fault" },...
};do_translation_fault() / do_page_fault()...__do_kernel_fault(mm, addr, fsr, regs)

/* arch/arm/mm/fault.c */static void
__do_kernel_fault(struct mm_struct *mm, unsigned long addr, unsigned int fsr,struct pt_regs *regs)
{...bust_spinlocks(1);pr_alert("Unable to handle kernel %s at virtual address %08lx\n",(addr < PAGE_SIZE) ? "NULL pointer dereference" :"paging request", addr);show_pte(mm, addr); /* 打印地址 @addr 的 pgd, pmd, ... pte */die("Oops", regs, fsr); /* arch/arm/kernel/trap.c */...oops_end(flags, regs, sig)...if (panic_on_oops) /* 促使 oops 导致内核 panic */panic("Fatal exception");...bust_spinlocks(0);do_exit(SIGKILL);
}

3.2.2.3 应用场景

有时候程序卡住了，可能想知道当前的调用链，可以通过 c 触发 panic 导出堆栈记录。

3.2.3 其它 SysRq 请求

感兴趣的读者可自行分析。

3.3 通过特殊按键发起 SysRq 请求

发起 SysRq 请求的具体按键，各个硬件平台各有不同，读者可参考文章末尾资料，或查阅相关资料了解。本小节对通过 特殊按键 的方式发起 SysRq 请求的过程做简要分析，如下：

/* drivers/input/input.c */
input_report_key()input_handle_event(dev, type, code, value)input_pass_values(dev, dev->vals, dev->num_vals)/* 将事件数据传递给挂接在输入设备 input_dev 上 input_handler 处理 */list_for_each_entry_rcu(handle, &dev->h_list, d_node)if (handle->open) {count = input_to_handler(handle, vals, count);if (handler->filter) {for (v = vals; v != vals + count; v++) {if (handler->filter(handle, v->type, v->code, v->value)) /* sysrq_filter(), ... */continue;if (end != v)*end = *v;end++;}count = end - vals;}/* 所有按键事件已被过滤处理(如 SysRq 按键事件)，没有按键事件需做进一步处理 */if (!count)return 0;/* 按键事件处理 */if (handler->events)handler->events(handle, vals, count);else if (handler->event)for (v = vals; v != vals + count; v++)handler->event(handle, v->type, v->code, v->value);return count;}/* drivers/tty/sysrq.c */
sysrq_filter()sysrq_handle_keypress(sysrq, code, value)static bool sysrq_handle_keypress(struct sysrq_state *sysrq,unsigned int code, int value)
{...switch (code) {case KEY_LEFTALT:case KEY_RIGHTALT:...break;case KEY_SYSRQ:...break;default:if (sysrq->active && value && value != 2) {sysrq->need_reinject = false;__handle_sysrq(sysrq_xlate[code], true); /* 处理 SYSRQ 按键事件 */}break;}...
}