kernel misc

kernel misc

. include/linux/string.h

在kernel里使用string类的函数，比如strstr()等，这些函数声明在include/linux/string.h，include <linux/string.h>即可

. /dev/tty, tty driver blog

http://www.mysixue.com/?p=127#devconsole_devtty0_devtty1

. log ratelimit

ratelimit_state_init()

例如如下ratelimit_state_init()是设置5s内打印10条msg：

#define DEFAULT_RATELIMIT_INTERVAL (5 * HZ)
#define DEFAULT_RATELIMIT_BURST 10
static inline void ratelimit_default_init(struct ratelimit_state *rs)
{
return ratelimit_state_init(rs, DEFAULT_RATELIMIT_INTERVAL,
DEFAULT_RATELIMIT_BURST);
}

. kernel makefile desc

https://blog.51cto.com/weiguozhihui/1591397

. set_task_comm(tsk, "kthreadd");

. android在init.rc里将panic_on_oops默认设置为了1

write /proc/sys/kernel/panic_on_oops 1

. __ffs

获取二进制形式数值第一个非0bit，从低位往高位检查，index从0开始：

	unsigned long bitmap = 0x90911010;
	/* first bit */
	printk("Bitmap: %#lx __ffs(): %ld
", bitmap, __ffs(bitmap));

执行结果：
Bitmap: 0x90911010 __ffs(): 4

size_t/ssize_t type definition

4.19includelinux	ypes.h
#ifndef _SIZE_T
#define _SIZE_T
typedef __kernel_size_t        size_t;
#endif

#ifndef _SSIZE_T
#define _SSIZE_T
typedef __kernel_ssize_t    ssize_t;
#endif

4.19includeuapiasm-genericposix_types.h
#ifndef __kernel_size_t
#if __BITS_PER_LONG != 64
typedef unsigned int    __kernel_size_t;
typedef int        __kernel_ssize_t;
typedef int        __kernel_ptrdiff_t;
#else
typedef __kernel_ulong_t __kernel_size_t;
typedef __kernel_long_t    __kernel_ssize_t;
typedef __kernel_long_t    __kernel_ptrdiff_t;
#endif
#endif

 CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT

CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT: Temporary per-CPU printk log buffer size (12 => 4KB, 13 => 8KB)

The Linux kernel configuration item CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT:

• prompt: Temporary per-CPU printk log buffer size (12 => 4KB, 13 => 8KB)
• type: int
• depends on: CONFIG_PRINTK
• defined in init/Kconfig
• found in Linux kernels: 4.11–4.20, 5.0–5.13, 5.13+HEAD

Help text

Select the size of an alternate printk per-CPU buffer where messages printed from usafe contexts are temporary stored. One example would be NMI messages, another one - printk recursion. The messages are copied to the main log buffer in a safe context to avoid a deadlock. The value defines the size as a power of 2.

Those messages are rare and limited. The largest one is when a backtrace is printed. It usually fits into 4KB. Select 8KB if you want to be on the safe side.

Examples: 17 => 128 KB for each CPU 16 => 64 KB for each CPU 15 => 32 KB for each CPU 14 => 16 KB for each CPU 13 => 8 KB for each CPU 12 => 4 KB for each CPU

from: https://cateee.net/lkddb/web-lkddb/PRINTK_SAFE_LOG_BUF_SHIFT.html

Linux kernel中的IS_ENABLED

首先在Kconfig中选中某个选项为y或m时, 在.config文件中就会由一个CONFIG_XXXXX=y或CONFIG_XXXXX=m, 并且会自动生成一个头文件autoconfig.h. 当选中为y时, 头文件中包含#define CONFIG_XXXXX 1, 当选中为m时, 头文件中包含#define CONFIG_XXXXX_MODULE 1, 当不选中是, 头文件中不包含相关语句.

from: https://blog.csdn.net/gngshn/article/details/69790798

在kernel里读一个文件（比如一个ko）

使用kernel_read_file_from_fd()，根据fd来read：

SYSCALL_DEFINE3(finit_module, int, fd, const char __user *, uargs, int, flags)
{
    struct load_info info = { };
    void *hdr = NULL;
    int err;

    err = may_init_module();
    if (err)
        return err;

    pr_debug("finit_module: fd=%d, uargs=%p, flags=%i
", fd, uargs, flags);

    if (flags & ~(MODULE_INIT_IGNORE_MODVERSIONS
              |MODULE_INIT_IGNORE_VERMAGIC))
        return -EINVAL;

    err = kernel_read_file_from_fd(fd, 0, &hdr, INT_MAX, NULL,
                       READING_MODULE);
    if (err < 0)
        return err;
    info.hdr = hdr;
    info.len = err;

    return load_module(&info, uargs, flags);
}

kernel延时函数

mdelay()

这个属于忙等待，不会让出CPU

msleep()

这个调用了后会让出CPU，不会忙等待

usleep_range(min, max)

这个调用了后会让出CPU，不会忙等待

/proc/kallsyms

如果只开了CONFIG_KALLSYMS，CONFIG_KALLSYMS_ALL没开，则/proc/kallsyms里将只会有函数symbol，全局变量symbol将没有。

kernel里读memory，比如dump一个寄存器指向的memory前后的数据

如下是一个示例，一次读16byte：

    unsigned char buf[16] = {0};
    if(!probe_kernel_read(buf,(const void *)task_struct_addr,sizeof(buf)))

list_for_each_entry()相关的操作都需要放在这个循环体里，如果是遍历了一遍，它会返回head，此时如果指向回了head，此循环将会结束，此时如果在循环体外有对p的相关操作，会导致debug异常call到brk_handler：

        list_for_each_entry(p, &mali_cmar_files_list, node) {
            if(p->pid == tsk->pid)
            {
                if(once)
                {
                    once = 0;
                    pr_info("unregister 0x%px files_struct ptr.
", &tsk->files);
                    unregister_wp_bp((unsigned long)&tsk->files);
                }
                
                list_del_init(&p->node);
                kfree(p);
                p = NULL;
                
                break;
            }
        }

打印调用者

    printk("caller is %pS
", __builtin_return_address(0));

打印结果：

[ 102.549872] caller is exit_files+0x150/0x310