从 Linux 上的 root 程序获取对 tty 设备的独占访问

发布于 2024-07-16 03:40:37 字数 1174 浏览 11 评论 0原文

我有一个程序在 Linux 上以 root 身份运行，与 tty 通信（实际上是作为 tty 实现的 LCD）。该设备的价值是/dev/ttyUSB0。我希望写入该设备的程序能够独占访问该设备，以免受到同时运行的程序的其他实例的任何干扰。

我看到有一个名为 TIOCEXCL 的 ioctl 选项，它将阻止设备的额外打开，即“除非发出 TIOCEXCL ioctl，否则对同一文件的多个 open() 调用将会成功。这将阻止除 root 拥有的进程之外的额外打开。 ” 我对此进行了测试，它的工作原理与广告中的一样：如果非 root 用户尝试打开 /dev/ttyUSB0 （一旦我更改了权限），则打开会失败，并显示“设备繁忙”之类的内容，并且如果 root 用户尝试打开它，有用。

我理想地想要的是一种让 root 用户能够独占 tty 访问的方法。因此，我有多个 root 用户使用写入 LCD 的程序，但不知何故，他们对 LCD (tty) 的访问将被序列化。显然 TIOCEXCL ioctl 选项对我不起作用，因为它不会阻止 root 用户打开已打开的 tty 设备。

我想这里有很多选择，但我正在联系你们所有人，看看你们是否有其他想法或建议。

也许我错过了一些关于使用 TIOCEXCL 的内容...
也许还有其他方法通过 open() 或 ioctl() 或什么-不获得独占访问权。
如果有某种方法我可以检测到其他进程已打开设备，我可以等待并重试。我知道 lsof 但我不愿意从这个程序内部调用它只是为了了解这一点。并且存在竞争条件。（也许我可以克服这个问题？:) ）
我可以实现锁定，就像显然用来获得对 tty 设备的独占访问权限一样。

更新 1：

由于写入 LCD 设备的唯一程序是我的，因此我倾向于执行以下操作（伪代码）来锁定代码内部：

f = open("/dev/ttyUSB0", O_RDWR)
flock(f, LOCK_EX)

// do any ioctl's, etc.

// do any write's

// sleep a tad to not flash messages too fast on LCD
nanosleep({0, 250000000}, NULL)

flock(f, LOCK_UN)
close(f)

原文

I have a program running as root on Linux, talking to a tty (actually an LCD implemented as a tty). The device for what it's worth is /dev/ttyUSB0. I'd like to have my program that writes to this device be able to have exclusive access to the device, so as to not have any interference from other instances of the program running at the same time.

I see that there's a ioctl option called TIOCEXCL which will prevent additonal open's of the device to wit "multiple open() calls to the same file will succeed unless the TIOCEXCL ioctl is issued. This will prevent additional opens except by root-owned processes." I tested this and it works just as advertised: if a non-root user tries to open /dev/ttyUSB0 (once I changed the permissions) then the open fails with something like "device busy" and if a root user tries to open it, it works.

What I ideally want is a way to this exclusive access to the tty to work for root users. So I'd have multiple root users using the program that writes to the LCD, but somehow their access to the LCD (tty) would be serialized. Apparently the TIOCEXCL ioctl option will not work for me since it doesn't stop root users from opening an already-opened tty device.

I guess there are a number of options here, but I am reaching out to all ya'll to see if you might have other thoughts or suggestions.

Maybe I'm missing something about using TIOCEXCL...
Maybe there's some other way via open() or ioctl() or what-not to get exclusive access.
If there was some way I could detect that some other process has the device open, I could just wait and retry. I know about lsof but I'm loath to invoke it from inside this program just to learn this. And there are race conditions with that. (Maybe I can get over that? :) )
I could implement locking like apparently used to be done to get exclusive access to tty devices.

Update 1:

Since the only program writing to the LCD device is mine, I am inclined to do something like the following (pseudo-code) to lock inside the code:

f = open("/dev/ttyUSB0", O_RDWR)
flock(f, LOCK_EX)

// do any ioctl's, etc.

// do any write's

// sleep a tad to not flash messages too fast on LCD
nanosleep({0, 250000000}, NULL)

flock(f, LOCK_UN)
close(f)

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ゞ花落谁相伴 2024-07-23 03:40:37

也许这个关于LKML的讨论：[TTY]独占模式问题可以帮你！

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我乃一代侩神 2024-07-23 03:40:37

我有一些像你一样的问题。
如果你能破解你的 Linux 内核，你就可以像这样进行肮脏的黑客攻击。
在文件

linux/drivers/char/tty_io.c

中向函数添加新命令

long tty_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
    ...
    switch (cmd) {
    ...
    case 0x54FF:
        return put_user(tty->count, (int __user *)p);
    ...
}

tty->count的值是当前打开描述符的计数。

在您的应用程序中，您可以尝试此代码

int fd, count, res;
fd = open("/dev/ttyS0", O_RDWR|O_NOCTTY|O_NONBLOCK);
if (fd >= 0)
{
    count = 0;
    res = ioctl(fd, 0x54FF, &count);
    if (res>=0)
    {
        if (count > 1)
        {
            printf("Device already in use.\n")
            close(fd);
            fd = -1;
        }
    }
}

i have some issue like you.
If you can hack you linux kernel, you can to do dirty hack like this.
In file

linux/drivers/char/tty_io.c

add new command to function

long tty_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
    ...
    switch (cmd) {
    ...
    case 0x54FF:
        return put_user(tty->count, (int __user *)p);
    ...
}

The value of tty->count is a count of current open descriptors.

In your application you can try this code

int fd, count, res;
fd = open("/dev/ttyS0", O_RDWR|O_NOCTTY|O_NONBLOCK);
if (fd >= 0)
{
    count = 0;
    res = ioctl(fd, 0x54FF, &count);
    if (res>=0)
    {
        if (count > 1)
        {
            printf("Device already in use.\n")
            close(fd);
            fd = -1;
        }
    }
}

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