Re: user space program from keyboard driver

[Payasam Manohar]

> > hai all,
> >   I am a newbie to kernel, I want to work on linux kernel modules.
> > My task is to call a user program from keyboard driver under certain 
> > conditions. I know that we can call user program using 
> > call_usermodehelper(), but we can not call it direcly from driver as it is 
> > a interrupt context. So we need to call using schedule_work(). But I need 
> > more clarification on these points. How to call user program from the 
> > keyboard driver. Please give ur ideas for doing this.
> >
> > Any small help is appreciated.
>
> You don't call a user-mode program from a driver period. It is
> possible to screw with kernel internals in such as way to do this,
> then unscrew what you've screwed up (sometimes) then state; "See
> it can be done.....!". Wrong! Learn that the kernel is designed
> to perform tasks on behalf of a user-mode caller. That's what it's
> designed for. The kernel doesn't have its own context which is
> needed to open files, etc. It has to steal somebody else's context
> to do that. It results in a very precarious situation where the
> stolen task's context can exit at any time and leave the kernel
> in a mess or one steals the context of `init` assuming that
> it's not going to quit. The result is a kludge which is
> filthy with races.
>
> You should NEVER design anything that runs in the kernel that
> expects to call user-mode code. The usermodehelper() code
> was used ONLY to get a module for automatic loading of modules.
> Even that shouldn't exist and it doesn't need to exist for
> the purpose being used. It creates a 'child' of the kernel-
> event daemon. If that code was done correctly, none of that
> bloat would be in the kernel and fewer persons might expect
> to make calls to user-mode code.
>
> The CORRECT design for a user-mode helper requires a user-mode
> task called a daemon. That daemon normally sleeps. You can
> usually see a lot of such daemons when you execute `ps lae`.
> So, you write a program that waits for a signal to wake it
> up, perhaps using select() or poll(). It has opened your
> driver that exists in the kernel. When your driver wants
> your user-mode helper to do something it wakes it up, perhaps
> using wake_up_interruptible() or other such kernel functions.
> The user-mode helper might make an ioctl() call to find out
> what the driver wants done (like reading/writing file data).
> The data goes across the driver/user I/O interface like
> read() write() or ioctl(). The actual file is open/created/
> read/write/closed by the user-mode daemon.

Hai thank u for ur information. If possible can u please give some 
reference for the above task of creating a daemon and waking up in demand.

Is it possible to call a daemon from keyboard driver on pressing certain 
key, if so how to kill the daemon from the driver itself with some other 
condition.
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