Re: user-defined function call in interrupt handler

[Iqbal AMS <iqbal@naturesoft.net>]

Never copy a buffer from the interrupt handler context to the user
space.

Copy the required data and schedule it accordingly.

Iqbal


On Tue, 2003-08-05 at 06:45, kjw75799@etri.re.kr wrote:
> Let me have time to ask some questions about Linux.
> 
> we can access user buffer within interrupt handler (kernel mode) if we
> tocuch segment selectors.
> Like these:
>                 mm_segment_t fs;
>                 fs = get_fs();
>                 set_fs(get_dsc());
>                 call user space function .
>                 .....
>         set_fs(fs);
> 
> My question is:
> Can we execute(call) a function of user space within the interrupt
> handler?
> 
> reference site http://www.cs.unm.edu/~jotto/linux/linux.html
> 
> Something similar happens with segment selectors when an interrupt
> takes place. But in this case there is no assurance that the handler
> is executing in the context of the interested process. Suppose though
> that by some bit of magic the interrupt handler knows the process's
> pid and a buffer address. Then in the case that the handler needs to
> write data to the buffer we could do something like the following: 
> 
>     struct task_struct* pid2task(int);
>     void in_thehandler();
> 
>     int pid;
>     char* user_buf;
>     char ch;
> 
>     unsigned long pgdir;
>     struct task_struct *temp; 
>   
>     void in_thehandler()
>     {
>       temp = current_set[0];
>       current_set[0] = pid2task(pid);
>       if (!current_set[0]) {
>         printk("error: couldn't find process.\n");
>       }
>       else {
> 
>         /* make CR3 point to this guy's page directory */
>         pgdir = current_set[0]->tss.cr3;
>         __asm__ __volatile__("movl %0,%%cr3": :"r" (pgdir));
> 
>         memcpy_tofs(user_buf, &ch, 1);
>       }
>       current_set[0] = temp;
> 
>       /* restore CR3 to point to the original page directory */
>       pgdir = current_set[0]->tss.cr3;
>       __asm__ __volatile__("movl %0,%%cr3": :"r" (pgdir));
>     }
> 
>     struct task_struct* pid2task(int pid)
>     {
>       /* see the section on Virtual to Physical address translation */
>     }
> 
> Here, we do a bit of a context switch by finding out the task
> structure associated with the pid (by calling pid2task()and then
> loading the page directory pointer for that task into the CR3 register
> (the latter is the hardware's entry point to a given process's paging
> structures). Then we can use the _fs functions. When we are done we
> restore the saved context. The code for updating CR3 was lifted from
> the SET_PAGE_DIR macro in include/asm. 
> 
> Thank you in advance
> 
> 
>  
> 
>  
> 

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