ioctl, copy string from user

[Lukáš Czerner]

On Thu, 29 Apr 2010, John Baldwin wrote:

> Date: Thu, 29 Apr 2010 14:18:09 -0400
> From: John Baldwin <jhb at freebsd.org>
> To: freebsd-hackers at freebsd.org
> Cc: Lukáš Czerner <czerner.lukas at gmail.com>
> Subject: Re: ioctl, copy string from user
> 
> On Thursday 29 April 2010 1:52:45 pm Lukáš Czerner wrote:
> > Hi,
> > 
> > I know that there are plenty of examples in the kernel code, but I
> > just can not get it working, so maybe I am doing some stupid mistake
> > I am not aware of. Please give me a hint if you can.
> > 
> > What I want to do is simply call the ioctl from the userspace with
> > (char *) argument. Then, in kernel ioctl handling function copy the
> > string argument into the kernel space. I have tried it various ways,
> > everything without any success.
> > 
> > *** Userspace ***
> > char name[MAXLEN];
> > 
> > strncpy(name, argv[1], MAXLEN);
> > fprintf(stdout,"Name: %s\n",name);
> > 
> > if (ioctl(fd, MYIOCTL, name)) {
> 
> On BSD systems, ioctl() copies the data into the kernel for you ahead of time.  
> What does the definition of MYIOCTL look like?

#define MYIOCTL _IOW('M', 0, char *)

> 
> > And the second question. I have commented that I can allocate buffer
> > dynamically, but I suppose that there will be some locks involved so
> > I think I can not just use M_WAITOK, am I right ?
> 
> malloc() and free() acquire their own locks internally, you do not need to 
> hold any locks to call them.

I probably does not express what I meant very clearly. My concern is
that when I am calling malloc with M_WAITOK I can sleep (be
rescheduled) and it may be bad thing if I am holding some lock,
because I can block others, am I right ?