ioctl, copy string from user

[John Baldwin]

On Thursday 29 April 2010 3:21:00 pm Lukáš Czerner wrote:
> 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 *)

Ok.  In that case the argument to ioctl needs to be a pointer to a char *,
not the raw char * itself.  Try doing 'ioctl(fd, MYIOCTL, &name)' from 
userland to see if that fixes it.

> > > 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 ?

Generally yes, but it depends on the lock.  If it is the vn_lock lock then it 
is ok to do a blocking malloc().  As a general rule I do try to call malloc()
before acquiring locks (basically preallocating) whenever possible.

-- 
John Baldwin