ioctl, copy string from user

[John Baldwin]

On Thursday 29 April 2010 4:28:31 pm Lukáš Czerner wrote:
> On Thu, 29 Apr 2010, John Baldwin wrote:
> > > > 
> > > > 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.
> 
> I have already tried that, but still without any success. The buffer
> remains unchanged (which is weird IMO).

Can you print out the value of 'ap->a_data' from the kernel and the value of
'&name' in userland?

> > 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.
> 
> So I suppose M_NOWAIT will do the trick when there is no other way
> (preallocations etc..) ? Of course I should test if it does not
> return NULL then.

Yes, but in a VOP_IOCTL() handler it should be safe to M_WAITOK malloc() as
long as you do it before you acquire any mutexes.

-- 
John Baldwin