ioctl, copy string from user

[Lukáš Czerner]

On Thu, 29 Apr 2010, Ryan Stone wrote:

> Date: Thu, 29 Apr 2010 17:19:41 -0400
> From: Ryan Stone <rysto32 at gmail.com>
> To: Lukáš Czerner <czerner.lukas at gmail.com>
> Cc: freebsd-hackers at freebsd.org
> Subject: Re: ioctl, copy string from user
> 
> > Apparently I need to tell ioctl how big is the variable I am
> > providing to it ([MAXLEN]). The odd thing is, when I have a structure
> > like this:
> >
> > struct lrfs_attach_info {
> >        char *name;
> >        int priority;
> > };
> >
> > and I pass the pointer to that structure to the ioctl, it just
> > works. I can even use the 'name' string from the structure without
> > any problems, apparently it translates the pointer properly, but I
> > did not expect this...
> 
> I think that you'll find that what actually happens is that you're
> reading from the userspace pointer.  That will work as long as the
> pointer is mapped, but you'll panic if what it points to gets swapped
> out or is invalid.
> 

Hmmm, I can prevent the pointer to be invalidated, but I doubt I can
prevent to the pointer to been swapped out. Is there some better way
? Allocate it statically (char name[MAXLEN]), then it gets copied
as whole to the kernel space, isn't it ?