Locking Memory Question

[Laurie Jennings]

--------------------------------------------
On Wed, 7/29/15, John-Mark Gurney <jmg at funkthat.com> wrote:

 Subject: Re: Locking Memory Question
 To: "Laurie Jennings" <laurie_jennings_1977 at yahoo.com>
 Cc: "John Baldwin" <jhb at freebsd.org>, freebsd-net at freebsd.org
 Date: Wednesday, July 29, 2015, 7:25 PM
 
 Laurie Jennings via
 freebsd-net wrote this message on Wed, Jul 29, 2015 at 15:26
 -0700:
 > 
 > I have a problem and
 I can't quite figure out where to look. This is what Im
 doing:
 > 
 > I have an
 IOCTL to read a block of data, but the data is too large to
 return via ioctl. So to get the data,
 > I
 allocate a block in a kernel module:
 >
 
 > foo =
 malloc(1024000,M_DEVBUF,M_WAITOK);
 > 
 >  I pass up a pointer and in user space
 map it using /dev/kmem:
 
 An easier solution would be for your ioctl to
 pass in a userland
 pointer and then use
 copyout(9) to push the data to userland...  This
 means the userland process doesn't have to
 have /dev/kmem access...
 
 Is
 there a reason you need to use kmem?  The only reason you
 list above
 is that it's too large via
 ioctl, but a copyout is fine, and would
 handle all page faults for you..
 
 __________________________________
I'm using kmem because the only options I could think of was to

1) use shared memory
2) use kmem
3) use a huge ioctl structure.

Im not clear how I'd do that. the data being passed up from the kernel is a variable size. To use copyout I'd have to pass a
pointer with a static buffer, right? Is there a way to malloc user space memory from within an ioctl call? Or
would I just have to pass down a pointer to a huge buffer large enough for the largest possible answer?

thanks

Laurie