Locking Memory Question

[Hooman Fazaeli]

On 7/30/2015 5:22 AM, Laurie Jennings via freebsd-net wrote:
> --------------------------------------------
> 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

You can use two IOCTLs. Get the block size from kernel module with the first ioctl,
and malloc(3) a buffer in userland with that size. Then use a second ioctl to pass the
address of allocated buffer to kernel module. The module may use copyout(9) to copy
in-kernel data to user space buffer.


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-- 
Best regards
Hooman Fazaeli