On 7/30/2015 5:22 AM, Laurie Jennings via freebsd-net wrote:
--------------------------------------------
On Wed, 7/29/15, John-Mark Gurney <j...@funkthat.com> wrote:
Subject: Re: Locking Memory Question
To: "Laurie Jennings" <laurie_jennings_1...@yahoo.com>
Cc: "John Baldwin" <j...@freebsd.org>, freebsd-net@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
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