linux-kernel - Re: [PATCH 5/7] FUSE: implement ioctl support

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Message-Id: <E1KYm0D-0003q0-IQ@pomaz-ex.szeredi.hu>
Date:	Thu, 28 Aug 2008 20:13:49 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	tj@...nel.org
CC:	greg@...ah.com, fuse-devel@...ts.sourceforge.net,
	miklos@...redi.hu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/7] FUSE: implement ioctl support

On Thu, 28 Aug 2008, Tejun Heo wrote:
> Tejun Heo wrote:
> > Greg KH wrote:
> >> Why do we need ioctls?  For CUSE?  In that case, would we need to copy
> >> the memory from userspace, into the kernel, and then back out into
> >> userspace again?  Can't we just have a "pass-through" type fixed ioctl
> >> instead?
> > 
> > Can you elaborate a bit?  How the fixed ioctl would know how much to
> > copy in and out and from where?
> 
> If you're worried about the double copying due to performance reasons,
> what we can do is implementing userspace to userspace copying.  I just
> took easier path of copying twice as u-u copy can't be done using the
> existing FUSE mechanics.

How about not copying anything in the kernel, just passing the virtual
address to the filesystem?  It can get/modify the data of the calling
task by mapping /proc/pid/mem.  Ugly, yes, but ioctls are inherently
ugly.

Miklos
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