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

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Message-ID: <48B7183E.3010100@kernel.org>
Date:	Thu, 28 Aug 2008 14:27:26 -0700
From:	"H. Peter Anvin" <hpa@...nel.org>
To:	Miklos Szeredi <miklos@...redi.hu>
CC:	tj@...nel.org, fuse-devel@...ts.sourceforge.net, greg@...ah.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/7] FUSE: implement ioctl support

Miklos Szeredi wrote:
> On Thu, 28 Aug 2008, H. Peter Anvin wrote:
>> This is *hard* to get right, and we screw this up in the kernel with 
>> painful regularity.  The throught of having user-space processes, which 
>> don't have access to the kernel locking primitives and functions like 
>> copy_from_user() dealing with this stuff scares me crazy.
> 
> What issues exactly are you thinking of?

Memory changing underneath you.  It can be dealt with by very careful 
sequencing only.

>> That is why I'm suggesting using an in-kernel linearizer.
> 
> Lots of complexity, ugh...  Even Tejun's current scheme is better IMO.

And then you get *no* privilege separation, for one thing, so why even 
bother doing it in userspace?

	-hpa
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