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

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Message-ID: <48B708C4.4000405@kernel.org>
Date:	Thu, 28 Aug 2008 13:21:24 -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:
>> Tejun Heo wrote:
>>> Ah.... funky.  If this retry thing is too repulsive, I guess the best
>>> alternative would be directly accessing caller's memory as Miklos suggested.
>>>
>> Be careful -- there are some serious dragons there in the presence of 
>> multiple threads.
> 
> OK, it should map /proc/pid/task/tid/mem.  Or rather
> /proc/tid/task/tid/mem, as the pid (tgid) of the caller is not
> currently passed to the filesystem.
> 

Uhm, no.  You can still have it change underneath you as long as you 
have any thread of execution with access to the same memory.

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.

That is why I'm suggesting using an in-kernel linearizer.

	-hpa
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