linux-kernel - Re: JIT emulator needs

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Date:	Fri, 22 Jun 2007 01:56:28 -0400
From:	"Albert Cahalan" <acahalan@...il.com>
To:	"Arjan van de Ven" <arjan@...radead.org>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: JIT emulator needs

On 6/21/07, Arjan van de Ven <arjan@...radead.org> wrote:
> On Fri, 2007-06-08 at 02:35 -0400, Albert Cahalan wrote:
> > Right now, Linux isn't all that friendly to JIT emulators.
> > Here are the problems and suggestions to improve the situation.
> >
> > There is an SE Linux execmem restriction that enforces W^X.
> > Assuming you don't wish to just disable SE Linux, there are
> > two ugly ways around the problem. You can mmap a file twice,
> > or you can abuse SysV shared memory. The mmap method requires
> > that you know of a filesystem mounted rw,exec where you can
> > write a very large temporary file. This arbitrary filesystem,
> > rather than swap space, will be the backing store. The SysV
> > shared memory method requires an undocumented flag and is
> > subject to some annoying size limits. Both methods create
> > objects that will fail to be deleted if the program dies
> > before marking the objects for deletion.
>
> and these methods also destroy yourself on any machine with a looser
> cache coherency between I and D-cache....
>
> for all but x86 you pretty much have to do the mprotect() between the
> two states to deal with the cache flushing properly...

If the instructions to force data write-back and/or to
invalidate the instruction cache are priveleged, yes.
AFAIK, only ARM is that lame.

For example, PowerPC lets unprivileged code run
the required instructions.
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