Abusing region 0

Abusing region 0

[Menyhart, Zoltan]

I am going to do some experiments: I am going to map some of the
virtual address ranges of a process into the address space of another
one. They are real 64 bit processes, i.e. the region 0 is not used.
Can I abuse this region 0 for mapping the addresses of the neighbor
process into this region ? E.g. if process A has got an address at
0x6000000000001000, then I am going to map it for B at address
0x0000000000001000 ? Wont anything break ?

(I need this trick because the source address can correspond to
anything, like initialized data, BSS, anonymous allocation, etc, i.e.
some SHM just wont work.)

Any advice will be appreciated.

Zoltan Menyhart

Re: Abusing region 0

[Jack Steiner]

On Thu, Feb 03, 2005 at 02:03:01AM +0100, Menyhart, Zoltan wrote:
> I am going to do some experiments: I am going to map some of the
> virtual address ranges of a process into the address space of another
> one. They are real 64 bit processes, i.e. the region 0 is not used.
> Can I abuse this region 0 for mapping the addresses of the neighbor
> process into this region ? E.g. if process A has got an address at
> 0x6000000000001000, then I am going to map it for B at address
> 0x0000000000001000 ? Wont anything break ?
> 
> (I need this trick because the source address can correspond to
> anything, like initialized data, BSS, anonymous allocation, etc, i.e.
> some SHM just wont work.)
> 
> Any advice will be appreciated.

FWIW, I have done the same thing that you are trying except that I used
address 0x0000000010000000 instead of 0x0000000000001000. No problems.


> 
> Zoltan Menyhart
> 
> 
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-- 
Thanks

Jack Steiner (steiner@sgi.com)          651-683-5302
Principal Engineer                      SGI - Silicon Graphics, Inc.

Re: Abusing region 0

[David Mosberger]

>>>>> On Thu, 03 Feb 2005 02:03:01 +0100, "Menyhart, Zoltan" <Zoltan.Menyhart@free.fr> said:

  Zoltan> I am going to do some experiments: I am going to map some of
  Zoltan> the virtual address ranges of a process into the address
  Zoltan> space of another one. They are real 64 bit processes,
  Zoltan> i.e. the region 0 is not used.  Can I abuse this region 0
  Zoltan> for mapping the addresses of the neighbor process into this
  Zoltan> region ? E.g. if process A has got an address at
  Zoltan> 0x6000000000001000, then I am going to map it for B at
  Zoltan> address 0x0000000000001000 ? Wont anything break ?

  Zoltan> (I need this trick because the source address can correspond
  Zoltan> to anything, like initialized data, BSS, anonymous
  Zoltan> allocation, etc, i.e.  some SHM just wont work.)

  Zoltan> Any advice will be appreciated.

Yes, you can do this.  There is nothing special about region 0 as far
as the kernel is concerned.  Well, I'm lying: the one thing that's
special is that there is a NaT page mapped at address 0 to catch
speculative accesses through a NULL-pointer.  Because of that, I'd
recommend to use an address greater than 64KB (the largest base
page-size supported by Linux).

	--david