Re: [PATCH] x86_64: Implement personality ADDR_LIMIT_32BIT

[Andi Kleen <andi@firstfloor.org>]

On Mon, Oct 06, 2008 at 11:17:23AM +0300, Kirill A. Shutemov wrote:
> On Mon, Oct 06, 2008 at 08:13:19AM +0200, Andi Kleen wrote:
> > "Kirill A. Shutemov" <kirill@shutemov.name> writes:
> > >
> > >> 
> > >> but more generally, we already have ADDR_LIMIT_3GB support on x86.
> > >
> > > Does ADDR_LIMIT_3GB really work?
> > 
> > As Arjan pointed out it only takes effect on exec()
> > 
> > andi@basil:~/tsrc> cat tstack2.c
> > #include <stdio.h>
> > int main(void)
> > {
> >         void *p = &p;
> >         printf("%p\n", &p);
> >         return 0;
> > }
> > andi@basil:~/tsrc> gcc -m32 tstack2.c  -o tstack2
> > andi@basil:~/tsrc> ./tstack2 
> > 0xff807d70
> > andi@basil:~/tsrc> linux32 --3gb ./tstack2 
> > 0xbfae2840
> 
> Which kernel do you use?

This was 2.6.26 (+ some irrelevant patches)

> Does it work only when compiled with -m32?

Yes. For 64bit processes you use the method described below for mmap.

> mmap() has MAP_32BIT flag on x86_64.

MAP_32BIT is just a short form for this, it's internally the same.
But it's actually MAP_31BIT. If you want the full 4GB you should not use it.

> 
> > Unfortunately that doesn't work for shmat() because the address argument
> > is not a search hint, but a fixed address. 
> > 
> > I presume you need this for the qemu syscall emulation. For a standard
> > application I would just recommend to use mmap with tmpfs instead
> > (sysv shm is kind of obsolete). For shmat() emulation the cleanest way
> > would be probably to add a new flag to shmat() that says that address
> > is a search hint, not a fixed address. Then implement it the way recommended
> > above.
> 
> I prefer one handle to switch application to 32-bit address mode. Why is it
> wrong?

"32 bit mode" really has to be set at exec time, otherwise it is not
(e.g. the stack will be beyond).

And personality() is not thread local/safe, so it's not a particularly
good interface to use later. Per system call switches are preferable
and more flexible.

-Andi
-- 
ak@linux.intel.com

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