Messages in this thread | | | Date | Mon, 6 Oct 2008 10:42:46 +0200 | From | Andi Kleen <> | Subject | Re: [PATCH] x86_64: Implement personality ADDR_LIMIT_32BIT |
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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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