Re: [PATCH] x86_64: Implement personality ADDR_LIMIT_32BIT

[Andi Kleen <andi@firstfloor.org>]

"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

>> Why 
>> should support for ADDR_LIMIT_32BIT be added?
>
> It's useful for user mode qemu when you try emulate 32-bit target on 
> x86_64. For example, if shmat(2) return addres above 32-bit, target will
> get SIGSEGV on access to it.

The traditional way in mmap() to handle this is to give it a search
hint < 4GB and then free the memory again/fail if the result was >4GB.

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.

-Andi

-- 
ak@linux.intel.com

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