Re: [PATCH] x86_64: Implement personality ADDR_LIMIT_32BIT
From: Andi Kleen
Date: Mon Oct 06 2008 - 02:14:04 EST
"Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> 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
void *p = &p;
andi@basil:~/tsrc> gcc -m32 tstack2.c -o tstack2
andi@basil:~/tsrc> linux32 --3gb ./tstack2
>> 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
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/