Messages in this thread |  | | | Subject | Re: [PATCH] x86_64: Implement personality ADDR_LIMIT_32BIT | | From | Andi Kleen <> | | Date | Mon, 06 Oct 2008 08:13:19 +0200 |
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"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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