From mboxrd@z Thu Jan 1 00:00:00 1970 Subject: Re: [PATCH] x86_64: Implement personality ADDR_LIMIT_32BIT From: Andi Kleen References: <1223017469-5158-1-git-send-email-kirill@shutemov.name> <20081003080244.GC25408@elte.hu> <20081003092550.GA8669@localhost.localdomain> Date: Mon, 06 Oct 2008 08:13:19 +0200 In-Reply-To: <20081003092550.GA8669@localhost.localdomain> (Kirill A. Shutemov's message of "Fri, 3 Oct 2008 12:25:52 +0300") Message-ID: <87abdintds.fsf@basil.nowhere.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: owner-linux-mm@kvack.org Return-Path: To: "Kirill A. Shutemov" Cc: Ingo Molnar , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Andrew Morton List-ID: "Kirill A. Shutemov" 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 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org