Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756750AbbLASTi (ORCPT ); Tue, 1 Dec 2015 13:19:38 -0500 Received: from mail-pa0-f45.google.com ([209.85.220.45]:36663 "EHLO mail-pa0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756358AbbLASTf (ORCPT ); Tue, 1 Dec 2015 13:19:35 -0500 Subject: Re: [PATCH v4 4/4] x86: mm: support ARCH_MMAP_RND_BITS. To: Kees Cook References: <1448578785-17656-1-git-send-email-dcashman@android.com> <1448578785-17656-2-git-send-email-dcashman@android.com> <1448578785-17656-3-git-send-email-dcashman@android.com> <1448578785-17656-4-git-send-email-dcashman@android.com> <1448578785-17656-5-git-send-email-dcashman@android.com> Cc: LKML , Russell King - ARM Linux , Andrew Morton , Ingo Molnar , "linux-arm-kernel@lists.infradead.org" , Jonathan Corbet , Don Zickus , "Eric W. Biederman" , Heinrich Schuchardt , jpoimboe@redhat.com, "Kirill A. Shutemov" , n-horiguchi@ah.jp.nec.com, Andrea Arcangeli , Mel Gorman , Thomas Gleixner , David Rientjes , Linux-MM , "linux-doc@vger.kernel.org" , Mark Salyzyn , Jeffrey Vander Stoep , Nick Kralevich , Catalin Marinas , Will Deacon , "H. Peter Anvin" , "x86@kernel.org" , Hector Marco , Borislav Petkov , Daniel Cashman From: Daniel Cashman Message-ID: <565DE4B4.5050305@android.com> Date: Tue, 1 Dec 2015 10:19:32 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2157 Lines: 58 On 11/30/2015 04:03 PM, Kees Cook wrote: > On Thu, Nov 26, 2015 at 2:59 PM, Daniel Cashman wrote: >> diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c >> index 844b06d..647fecf 100644 >> --- a/arch/x86/mm/mmap.c >> +++ b/arch/x86/mm/mmap.c >> @@ -69,14 +69,14 @@ unsigned long arch_mmap_rnd(void) >> { >> unsigned long rnd; >> >> - /* >> - * 8 bits of randomness in 32bit mmaps, 20 address space bits >> - * 28 bits of randomness in 64bit mmaps, 40 address space bits >> - */ >> if (mmap_is_ia32()) >> - rnd = (unsigned long)get_random_int() % (1<<8); >> +#ifdef CONFIG_COMPAT >> + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits); >> +#else >> + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits); >> +#endif >> else >> - rnd = (unsigned long)get_random_int() % (1<<28); >> + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits); >> >> return rnd << PAGE_SHIFT; >> } >> -- >> 2.6.0.rc2.230.g3dd15c0 >> > > Can you rework this logic to look more like the arm64 one? I think > it's more readable as: > > #ifdef CONFIG_COMPAT > if (mmap_is_ia32()) > rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits); > else > #endif > rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits); > > -Kees > There is a subtle difference between the two that requires this difference. the x86 code was written to be used by both 32-bit and 64-bit kernels, whereas the arm64 code runs only for 64-bit. The assumption I've made with arm64 is that TIF_32BIT should never be set if CONFIG_COMPAT is not set, but with x86 we could encounter a 32-bit application without CONFIG_COMPAT, in which case it should use the default mmap_rnd_bits, not compat, since there is no compat. -Dan -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/