Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932273AbbLNTCP (ORCPT ); Mon, 14 Dec 2015 14:02:15 -0500 Received: from terminus.zytor.com ([198.137.202.10]:59146 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932175AbbLNTCN (ORCPT ); Mon, 14 Dec 2015 14:02:13 -0500 Subject: Re: [PATCH v6 4/4] x86: mm: support ARCH_MMAP_RND_BITS. To: Daniel Cashman , linux-kernel@vger.kernel.org References: <1449856338-30984-1-git-send-email-dcashman@android.com> <1449856338-30984-2-git-send-email-dcashman@android.com> <1449856338-30984-3-git-send-email-dcashman@android.com> <1449856338-30984-4-git-send-email-dcashman@android.com> <1449856338-30984-5-git-send-email-dcashman@android.com> Cc: linux@arm.linux.org.uk, akpm@linux-foundation.org, keescook@chromium.org, mingo@kernel.org, linux-arm-kernel@lists.infradead.org, corbet@lwn.net, dzickus@redhat.com, ebiederm@xmission.com, xypron.glpk@gmx.de, jpoimboe@redhat.com, kirill.shutemov@linux.intel.com, n-horiguchi@ah.jp.nec.com, aarcange@redhat.com, mgorman@suse.de, tglx@linutronix.de, rientjes@google.com, linux-mm@kvack.org, linux-doc@vger.kernel.org, salyzyn@android.com, jeffv@google.com, nnk@google.com, catalin.marinas@arm.com, will.deacon@arm.com, x86@kernel.org, hecmargi@upv.es, bp@suse.de, dcashman@google.com, arnd@arndb.de, jonathanh@nvidia.com From: "H. Peter Anvin" Message-ID: <566F1154.7030703@zytor.com> Date: Mon, 14 Dec 2015 10:58:28 -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: <1449856338-30984-5-git-send-email-dcashman@android.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2063 Lines: 57 On 12/11/15 09:52, Daniel Cashman wrote: > From: dcashman > > x86: arch_mmap_rnd() uses hard-coded values, 8 for 32-bit and 28 for > 64-bit, to generate the random offset for the mmap base address. > This value represents a compromise between increased ASLR > effectiveness and avoiding address-space fragmentation. Replace it > with a Kconfig option, which is sensibly bounded, so that platform > developers may choose where to place this compromise. Keep default > values as new minimums. > > Signed-off-by: Daniel Cashman OK, this is around the time when I make a lecture about the danger of expecting the compiler to make certain transformations: > 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; > } > Now, you and I know that both variants can be implemented with a simple AND, but I have a strong suspicion that once this is turned into a variable, this will in fact be changed from an AND to a divide. So I'd prefer to use the "get_random_int() & ((1UL << mmap_rnd_bits) - 1)" construct instead. -hpa -- 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/