Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752587AbdLHUra (ORCPT <rfc822;w@1wt.eu>);
        Fri, 8 Dec 2017 15:47:30 -0500
Received: from mx1.redhat.com ([209.132.183.28]:33748 "EHLO mx1.redhat.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1751474AbdLHUr1 (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 8 Dec 2017 15:47:27 -0500
Subject: Re: [PATCH 0/2] mm: introduce MAP_FIXED_SAFE
To: Pavel Machek <pavel@ucw.cz>, Michael Ellerman <mpe@ellerman.id.au>
Cc: Matthew Wilcox <willy@infradead.org>,
        Kees Cook <keescook@chromium.org>, Cyril Hrubis <chrubis@suse.cz>,
        Michal Hocko <mhocko@kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Khalid Aziz <khalid.aziz@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Russell King - ARM Linux <linux@armlinux.org.uk>,
        Andrea Arcangeli <aarcange@redhat.com>, Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        John Hubbard <jhubbard@nvidia.com>,
        Abdul Haleem <abdhalee@linux.vnet.ibm.com>,
        Joel Stanley <joel@jms.id.au>
References: <CAGXu5jLa=b2HhjWXXTQunaZuz11qUhm5aNXHpS26jVqb=G-gfw@mail.gmail.com>
 <20171130065835.dbw4ajh5q5whikhf@dhcp22.suse.cz> <20171201152640.GA3765@rei>
 <87wp20e9wf.fsf@concordia.ellerman.id.au>
 <20171206045433.GQ26021@bombadil.infradead.org>
 <20171206070355.GA32044@bombadil.infradead.org>
 <87bmjbks4c.fsf@concordia.ellerman.id.au>
 <CAGXu5jLWRQn6EaXEEvdvXr+4gbiJawwp1EaLMfYisHVfMiqgSA@mail.gmail.com>
 <20171207195727.GA26792@bombadil.infradead.org>
 <87shclh3zc.fsf@concordia.ellerman.id.au> <20171208142714.GB7793@amd>
From: Florian Weimer <fweimer@redhat.com>
Message-ID: <289f0cc8-9e24-fc38-cb83-4223f9923940@redhat.com>
Date: Fri, 8 Dec 2017 21:47:21 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.4.0
MIME-Version: 1.0
In-Reply-To: <20171208142714.GB7793@amd>
Content-Type: text/plain; charset=windows-1252; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.38]); Fri, 08 Dec 2017 20:47:27 +0000 (UTC)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 974
Lines: 22

On 12/08/2017 03:27 PM, Pavel Machek wrote:
> On Fri 2017-12-08 22:08:07, Michael Ellerman wrote:
>> If we had a time machine, the right set of flags would be:
>>
>>    - MAP_FIXED:   don't treat addr as a hint, fail if addr is not free
>>    - MAP_REPLACE: replace an existing mapping (or force or clobber)

> Actually, if we had a time machine... would we even provide
> MAP_REPLACE functionality?

Probably yes.  ELF loading needs to construct a complex set of mappings 
from a single file.  munmap (to create a hole) followed by mmap would be 
racy because another thread could have reused the gap in the meantime. 
The only alternative to overriding existing mappings would be mremap 
with MREMAP_FIXED, and that doesn't look like an improvement API-wise.

(The glibc dynamic linker uses an mmap call with an increased length to 
reserve address space and then loads additional segments with MAP_FIXED 
at the offsets specified in the program header.)

Thanks,
Florian