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Thu, 12 Apr 2018 14:32:47 -0400 Received: from mail-wm0-f65.google.com ([74.125.82.65]:38710 "EHLO mail-wm0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752937AbeDLScp (ORCPT ); Thu, 12 Apr 2018 14:32:45 -0400 X-Google-Smtp-Source: AIpwx48gKvFYuuAdObpIaxBfPL7Na/OHz8wUWizsxLDupy0W5y8zfW5s2Wie/NRTAy0EmZgn5qLdIw== Cc: mtk.manpages@gmail.com Subject: Re: [PATCH] mmap.2: MAP_FIXED is okay if the address range has been reserved To: Jann Horn , linux-man@vger.kernel.org, mhocko@kernel.org, jhubbard@nvidia.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-api@vger.kernel.org References: <20180412153941.170849-1-jannh@google.com> From: "Michael Kerrisk (man-pages)" Message-ID: <3ff7472c-23c8-402b-ddfb-871a749c5016@gmail.com> Date: Thu, 12 Apr 2018 20:32:40 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 MIME-Version: 1.0 In-Reply-To: <20180412153941.170849-1-jannh@google.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-api-owner@vger.kernel.org X-Mailing-List: linux-api@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: Hello Jann, On 04/12/2018 05:39 PM, Jann Horn wrote: > Clarify that MAP_FIXED is appropriate if the specified address range has > been reserved using an existing mapping, but shouldn't be used otherwise. > > Signed-off-by: Jann Horn > --- > man2/mmap.2 | 19 +++++++++++-------- > 1 file changed, 11 insertions(+), 8 deletions(-) > > diff --git a/man2/mmap.2 b/man2/mmap.2 > index bef8b4432..80c9ec285 100644 > --- a/man2/mmap.2 > +++ b/man2/mmap.2 > @@ -253,8 +253,9 @@ Software that aspires to be portable should use this option with care, > keeping in mind that the exact layout of a process's memory mappings > is allowed to change significantly between kernel versions, > C library versions, and operating system releases. > -Furthermore, this option is extremely hazardous (when used on its own), > -because it forcibly removes preexisting mappings, > +This option should only be used when the specified memory region has > +already been reserved using another mapping; otherwise, it is extremely > +hazardous because it forcibly removes preexisting mappings, > making it easy for a multithreaded process to corrupt its own address space. > .IP > For example, suppose that thread A looks through > @@ -284,13 +285,15 @@ and the PAM libraries > .UR http://www.linux-pam.org > .UE . > .IP > -Newer kernels > -(Linux 4.17 and later) have a > +For cases in which the specified memory region has not been reserved using an > +existing mapping, newer kernels (Linux 4.17 and later) provide an option > .B MAP_FIXED_NOREPLACE > -option that avoids the corruption problem; if available, > -.B MAP_FIXED_NOREPLACE > -should be preferred over > -.BR MAP_FIXED . > +that should be used instead; older kernels require the caller to use > +.I addr > +as a hint (without > +.BR MAP_FIXED ) > +and take appropriate action if the kernel places the new mapping at a > +different address. > .TP > .BR MAP_FIXED_NOREPLACE " (since Linux 4.17)" > .\" commit a4ff8e8620d3f4f50ac4b41e8067b7d395056843 Thanks! Nice patch! Applied. Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/