From mboxrd@z Thu Jan 1 00:00:00 1970 From: John Hubbard Subject: Re: [PATCH] mmap.2: MAP_FIXED is okay if the address range has been reserved Date: Thu, 12 Apr 2018 11:33:04 -0700 Message-ID: References: <20180412153941.170849-1-jannh@google.com> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20180412153941.170849-1-jannh@google.com> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Jann Horn , mtk.manpages@gmail.com, linux-man@vger.kernel.org, mhocko@kernel.org, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-api@vger.kernel.org List-Id: linux-man@vger.kernel.org On 04/12/2018 08:39 AM, 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. Yes, that's clearer and provides more information than before. > .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 ) Here, I got lost: the sentence suddenly jumps into explaining non-MAP_FIXED behavior, in the MAP_FIXED section. Maybe if you break up the sentence, and possibly omit non-MAP_FIXED discussion, it will help. > +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, -- John Hubbard NVIDIA