From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f197.google.com (mail-qk0-f197.google.com [209.85.220.197]) by kanga.kvack.org (Postfix) with ESMTP id E21946B0009 for ; Thu, 12 Apr 2018 11:39:51 -0400 (EDT) Received: by mail-qk0-f197.google.com with SMTP id v74so3795791qkl.9 for ; Thu, 12 Apr 2018 08:39:51 -0700 (PDT) Received: from mail-sor-f73.google.com (mail-sor-f73.google.com. [209.85.220.73]) by mx.google.com with SMTPS id y10sor3088346qkl.24.2018.04.12.08.39.50 for (Google Transport Security); Thu, 12 Apr 2018 08:39:50 -0700 (PDT) MIME-Version: 1.0 Date: Thu, 12 Apr 2018 17:39:41 +0200 Message-Id: <20180412153941.170849-1-jannh@google.com> Subject: [PATCH] mmap.2: MAP_FIXED is okay if the address range has been reserved From: Jann Horn Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: To: mtk.manpages@gmail.com, 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, jannh@google.com 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 -- 2.17.0.484.g0c8726318c-goog