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From: Jann Horn <jannh@google.com>
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
Cc: linux-man@vger.kernel.org, Michal Hocko <mhocko@kernel.org>,
	John Hubbard <jhubbard@nvidia.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	linux-api@vger.kernel.org
Subject: [PATCH] mmap.2: MAP_FIXED is okay if the address range has been reserved
Date: Thu, 12 Apr 2018 17:39:41 +0200	[thread overview]
Message-ID: <20180412153941.170849-1-jannh@google.com> (raw)

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 <jannh@google.com>
---
 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


             reply	other threads:[~2018-04-12 15:39 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-12 15:39 Jann Horn [this message]
2018-04-12 18:32 ` Michael Kerrisk (man-pages)
2018-04-12 18:33 ` John Hubbard
2018-04-12 18:37   ` Michael Kerrisk (man-pages)
2018-04-12 18:49     ` Jann Horn
2018-04-12 18:59       ` John Hubbard
2018-04-12 19:18         ` Jann Horn
2018-04-12 19:24           ` John Hubbard
2018-04-13  6:43             ` Michael Kerrisk (man-pages)
2018-04-13  6:49               ` Michal Hocko
2018-04-13 15:04                 ` Jann Horn
2018-04-13 16:04                   ` Michal Hocko
2018-04-13 16:05                     ` Jann Horn
2018-04-13 16:17                       ` Jann Horn
2018-04-16 10:07                         ` Michal Hocko
2018-04-16 13:55                           ` Jann Horn
2018-04-16 19:18                             ` Michal Hocko
2018-04-16 19:30                               ` Jann Horn
2018-04-16 19:57                                 ` Michal Hocko
2018-04-16 20:17                                   ` Jann Horn
2018-04-16 21:11                                     ` Michal Hocko
2018-04-16 21:12                                       ` Jann Horn
2018-04-17  6:23                                         ` Michal Hocko
2018-05-02 13:06                                       ` Michael Kerrisk (man-pages)

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