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Subject[PATCH] mmap.2: MAP_FIXED is okay if the address range has been reserved
From
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
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