mmap.2: MAP_FIXED is okay if the address range has been reserved

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>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

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