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Explain PROT_MPROTECT better, add references to mremap(2), document the 

unimplemented flags, documented the alignment pre-defined constants.
This commit is contained in:
christos 2018-05-02 16:00:20 +00:00
parent 652623d133
commit 3e4df0d384
1 changed files with 48 additions and 5 deletions
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53
lib/libc/sys/mmap.2
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@ -1,4 +1,4 @@
.\" $NetBSD: mmap.2,v 1.51 2017/06/12 14:59:04 abhinav Exp $
.\" $NetBSD: mmap.2,v 1.52 2018/05/02 16:00:20 christos Exp $
.\"
.\" Copyright (c) 1991, 1993
.\" The Regents of the University of California. All rights reserved.
@ -29,7 +29,7 @@
.\"
.\" @(#)mmap.2 8.4 (Berkeley) 5/11/95
.\"
.Dd April 27, 2017
.Dd May 2, 2018
.Dt MMAP 2
.Os
.Sh NAME
@ -89,12 +89,22 @@ Placeholder when requesting no access permission.
.Pp
As a
.Nx
extension,
extension, the
.Dv PROT_MPROTECT
can be used to request additional permissions for later use with
macro can be used to request additional permissions for later use with
.Fn mprotect 2 .
For example
.Dv PROT_MPROTECT(PROT_READ)
requests that future
.Dv PROT_READ
mappings are allowed and can be enabled using
.Xr mprotect 2 ,
but does not currently grant read mappings to the returned memory segment.
This is necessary for switching pages between writable and executable
when PAX mprotect restrictions are in place.
when PaX MPROTECT restrictions are in place.
See
.Xr mremap 2
for a sample use case.
.Pp
.Bf -symbolic
Note that, due to hardware limitations, on some platforms
@ -137,6 +147,21 @@ size as returned by
with the
.Dv _SC_PAGESIZE
request.
The following constants are defined for convenience:
.Bl -bullet -compact -offset indent
.It
.Dv MAP_ALIGNMENT_64KB
.It
.Dv MAP_ALIGNMENT_16MB
.It
.Dv MAP_ALIGNMENT_4GB
.It
.Dv MAP_ALIGNMENT_1TB
.It
.Dv MAP_ALIGNMENT_256TB
.It
.Dv MAP_ALIGNMENT_64PB
.El
.It Dv MAP_ANON
Map anonymous memory not associated with any specific file.
The file descriptor is not used for creating
@ -170,6 +195,23 @@ handling may be necessary.
Permit regions to be inherited across
.Xr execve 2
system calls.
.It Dv MAP_NORESERVE
Only reserve address space, but do not reserve swap space or any other
resources for this mapping.
Access to the address space is not guaranteed and may result in a segmentation
violation.
Unimplemented.
.It Dv MAP_REMAPDUP
Only valid for
.Xr mremap 2 .
.It Dv MAP_RENAME
Assign the referenced private pages to the file descriptor provided.
Unimplemented.
.It Dv MAP_STACK
Allocate a memory segment that can be used either for a process or thread stack.
This currently has no effect, but its use is reserved for architectures
that might require special treatment of that address space.
Unimplemented.
.It Dv MAP_TRYFIXED
Attempt to use the address
.Fa addr
@ -303,6 +345,7 @@ would exceed the offset maximum established in its open file description.
.Xr mincore 2 ,
.Xr mlock 2 ,
.Xr mprotect 2 ,
.Xr mremap 2 ,
.Xr msync 2 ,
.Xr munmap 2 ,
.Xr getpagesize 3 ,