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Revise the description of MAP_STACK. In particular, describe the guard

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in more detail.

Reviewed by:	bcr, kib, markj
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D26908
This commit is contained in:
Alan Cox 2020-10-27 18:08:33 +00:00
parent f9067cf591
commit 76c7af51ab
1 changed files with 23 additions and 19 deletions
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42
lib/libc/sys/mmap.2
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@ -28,7 +28,7 @@
.\" @(#)mmap.2 8.4 (Berkeley) 5/11/95
.\" $FreeBSD$
.\"
.Dd October 21, 2020
.Dd October 27, 2020
.Dt MMAP 2
.Os
.Sh NAME
@ -319,9 +319,30 @@ Modifications are private.
.It Dv MAP_SHARED
Modifications are shared.
.It Dv MAP_STACK
Creates both a mapped region that grows downward on demand and an
adjoining guard that both reserves address space for the mapped region
to grow into and limits the mapped region's growth.
Together, the mapped region and the guard occupy
.Fa len
bytes of the address space.
The guard starts at the returned address, and the mapped region ends at
the returned address plus
.Fa len
bytes.
Upon access to the guard, the mapped region automatically grows in size,
and the guard shrinks by an equal amount.
Essentially, the boundary between the guard and the mapped region moves
downward so that the access falls within the enlarged mapped region.
However, the guard will never shrink to less than the number of pages
specified by the sysctl
.Dv security.bsd.stack_guard_page ,
thereby ensuring that a gap for detecting stack overflow always exists
between the downward growing mapped region and the closest mapped region
beneath it.
.Pp
.Dv MAP_STACK
implies
.Dv MAP_ANON ,
.Dv MAP_ANON
and
.Fa offset
of 0.
@ -334,23 +355,6 @@ must include at least
.Dv PROT_READ
and
.Dv PROT_WRITE .
.Pp
This option creates
a memory region that grows to at most
.Fa len
bytes in size, starting from the stack top and growing down.
The
stack top is the starting address returned by the call, plus
.Fa len
bytes.
The bottom of the stack at maximum growth is the starting
address returned by the call.
.Pp
Stacks created with
.Dv MAP_STACK
automatically grow.
Guards prevent inadvertent use of the regions into which those
stacks can grow without requiring mapping the whole stack in advance.
The size of the guard, in pages, is specified by sysctl
.Dv security.bsd.stack_guard_page .
.El