Document how MAP_STACK will be used. All stacks must be mmap'd with

this attribute.  The kernel does so for main-process stacks at execve() time,
pthread stack functions do so for new stacks, and stacks provided to
sigaltstack() and other user-provided stacks will need to be allocated
in that way.
Not required yet, but paving the way.
Work done with stefan

2 files changed, 20 insertions, 5 deletions

diff --git a/lib/libc/sys/mmap.2 b/lib/libc/sys/mmap.2
index 37c82d786df..4ff05ab6d27 100644
--- a/lib/libc/sys/mmap.2
+++ b/lib/libc/sys/mmap.2
@@ -1,4 +1,4 @@
-.\"	$OpenBSD: mmap.2,v 1.57 2018/01/12 04:36:44 deraadt Exp $
+.\"	$OpenBSD: mmap.2,v 1.58 2018/02/11 04:50:25 deraadt Exp $
 .\"	$NetBSD: mmap.2,v 1.5 1995/06/24 10:48:59 cgd Exp $
 .\"
 .\" Copyright (c) 1991, 1993
@@ -30,7 +30,7 @@
 .\"
 .\"	@(#)mmap.2	8.1 (Berkeley) 6/4/93
 .\"
-.Dd $Mdocdate: January 12 2018 $
+.Dd $Mdocdate: February 11 2018 $
 .Dt MMAP 2
 .Os
 .Sh NAME
@@ -153,6 +153,11 @@ mappings)
 must be multiples of the page size.
 Existing mappings in the address range will be replaced.
 Use of this option is discouraged.
+.It Dv MAP_STACK
+Indicate that the mapping is used as a stack.
+This flag must be used in combination with
+.Dv MAP_ANON and
+.Dv MAP_PRIVATE .
 .El
 .Pp
 Finally, the following flags are also provided for
diff --git a/lib/libc/sys/sigaltstack.2 b/lib/libc/sys/sigaltstack.2
index 01f47dd4ef4..d00cdac48fc 100644
--- a/lib/libc/sys/sigaltstack.2
+++ b/lib/libc/sys/sigaltstack.2
@@ -1,4 +1,4 @@
-.\"	$OpenBSD: sigaltstack.2,v 1.19 2015/05/31 23:54:25 schwarze Exp $
+.\"	$OpenBSD: sigaltstack.2,v 1.20 2018/02/11 04:50:25 deraadt Exp $
 .\"	$NetBSD: sigaltstack.2,v 1.3 1995/02/27 10:41:52 cgd Exp $
 .\"
 .\" Copyright (c) 1983, 1991, 1992, 1993
@@ -30,7 +30,7 @@
 .\"
 .\"     @(#)sigaltstack.2	8.1 (Berkeley) 6/4/93
 .\"
-.Dd $Mdocdate: May 31 2015 $
+.Dd $Mdocdate: February 11 2018 $
 .Dt SIGALTSTACK 2
 .Os
 .Sh NAME
@@ -92,6 +92,15 @@ field will contain the value
 if the thread is currently on a signal stack and
 .Dv SS_DISABLE
 if the signal stack is currently disabled.
+.Pp
+The stack must be allocated using
+.Xr mmap 2
+with
+.Ar MAP_STACK
+to inform the kernel that the memory is being used as a stack.
+Otherwise, the first system call performed while operating on
+that stack will deliver
+.Dv SIGABRT .
 .Sh NOTES
 The value
 .Dv SIGSTKSZ
@@ -99,7 +108,8 @@ is defined to be the number of bytes/chars that would be used to cover
 the usual case when allocating an alternate stack area.
 The following code fragment is typically used to allocate an alternate stack.
 .Bd -literal -offset indent
-if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)
+if ((sigstk.ss_sp = mmap(NULL, SIGSTKSZ, PROT_WRITE | PROT_READ,
+    MAP_PRIVATE | MAP_ANON | MAP_STACK, -1, 0)) == NULL)
 	/* error return */
 sigstk.ss_size = SIGSTKSZ;
 sigstk.ss_flags = 0;