.\" Copyright (c) 2005 Christian Brueffer
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.Dd February 22, 2005
.Dt MEMGUARD 9
.Os
.Sh NAME
.Nm MemGuard
.Nd "memory allocator for debugging purposes"
.Sh SYNOPSIS
.Cd "options DEBUG_MEMGUARD"
.Sh DESCRIPTION
.Nm
is a simple and small replacement memory allocator designed
to help detect tamper-after-free scenarios.
These problems are more and more common and likely with
multithreaded kernels where race conditions are more prevalent.
.Pp
Currently,
.Nm
can only take over
.Fn malloc ,
.Fn realloc
and
.Fn free
for a particular malloc type.
.Nm
takes over
.Dv M_SUBPROC
allocations by default.
.Sh FILES
.Bl -tag -width ".Pa src/sys/kern/kern_malloc.c" -compact
.It Pa src/sys/kern/kern_malloc.c
File to replace the malloc type in
.El
.Sh EXAMPLES
The following steps are necessary to use
.Nm :
.Bl -enum
.It
Put the
.Dv DEBUG_MEMGUARD
option into your kernel config.
.It
Open
.Pa src/sys/kern/kern_malloc.c
in your favourite editor.
Look for lines containing
.Dq Li "XXX CHANGEME!"
and replace
.Dv M_SUBPROC
with the appropriate malloc type.
This might require additional but small/simple
code modifications
(e.g., if the malloc type is declared out of scope).
.It
Build and install your kernel.
Tune the
.Va vm.memguard_divisor
boot-time tunable, which is used to scale how much of
.Va kmem_map
you want to allot for
.Nm .
The default is 10, so
.Va kmem_size Ns /10
bytes will be used.
The
.Va kmem_size
value can be obtained via the
.Va vm.kmem_size
.Xr sysctl 8
variable.
.El
.Sh SEE ALSO
.Xr sysctl 8 ,
.Xr vmstat 8 ,
.Xr contigmalloc 9 ,
.Xr malloc 9
.Sh HISTORY
.Nm
first appeared in
.Fx 6.0 .
.Sh AUTHORS
.An -nosplit
.Nm
was written by
.An Bosko Milekic Aq bmilekic@FreeBSD.org .
This manual page was written by
.An Christian Brueffer Aq brueffer@FreeBSD.org .
.Sh BUGS
Currently, it is not possible to override UMA
.Xr zone 9
allocations.