.\" Copyright (c) 2005 Christian Brueffer
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.Dd January 31, 2006
.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.
.Sh EXAMPLES
To use
.Nm
for memory type compiled into the kernel, one has to add the
following line to the
.Pa /boot/loader.conf :
.Bd -literal -offset indent
vm.memguard.desc=<memory_type>
.Ed
.Pp
Where
.Ar memory_type
is a short description of memory type to monitor.
The short description of memory type is the second argument to
.Xr MALLOC_DEFINE 9 ,
so one has to find it in the kernel source.
.Pp
To use
.Nm
for memory type defined in a kernel module, one has to set
.Va vm.memguard.desc
.Xr sysctl 8
variable before loading the module:
.Bd -literal -offset indent
sysctl vm.memguard.desc=<memory_type>
.Ed
.Pp
The
.Va vm.memguard.divisor
boot-time tunable is used to scale how much of
.Va kmem_map
one wants to allocate 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.
.Sh SEE ALSO
.Xr sysctl 8 ,
.Xr vmstat 8 ,
.Xr contigmalloc 9 ,
.Xr malloc 9 ,
.Xr redzone 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.