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.\" $FreeBSD: release/9.1.0/share/man/man9/memguard.9 211194 2010-08-11 22:10:37Z mdf $
.\"
.Dd August 2, 2010
.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 take over
.Fn malloc ,
.Fn realloc
and
.Fn free
for a single malloc type.
.Nm
can also guard all allocations larger than
.Dv PAGE_SIZE ,
and can guard a random fraction of all allocations.
There is also a knob to prevent allocations smaller than a specified
size from being guarded, to limit memory waste.
.Sh EXAMPLES
To use
.Nm
for a memory type, either add an entry to
.Pa /boot/loader.conf :
.Bd -literal -offset indent
vm.memguard.desc=<memory_type>
.Ed
.Pp
Or set the
.Va vm.memguard.desc
.Xr sysctl 8
variable at run-time:
.Bd -literal -offset indent
sysctl vm.memguard.desc=<memory_type>
.Ed
.Pp
Where
.Ar memory_type
is a short description of the memory type to monitor.
Only allocations from that
.Ar memory_type
made after
.Va vm.memguard.desc
is set will potentially be guarded.
If
.Va vm.memguard.desc
is modified at run-time then only allocations of the new
.Ar memory_type
will potentially be guarded once the
.Xr sysctl 8
is set.
Existing guarded allocations will still be properly released by
.Xr free 9 .
.Pp
The short description of a
.Xr malloc 9
type is the second argument to
.Xr MALLOC_DEFINE 9 ,
so one has to find it in the kernel source.
.Pp
The
.Va vm.memguard.divisor
boot-time tunable is used to scale how much of the system's physical
memory
.Nm
is allowed to consume.
The default is 10, so up to
.Va cnt.v_page_count Ns /10
pages can be used.
.Nm
will reserve
.Va vm_kmem_max
/
.Va vm.memguard.divisor
bytes of virtual address space, limited by twice the physical memory
size.
The physical limit is reported as
.Va vm.memguard.phys_limit
and the virtual space reserved for
.Nm
is reported as
.Va vm.memguard.mapsize .
.Pp
.Nm
will not do page promotions for any allocation smaller than
.Va vm.memguard.minsize
bytes.
The default is 0, meaning all allocations can potentially be guarded.
.Nm
can guard sufficiently large allocations randomly, with average
frequency of every one in 100000 /
.Va vm.memguard.frequency
allocations.
The default is 0, meaning no allocations are randomly guarded.
.Pp
.Nm
can optionally add unmapped guard pages around each allocation to
detect overflow and underflow, if
.Va vm.memguard.options
has the 1 bit set.
This option is enabled by default.
.Nm
will optionally guard all allocations of
.Dv PAGE_SIZE
or larger if
.Va vm.memguard.options
has the 2 bit set.
This option is off by default.
.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 originally written by
.An Bosko Milekic Aq bmilekic@FreeBSD.org .
This manual page was originally written by
.An Christian Brueffer Aq brueffer@FreeBSD.org .
Additions have been made by
.An Matthew Fleming Aq mdf@FreeBSD.org
to both the implementation and the documentation.
.Sh BUGS
Currently, it is not possible to override UMA
.Xr zone 9
allocations.