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NAME

     MemGuard -- memory allocator for debugging purposes

SYNOPSIS

     options DEBUG_MEMGUARD

DESCRIPTION

     MemGuard 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.

     Currently, MemGuard can take over malloc(), realloc() and free() for a
     single malloc type.  MemGuard can also guard all allocations larger than
     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.

EXAMPLES

     To use MemGuard for a memory type, either add an entry to
     /boot/loader.conf:

           vm.memguard.desc=<memory_type>

     Or set the vm.memguard.desc sysctl(8) variable at run-time:

           sysctl vm.memguard.desc=<memory_type>

     Where memory_type is a short description of the memory type to monitor.
     Only allocations from that memory_type made after vm.memguard.desc is set
     will potentially be guarded.  If vm.memguard.desc is modified at run-time
     then only allocations of the new memory_type will potentially be guarded
     once the sysctl(8) is set.  Existing guarded allocations will still be
     properly released by free(9).

     The short description of a malloc(9) type is the second argument to
     MALLOC_DEFINE(9), so one has to find it in the kernel source.

     The vm.memguard.divisor boot-time tunable is used to scale how much of
     the system's physical memory MemGuard is allowed to consume.  The default
     is 10, so up to cnt.v_page_count/10 pages can be used.  MemGuard will
     reserve vm_kmem_max / vm.memguard.divisor bytes of virtual address space,
     limited by twice the physical memory size.  The physical limit is
     reported as vm.memguard.phys_limit and the virtual space reserved for
     MemGuard is reported as vm.memguard.mapsize.

     MemGuard will not do page promotions for any allocation smaller than
     vm.memguard.minsize bytes.  The default is 0, meaning all allocations can
     potentially be guarded.  MemGuard can guard sufficiently large
     allocations randomly, with average frequency of every one in 100000 /
     vm.memguard.frequency allocations.  The default is 0, meaning no
     allocations are randomly guarded.

     MemGuard can optionally add unmapped guard pages around each allocation
     to detect overflow and underflow, if vm.memguard.options has the 1 bit
     set.  This option is enabled by default.  MemGuard will optionally guard
     all allocations of PAGE_SIZE or larger if vm.memguard.options has the 2
     bit set.  This option is off by default.

SEE ALSO

     sysctl(8), vmstat(8), contigmalloc(9), malloc(9), redzone(9)

HISTORY

     MemGuard first appeared in FreeBSD 6.0.

AUTHORS

     MemGuard was originally written by Bosko Milekic <bmilekic@FreeBSD.org>.
     This manual page was originally written by Christian Brueffer
     <brueffer@FreeBSD.org>.  Additions have been made by Matthew Fleming
     <mdf@FreeBSD.org> to both the implementation and the documentation.

BUGS

     Currently, it is not possible to override UMA zone(9) allocations.