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.

Debian                                           August 2, 2010                                      MEMGUARD(9)