Ubuntu Manpage: MemGuard — memory allocator for debugging purposes

Provided by: freebsd-manpages_12.2-1_all

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.

     MemGuard can take over malloc(), realloc() and free() for a single malloc type.
     Alternatively MemGuard can take over uma_zalloc(), uma_zalloc_arg() and uma_free() for a
     single uma(9) zone.  Also MemGuard can 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 can be either a short description of the memory type to monitor, either
name of uma(9) zone. 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 either free(9) or
uma_zfree(9), depending on what kind of allocation was taken over.

To determine short description of a malloc(9) type one can either take it from the first
column of vmstat(8) -m output, or to find it in the kernel source. It is the second
argument to MALLOC_DEFINE(9) macro. To determine name of uma(9) zone one can either take it
from the first column of vmstat(8) -z output, or to find it in the kernel source. It is the
first argument to the uma_zcreate(9) function.

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 vm_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. By default MemGuard does not guard
uma(9) zones that have been initialized with the UMA_ZONE_NOFREE flag set, since it can
produce false positives on them. However, this safety measure can be turned off by setting
bit 3 of the vm.memguard.options tunable.

HISTORY

     MemGuard first appeared in FreeBSD 6.0.

AUTHORS