# Documentation - hardened_malloc

This documentation contains instructions to use hardened_malloc memory allocator as the system's default memory allocator via dynamic linking as a shared library. These instructions apply to both musl and glibc C libraries on Linux-based systems.

hardened_malloc can also be used per-application and/or per-user, in which case root permissions are not required; this documentation focuses on system-wide usage of hardened_malloc, assumes root privileges, and assumes the compiled library will readable and executable by all users of the system.

This documentation uses Linux Filesystem Hierarchy Standard paths, with the modern `/usr/` merge approach of most Linux distributions. For non-standard configurations, adjust the paths accordingly.

For the complete hardened_malloc documentation, visit its official documentation.

This documentation is also available in portable AsciiDoc format in my documentation source code repository.

## Increase Permitted Amount of Memory Pages

Add `vm.max_map_count = 1048576` to `/etc/sysctl.conf` to accommodate hardened_malloc's large amount of guard pages.

## Clone hardened_malloc Source Code

`$ git clone https://github.com/GrapheneOS/hardened_malloc.git`

## Enter hardened_malloc Local Git Repository

`$ cd hardened_malloc/`

## Compile hardened_malloc

`$ make <arguments>`

`CONFIG_N_ARENA=n` can be adjusted to increase parallel performance at the expense of memory usage, or decrease memory usage at the expense of parallel performance, where `n` is a non-negative integer. Higher values prefer parallel performance, whereas lower values prefer lower memory usage. Note that having too many arenas may cause memory fragmentation and decrease system performance. The number of arenas has no impact on the security properties of hardened_malloc.

Minimum | Maximum | Default
--- | --- | ---
1 | 256 | 4

For extra security, `CONFIG_SEAL_METADATA=true` can be used in order to control whether Memory Protection Keys are used to disable access to all writable allocator state outside of the memory allocator code. It's currently disabled by default due to a significant performance cost for this use case on current-generation hardware. Whether or not this feature is enabled, the metadata is all contained within an isolated memory region with high-entropy random guard regions around it.

For low-memory systems, `VARIANT=light` can be used to compile the light variant of hardened_malloc, which sacrifices some security for much less memory usage. This option still produces a more hardened memory allocator than both the default musl and glibc allocators, despite the security sacrifices over the full variant.

For all compile-time options, see the configuration section of hardened_malloc's official documentation.

## Copy Compiled hardened_malloc Library

`# cp out/libhardened_malloc.so /usr/local/lib/libhardened_malloc.so`

## Set System to Preload hardened_malloc on Boot

musl-based systems: Add `LD_PRELOAD=/usr/local/lib/libhardened_malloc.so` to `/etc/environment`

glibc-based systems: Add `/usr/local/lib/libhardened_malloc.so` to `/etc/ld.so.preload`

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