Project:Hardened musl

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Hardened musl
Description This subproject aims to port the hardened tool chain to musl based systems for a variety of architectures. The project treats musl as an alternative to glibc and uClibc, and not necessarily as "embedded".
Project email
IRC channel #gentoo-hardened (webchat)
Last elected: 2019-08-27
(and inherited member(s))
Parent Project Hardened
Project listing

The Hardened musl subproject has a goal similar to its sister subproject, Hardened uClibc, but with the intention of using musl to replace glibc as the system's "standard C library," or "libc" for short. A system's libc forms an integral part of the toolchain, but unlike the other components, it remains a runtime dependency of nearly every dynamically linked object in the system, or becomes incorporated into statically linked executables. For embedded systems, the size and speed of your libc become important issues which are better addressed by libc's designed with that purpose in mind. uClibc addresses at least the size issue by being very configurable, so any unneeded code can be turned off. Whether a function is required by POSIX standards or not doesn't matter if you are not using it for some targetted application. musl takes a different approach: it is written with static linking in mind, but also with fast dynamic linking capabilities, while remaining close to standards and conscious of security issues. However, unlike uClibc, it is not configurable. How glibc, uClibc and musl compare on the various points of interest is complex and something that will probably be debated forever. The musl team does provide a table of C/POSIX standard library implementations for Linux that you can browse. Since there are different needs for different folks, in Gentoo we are not afraid to target anything and everything: all arches, all libc's, hardened/vanilla userland, hardened/vanilla Linux kernel, and even different kernels.

musl's completeness, including a robust implementation of POSIX threads, means that we can include all of Gentoo's Hardened toolchain goodies without any problems:

  • Stack Smashing Protection (SSP), which requires threads but doesn't work with the old NPTL or LinuxThreads that uClibc provides.
  • Position Independent Execution (PIE).
  • Bind now and relro, linker hardening to protect the global offset table.

These are augmented by the kernel hardening, especially PaX's enhanced address space layout randomization (ASLR).

This subproject aims to treat musl more as a drop in alternative to glibc, and not necessarily as "embedded". This is not at the exclusion of the concerns of embedded systems, but rather to make our userland tarballs as flexible as possible. They can be used as development environments for native compiling on native hardware, starting points to build server or desktop systems, or they can be stripped down to just the essential apps for whatever purpose. The stages are not "embedded" in the sense that they use busybox as their "Swiss Army Knife" of common UNIX utilities. While not excluding this possibility, we aim at making most (all?) of Gentoo's packages both hardened and musl compatible.


The project goals can be best summarized by the following chart:

Arch Subarchs Tool Chain Hardening Status Downloads
amd64 Generic Yes Development Desktop stage3-amd64-musl-{hardened,vanilla}
arm armv7a-hardfp Yes Development stage3-armv7a_hardfp-musl-{hardened,vanilla}
mips mips32r2 No NA Not Yet
mips mipsel3 No Development stage3-mipsel3-musl-vanilla
ppc classic No Development stage3-ppc-musl-vanilla
ppc64 ppc64/ppc64le Yes Development stage3-ppc64-musl-hardened-openrc / stage3-ppc64le-musl-hardened-openrc
x86 i686 No Development stage3-i686-musl-vanilla

Working with musl

Unlike the situation with uClibc, where pretty much every package in the Gentoo portage tree "just builds," musl's adherence to standards means that many packages which deviate from those standards, primarily POSIX, need some patching. Most of this is minor, like the location of header files, but some is more substantial. So we maintain the musl overlay to house those patches, and this overlay must be added to the stage3's to be able to update and maintain them. Here's how:

0) Get your chroot ready as you would on any other stage3. See the Handbook.

1) Set up your favorite GENTOO_MIRRORS and sync-uri and sync the main ebuild repository. Obviously replace with your servers of choice:

root #echo GENTOO_MIRRORS= >> /etc/portage/make.conf
root #sed -i "/^sync-uri/d" /etc/portage/repos.conf/gentoo.conf
root #echo "sync-uri = rsync://" >> /etc/portage/repos.conf/gentoo.conf
root #emerge --sync

2) Set up the nameserver for DNS resolution:

root #echo nameserver >> /etc/resolv.conf

3) Install git to fetch the overlay and eselect-repository to setup the overlay:

root #emerge --ask app-eselect/eselect-repository dev-vcs/git

4) Let's add the overlay.

root #eselect repository enable musl

5) Now we can update! If we tried to update without the overlay, we get a bunch of downgrades to ebuilds that are slightly broken on musl and will not build.

root #emerge -uvNDq @world

6) In the future, update both the portage tree and the overlay before repeating step 5.

root #emerge --sync
root #emerge -uvDU @world


The following people are or have contributed to the project:

Contributor Email Still active?
Felix Janda Yes


To participate in the Hardened musl project join the mailing list at and visit our IRC channel at #gentoo-hardened (webchat) on Libera.Chat.

See also