User Environment – Gentoo Development Guide

User environment variables and make.conf settings get passed on to ebuilds. This can be useful — it's how CFLAGS and friends work, for example — but it can also result in nasty build-breaking variables like LANG and LC_ALL getting through. Currently no sanitisation is performed upon the environment.

Filtering Variables

Certain variables will really really upset certain build systems. A good example is the locale variables (LC_ALL et al), which if set to certain values will cause sed or grep expressions involving the likes of [A-Z] to fail. The easiest thing to do here is to unset or sanitise the offending variables inside pkg_setup.

The simplest way to unset all locale-related variables is:

pkg_setup() {
    # Unset all locale related variables, they can make the
    # build fail.

    eval unset ${!LC_*} LANG
}

Not Filtering Variables

On the other hand, it is extremely important that certain user preferences are honoured as far as possible. A good example is CFLAGS, which must be respected (selective filtering is fine, but outright ignoring is not). Ignoring CFLAGS when compiling can cause serious problems:

Ignoring march/mcpu may force kernel or software emulation for certain opcodes on some architectures. This can be very slow — for example, openssl built for SPARC v7 but run on v9 is around five times slower for RSA operations.
Stripping certain ABI-related flags will break linkage.
Stripping certain ABI-related flags will result in invalid code being produced for certain setups. In extreme cases, we could end up with daft things like big endian code being produced for little endian CPUs.
If a user's march/mcpu/mtune is ignored, and an auto-detected setting is used instead, GRP and stages will break. For example, i686 stages could no longer be produced on a pentium-4, and v8 stages could no longer be produced on an UltraSparc.

Some packages do this by accident. For example, one might see CFLAGS=-Wall in Makefile.am. To fix this, either sed in the user's CFLAGS, or (the better solution) change the variable to AM_CFLAGS, which will automatically be merged with the user's settings. LDFLAGS also should be respected.