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John Ericson 0578dda8e1 doc: Describe CC Wrapper in more detail
The main motivation for this is to have something to google for LD=$CC.

Eventually, this should probably be moved to another section, but we
can deal with that later.
2017-08-20 15:49:50 -04:00

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<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xml:id="chap-stdenv">
<title>The Standard Environment</title>
<para>The standard build environment in the Nix Packages collection
provides an environment for building Unix packages that does a lot of
common build tasks automatically. In fact, for Unix packages that use
the standard <literal>./configure; make; make install</literal> build
interface, you dont need to write a build script at all; the standard
environment does everything automatically. If
<literal>stdenv</literal> doesnt do what you need automatically, you
can easily customise or override the various build phases.</para>
<section xml:id="sec-using-stdenv"><title>Using
<literal>stdenv</literal></title>
<para>To build a package with the standard environment, you use the
function <varname>stdenv.mkDerivation</varname>, instead of the
primitive built-in function <varname>derivation</varname>, e.g.
<programlisting>
stdenv.mkDerivation {
name = "libfoo-1.2.3";
src = fetchurl {
url = http://example.org/libfoo-1.2.3.tar.bz2;
sha256 = "0x2g1jqygyr5wiwg4ma1nd7w4ydpy82z9gkcv8vh2v8dn3y58v5m";
};
}</programlisting>
(<varname>stdenv</varname> needs to be in scope, so if you write this
in a separate Nix expression from
<filename>pkgs/all-packages.nix</filename>, you need to pass it as a
function argument.) Specifying a <varname>name</varname> and a
<varname>src</varname> is the absolute minimum you need to do. Many
packages have dependencies that are not provided in the standard
environment. Its usually sufficient to specify those dependencies in
the <varname>buildInputs</varname> attribute:
<programlisting>
stdenv.mkDerivation {
name = "libfoo-1.2.3";
...
buildInputs = [libbar perl ncurses];
}</programlisting>
This attribute ensures that the <filename>bin</filename>
subdirectories of these packages appear in the <envar>PATH</envar>
environment variable during the build, that their
<filename>include</filename> subdirectories are searched by the C
compiler, and so on. (See <xref linkend="ssec-setup-hooks"/> for
details.)</para>
<para>Often it is necessary to override or modify some aspect of the
build. To make this easier, the standard environment breaks the
package build into a number of <emphasis>phases</emphasis>, all of
which can be overridden or modified individually: unpacking the
sources, applying patches, configuring, building, and installing.
(There are some others; see <xref linkend="sec-stdenv-phases"/>.)
For instance, a package that doesnt supply a makefile but instead has
to be compiled “manually” could be handled like this:
<programlisting>
stdenv.mkDerivation {
name = "fnord-4.5";
...
buildPhase = ''
gcc foo.c -o foo
'';
installPhase = ''
mkdir -p $out/bin
cp foo $out/bin
'';
}</programlisting>
(Note the use of <literal>''</literal>-style string literals, which
are very convenient for large multi-line script fragments because they
dont need escaping of <literal>"</literal> and <literal>\</literal>,
and because indentation is intelligently removed.)</para>
<para>There are many other attributes to customise the build. These
are listed in <xref linkend="ssec-stdenv-attributes"/>.</para>
<para>While the standard environment provides a generic builder, you
can still supply your own build script:
<programlisting>
stdenv.mkDerivation {
name = "libfoo-1.2.3";
...
builder = ./builder.sh;
}</programlisting>
where the builder can do anything it wants, but typically starts with
<programlisting>
source $stdenv/setup
</programlisting>
to let <literal>stdenv</literal> set up the environment (e.g., process
the <varname>buildInputs</varname>). If you want, you can still use
<literal>stdenv</literal>s generic builder:
<programlisting>
source $stdenv/setup
buildPhase() {
echo "... this is my custom build phase ..."
gcc foo.c -o foo
}
installPhase() {
mkdir -p $out/bin
cp foo $out/bin
}
genericBuild
</programlisting>
</para>
</section>
<section xml:id="sec-tools-of-stdenv"><title>Tools provided by
<literal>stdenv</literal></title>
<para>The standard environment provides the following packages:
<itemizedlist>
<listitem><para>The GNU C Compiler, configured with C and C++
support.</para></listitem>
<listitem><para>GNU coreutils (contains a few dozen standard Unix
commands).</para></listitem>
<listitem><para>GNU findutils (contains
<command>find</command>).</para></listitem>
<listitem><para>GNU diffutils (contains <command>diff</command>,
<command>cmp</command>).</para></listitem>
<listitem><para>GNU <command>sed</command>.</para></listitem>
<listitem><para>GNU <command>grep</command>.</para></listitem>
<listitem><para>GNU <command>awk</command>.</para></listitem>
<listitem><para>GNU <command>tar</command>.</para></listitem>
<listitem><para><command>gzip</command>, <command>bzip2</command>
and <command>xz</command>.</para></listitem>
<listitem><para>GNU Make. It has been patched to provide
<quote>nested</quote> output that can be fed into the
<command>nix-log2xml</command> command and
<command>log2html</command> stylesheet to create a structured,
readable output of the build steps performed by
Make.</para></listitem>
<listitem><para>Bash. This is the shell used for all builders in
the Nix Packages collection. Not using <command>/bin/sh</command>
removes a large source of portability problems.</para></listitem>
<listitem><para>The <command>patch</command>
command.</para></listitem>
</itemizedlist>
</para>
<para>On Linux, <literal>stdenv</literal> also includes the
<command>patchelf</command> utility.</para>
</section>
<section xml:id="ssec-stdenv-attributes"><title>Attributes</title>
<variablelist>
<title>Variables affecting <literal>stdenv</literal>
initialisation</title>
<varlistentry>
<term><varname>NIX_DEBUG</varname></term>
<listitem><para>If set, <literal>stdenv</literal> will print some
debug information during the build. In particular, the
<command>gcc</command> and <command>ld</command> wrapper scripts
will print out the complete command line passed to the wrapped
tools.</para></listitem>
</varlistentry>
</variablelist>
<variablelist>
<title>Variables specifying dependencies</title>
<varlistentry>
<term><varname>nativeBuildInputs</varname></term>
<listitem><para>
A list of dependencies used by the new derivation at <emphasis>build</emphasis>-time.
I.e. these dependencies should not make it into the package's runtime-closure, though this is currently not checked.
For each dependency <replaceable>dir</replaceable>, the directory <filename><replaceable>dir</replaceable>/bin</filename>, if it exists, is added to the <envar>PATH</envar> environment variable.
Other environment variables are also set up via a pluggable mechanism.
For instance, if <varname>buildInputs</varname> contains Perl, then the <filename>lib/site_perl</filename> subdirectory of each input is added to the <envar>PERL5LIB</envar> environment variable.
See <xref linkend="ssec-setup-hooks"/> for details.
</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>buildInputs</varname></term>
<listitem><para>
A list of dependencies used by the new derivation at <emphasis>run</emphasis>-time.
Currently, the build-time environment is modified in the exact same way as with <varname>nativeBuildInputs</varname>.
This is problematic in that when cross-compiling, foreign executables can clobber native ones on the <envar>PATH</envar>.
Even more confusing is static-linking.
A statically-linked library should be listed here because ultimately that generated machine code will be used at run-time, even though a derivation containing the object files or static archives will only be used at build-time.
A less confusing solution to this would be nice.
</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>propagatedNativeBuildInputs</varname></term>
<listitem><para>
Like <varname>nativeBuildInputs</varname>, but these dependencies are <emphasis>propagated</emphasis>:
that is, the dependencies listed here are added to the <varname>nativeBuildInputs</varname> of any package that uses <emphasis>this</emphasis> package as a dependency.
So if package Y has <literal>propagatedBuildInputs = [X]</literal>, and package Z has <literal>buildInputs = [Y]</literal>, then package X will appear in Zs build environment automatically.
</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>propagatedBuildInputs</varname></term>
<listitem><para>
Like <varname>buildInputs</varname>, but propagated just like <varname>propagatedNativeBuildInputs</varname>.
This inherits <varname>buildInputs</varname>'s flaws of clobbering native executables when cross-compiling and being confusing for static linking.
</para></listitem>
</varlistentry>
</variablelist>
<variablelist>
<title>Variables affecting build properties</title>
<varlistentry>
<term><varname>enableParallelBuilding</varname></term>
<listitem><para>If set, <literal>stdenv</literal> will pass specific
flags to <literal>make</literal> and other build tools to enable
parallel building with up to <literal>build-cores</literal>
workers.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>preferLocalBuild</varname></term>
<listitem><para>If set, specifies that the package is so lightweight
in terms of build operations (e.g. write a text file from a Nix string
to the store) that there's no need to look for it in binary caches --
it's faster to just build it locally. It also tells Hydra and other
facilities that this package doesn't need to be exported in binary
caches (noone would use it, after all).</para></listitem>
</varlistentry>
</variablelist>
<variablelist>
<title>Special variables</title>
<varlistentry>
<term><varname>passthru</varname></term>
<listitem><para>This is an attribute set which can be filled with arbitrary
values. For example:
<programlisting>
passthru = {
foo = "bar";
baz = {
value1 = 4;
value2 = 5;
};
}
</programlisting>
</para>
<para>Values inside it are not passed to the builder, so you can change
them without triggering a rebuild. However, they can be accessed outside of a
derivation directly, as if they were set inside a derivation itself, e.g.
<literal>hello.baz.value1</literal>. We don't specify any usage or
schema of <literal>passthru</literal> - it is meant for values that would be
useful outside the derivation in other parts of a Nix expression (e.g. in other
derivations). An example would be to convey some specific dependency of your
derivation which contains a program with plugins support. Later, others who
make derivations with plugins can use passed-through dependency to ensure that
their plugin would be binary-compatible with built program.</para></listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="sec-stdenv-phases"><title>Phases</title>
<para>The generic builder has a number of <emphasis>phases</emphasis>.
Package builds are split into phases to make it easier to override
specific parts of the build (e.g., unpacking the sources or installing
the binaries). Furthermore, it allows a nicer presentation of build
logs in the Nix build farm.</para>
<para>Each phase can be overridden in its entirety either by setting
the environment variable
<varname><replaceable>name</replaceable>Phase</varname> to a string
containing some shell commands to be executed, or by redefining the
shell function
<varname><replaceable>name</replaceable>Phase</varname>. The former
is convenient to override a phase from the derivation, while the
latter is convenient from a build script.
However, typically one only wants to <emphasis>add</emphasis> some
commands to a phase, e.g. by defining <literal>postInstall</literal>
or <literal>preFixup</literal>, as skipping some of the default actions
may have unexpected consequences.
</para>
<section xml:id="ssec-controlling-phases"><title>Controlling
phases</title>
<para>There are a number of variables that control what phases are
executed and in what order:
<variablelist>
<title>Variables affecting phase control</title>
<varlistentry>
<term><varname>phases</varname></term>
<listitem>
<para>Specifies the phases. You can change the order in which
phases are executed, or add new phases, by setting this
variable. If its not set, the default value is used, which is
<literal>$prePhases unpackPhase patchPhase $preConfigurePhases
configurePhase $preBuildPhases buildPhase checkPhase
$preInstallPhases installPhase fixupPhase $preDistPhases
distPhase $postPhases</literal>.
</para>
<para>Usually, if you just want to add a few phases, its more
convenient to set one of the variables below (such as
<varname>preInstallPhases</varname>), as you then dont specify
all the normal phases.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>prePhases</varname></term>
<listitem>
<para>Additional phases executed before any of the default phases.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>preConfigurePhases</varname></term>
<listitem>
<para>Additional phases executed just before the configure phase.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>preBuildPhases</varname></term>
<listitem>
<para>Additional phases executed just before the build phase.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>preInstallPhases</varname></term>
<listitem>
<para>Additional phases executed just before the install phase.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>preFixupPhases</varname></term>
<listitem>
<para>Additional phases executed just before the fixup phase.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>preDistPhases</varname></term>
<listitem>
<para>Additional phases executed just before the distribution phase.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>postPhases</varname></term>
<listitem>
<para>Additional phases executed after any of the default
phases.</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</section>
<section xml:id="ssec-unpack-phase"><title>The unpack phase</title>
<para>The unpack phase is responsible for unpacking the source code of
the package. The default implementation of
<function>unpackPhase</function> unpacks the source files listed in
the <envar>src</envar> environment variable to the current directory.
It supports the following files by default:
<variablelist>
<varlistentry>
<term>Tar files</term>
<listitem><para>These can optionally be compressed using
<command>gzip</command> (<filename>.tar.gz</filename>,
<filename>.tgz</filename> or <filename>.tar.Z</filename>),
<command>bzip2</command> (<filename>.tar.bz2</filename> or
<filename>.tbz2</filename>) or <command>xz</command>
(<filename>.tar.xz</filename> or
<filename>.tar.lzma</filename>).</para></listitem>
</varlistentry>
<varlistentry>
<term>Zip files</term>
<listitem><para>Zip files are unpacked using
<command>unzip</command>. However, <command>unzip</command> is
not in the standard environment, so you should add it to
<varname>buildInputs</varname> yourself.</para></listitem>
</varlistentry>
<varlistentry>
<term>Directories in the Nix store</term>
<listitem><para>These are simply copied to the current directory.
The hash part of the file name is stripped,
e.g. <filename>/nix/store/1wydxgby13cz...-my-sources</filename>
would be copied to
<filename>my-sources</filename>.</para></listitem>
</varlistentry>
</variablelist>
Additional file types can be supported by setting the
<varname>unpackCmd</varname> variable (see below).</para>
<para></para>
<variablelist>
<title>Variables controlling the unpack phase</title>
<varlistentry>
<term><varname>srcs</varname> / <varname>src</varname></term>
<listitem><para>The list of source files or directories to be
unpacked or copied. One of these must be set.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>sourceRoot</varname></term>
<listitem><para>After running <function>unpackPhase</function>,
the generic builder changes the current directory to the directory
created by unpacking the sources. If there are multiple source
directories, you should set <varname>sourceRoot</varname> to the
name of the intended directory.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>setSourceRoot</varname></term>
<listitem><para>Alternatively to setting
<varname>sourceRoot</varname>, you can set
<varname>setSourceRoot</varname> to a shell command to be
evaluated by the unpack phase after the sources have been
unpacked. This command must set
<varname>sourceRoot</varname>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>preUnpack</varname></term>
<listitem><para>Hook executed at the start of the unpack
phase.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>postUnpack</varname></term>
<listitem><para>Hook executed at the end of the unpack
phase.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>dontMakeSourcesWritable</varname></term>
<listitem><para>If set to <literal>1</literal>, the unpacked
sources are <emphasis>not</emphasis> made
writable. By default, they are made writable to prevent problems
with read-only sources. For example, copied store directories
would be read-only without this.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>unpackCmd</varname></term>
<listitem><para>The unpack phase evaluates the string
<literal>$unpackCmd</literal> for any unrecognised file. The path
to the current source file is contained in the
<varname>curSrc</varname> variable.</para></listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="ssec-patch-phase"><title>The patch phase</title>
<para>The patch phase applies the list of patches defined in the
<varname>patches</varname> variable.</para>
<variablelist>
<title>Variables controlling the patch phase</title>
<varlistentry>
<term><varname>patches</varname></term>
<listitem><para>The list of patches. They must be in the format
accepted by the <command>patch</command> command, and may
optionally be compressed using <command>gzip</command>
(<filename>.gz</filename>), <command>bzip2</command>
(<filename>.bz2</filename>) or <command>xz</command>
(<filename>.xz</filename>).</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>patchFlags</varname></term>
<listitem><para>Flags to be passed to <command>patch</command>.
If not set, the argument <option>-p1</option> is used, which
causes the leading directory component to be stripped from the
file names in each patch.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>prePatch</varname></term>
<listitem><para>Hook executed at the start of the patch
phase.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>postPatch</varname></term>
<listitem><para>Hook executed at the end of the patch
phase.</para></listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="ssec-configure-phase"><title>The configure phase</title>
<para>The configure phase prepares the source tree for building. The
default <function>configurePhase</function> runs
<filename>./configure</filename> (typically an Autoconf-generated
script) if it exists.</para>
<variablelist>
<title>Variables controlling the configure phase</title>
<varlistentry>
<term><varname>configureScript</varname></term>
<listitem><para>The name of the configure script. It defaults to
<filename>./configure</filename> if it exists; otherwise, the
configure phase is skipped. This can actually be a command (like
<literal>perl ./Configure.pl</literal>).</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>configureFlags</varname></term>
<listitem><para>A list of strings passed as additional arguments to the
configure script.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>configureFlagsArray</varname></term>
<listitem><para>A shell array containing additional arguments
passed to the configure script. You must use this instead of
<varname>configureFlags</varname> if the arguments contain
spaces.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>dontAddPrefix</varname></term>
<listitem><para>By default, the flag
<literal>--prefix=$prefix</literal> is added to the configure
flags. If this is undesirable, set this variable to
true.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>prefix</varname></term>
<listitem><para>The prefix under which the package must be
installed, passed via the <option>--prefix</option> option to the
configure script. It defaults to
<option>$out</option>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>dontAddDisableDepTrack</varname></term>
<listitem><para>By default, the flag
<literal>--disable-dependency-tracking</literal> is added to the
configure flags to speed up Automake-based builds. If this is
undesirable, set this variable to true.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>dontFixLibtool</varname></term>
<listitem><para>By default, the configure phase applies some
special hackery to all files called <filename>ltmain.sh</filename>
before running the configure script in order to improve the purity
of Libtool-based packages<footnote><para>It clears the
<varname>sys_lib_<replaceable>*</replaceable>search_path</varname>
variables in the Libtool script to prevent Libtool from using
libraries in <filename>/usr/lib</filename> and
such.</para></footnote>. If this is undesirable, set this
variable to true.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>dontDisableStatic</varname></term>
<listitem><para>By default, when the configure script has
<option>--enable-static</option>, the option
<option>--disable-static</option> is added to the configure flags.</para>
<para>If this is undesirable, set this variable to
true.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>configurePlatforms</varname></term>
<listitem><para>
By default, when cross compiling, the configure script has <option>--build=...</option> and <option>--host=...</option> passed.
Packages can instead pass <literal>[ "build" "host" "target" ]</literal> or a subset to control exactly which platform flags are passed.
Compilers and other tools should use this to also pass the target platform, for example.
Note eventually these will be passed when in native builds too, to improve determinism: build-time guessing, as is done today, is a risk of impurity.
</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>preConfigure</varname></term>
<listitem><para>Hook executed at the start of the configure
phase.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>postConfigure</varname></term>
<listitem><para>Hook executed at the end of the configure
phase.</para></listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="build-phase"><title>The build phase</title>
<para>The build phase is responsible for actually building the package
(e.g. compiling it). The default <function>buildPhase</function>
simply calls <command>make</command> if a file named
<filename>Makefile</filename>, <filename>makefile</filename> or
<filename>GNUmakefile</filename> exists in the current directory (or
the <varname>makefile</varname> is explicitly set); otherwise it does
nothing.</para>
<variablelist>
<title>Variables controlling the build phase</title>
<varlistentry>
<term><varname>dontBuild</varname></term>
<listitem><para>Set to true to skip the build phase.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>makefile</varname></term>
<listitem><para>The file name of the Makefile.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>makeFlags</varname></term>
<listitem><para>A list of strings passed as additional flags to
<command>make</command>. These flags are also used by the default
install and check phase. For setting make flags specific to the
build phase, use <varname>buildFlags</varname> (see
below).</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>makeFlagsArray</varname></term>
<listitem><para>A shell array containing additional arguments
passed to <command>make</command>. You must use this instead of
<varname>makeFlags</varname> if the arguments contain
spaces, e.g.
<programlisting>
makeFlagsArray=(CFLAGS="-O0 -g" LDFLAGS="-lfoo -lbar")
</programlisting>
Note that shell arrays cannot be passed through environment
variables, so you cannot set <varname>makeFlagsArray</varname> in
a derivation attribute (because those are passed through
environment variables): you have to define them in shell
code.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>buildFlags</varname> / <varname>buildFlagsArray</varname></term>
<listitem><para>A list of strings passed as additional flags to
<command>make</command>. Like <varname>makeFlags</varname> and
<varname>makeFlagsArray</varname>, but only used by the build
phase.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>preBuild</varname></term>
<listitem><para>Hook executed at the start of the build
phase.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>postBuild</varname></term>
<listitem><para>Hook executed at the end of the build
phase.</para></listitem>
</varlistentry>
</variablelist>
<para>
You can set flags for <command>make</command> through the
<varname>makeFlags</varname> variable.</para>
<para>Before and after running <command>make</command>, the hooks
<varname>preBuild</varname> and <varname>postBuild</varname> are
called, respectively.</para>
</section>
<section xml:id="ssec-check-phase"><title>The check phase</title>
<para>The check phase checks whether the package was built correctly
by running its test suite. The default
<function>checkPhase</function> calls <command>make check</command>,
but only if the <varname>doCheck</varname> variable is enabled.</para>
<variablelist>
<title>Variables controlling the check phase</title>
<varlistentry>
<term><varname>doCheck</varname></term>
<listitem><para>If set to a non-empty string, the check phase is
executed, otherwise it is skipped (default). Thus you should set
<programlisting>
doCheck = true;</programlisting>
in the derivation to enable checks.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>makeFlags</varname> /
<varname>makeFlagsArray</varname> /
<varname>makefile</varname></term>
<listitem><para>See the build phase for details.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>checkTarget</varname></term>
<listitem><para>The make target that runs the tests. Defaults to
<literal>check</literal>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>checkFlags</varname> / <varname>checkFlagsArray</varname></term>
<listitem><para>A list of strings passed as additional flags to
<command>make</command>. Like <varname>makeFlags</varname> and
<varname>makeFlagsArray</varname>, but only used by the check
phase.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>preCheck</varname></term>
<listitem><para>Hook executed at the start of the check
phase.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>postCheck</varname></term>
<listitem><para>Hook executed at the end of the check
phase.</para></listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="ssec-install-phase"><title>The install phase</title>
<para>The install phase is responsible for installing the package in
the Nix store under <envar>out</envar>. The default
<function>installPhase</function> creates the directory
<literal>$out</literal> and calls <command>make
install</command>.</para>
<variablelist>
<title>Variables controlling the install phase</title>
<varlistentry>
<term><varname>makeFlags</varname> /
<varname>makeFlagsArray</varname> /
<varname>makefile</varname></term>
<listitem><para>See the build phase for details.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>installTargets</varname></term>
<listitem><para>The make targets that perform the installation.
Defaults to <literal>install</literal>. Example:
<programlisting>
installTargets = "install-bin install-doc";</programlisting>
</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>installFlags</varname> / <varname>installFlagsArray</varname></term>
<listitem><para>A list of strings passed as additional flags to
<command>make</command>. Like <varname>makeFlags</varname> and
<varname>makeFlagsArray</varname>, but only used by the install
phase.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>preInstall</varname></term>
<listitem><para>Hook executed at the start of the install
phase.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>postInstall</varname></term>
<listitem><para>Hook executed at the end of the install
phase.</para></listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="ssec-fixup-phase"><title>The fixup phase</title>
<para>The fixup phase performs some (Nix-specific) post-processing
actions on the files installed under <filename>$out</filename> by the
install phase. The default <function>fixupPhase</function> does the
following:
<itemizedlist>
<listitem><para>It moves the <filename>man/</filename>,
<filename>doc/</filename> and <filename>info/</filename>
subdirectories of <envar>$out</envar> to
<filename>share/</filename>.</para></listitem>
<listitem><para>It strips libraries and executables of debug
information.</para></listitem>
<listitem><para>On Linux, it applies the <command>patchelf</command>
command to ELF executables and libraries to remove unused
directories from the <literal>RPATH</literal> in order to prevent
unnecessary runtime dependencies.</para></listitem>
<listitem><para>It rewrites the interpreter paths of shell scripts
to paths found in <envar>PATH</envar>. E.g.,
<filename>/usr/bin/perl</filename> will be rewritten to
<filename>/nix/store/<replaceable>some-perl</replaceable>/bin/perl</filename>
found in <envar>PATH</envar>.</para></listitem>
</itemizedlist>
</para>
<variablelist>
<title>Variables controlling the fixup phase</title>
<varlistentry>
<term><varname>dontStrip</varname></term>
<listitem><para>If set, libraries and executables are not
stripped. By default, they are.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>dontMoveSbin</varname></term>
<listitem><para>If set, files in <filename>$out/sbin</filename> are not moved
to <filename>$out/bin</filename>. By default, they are.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>stripAllList</varname></term>
<listitem><para>List of directories to search for libraries and
executables from which <emphasis>all</emphasis> symbols should be
stripped. By default, its empty. Stripping all symbols is
risky, since it may remove not just debug symbols but also ELF
information necessary for normal execution.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>stripAllFlags</varname></term>
<listitem><para>Flags passed to the <command>strip</command>
command applied to the files in the directories listed in
<varname>stripAllList</varname>. Defaults to <option>-s</option>
(i.e. <option>--strip-all</option>).</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>stripDebugList</varname></term>
<listitem><para>List of directories to search for libraries and
executables from which only debugging-related symbols should be
stripped. It defaults to <literal>lib bin
sbin</literal>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>stripDebugFlags</varname></term>
<listitem><para>Flags passed to the <command>strip</command>
command applied to the files in the directories listed in
<varname>stripDebugList</varname>. Defaults to
<option>-S</option>
(i.e. <option>--strip-debug</option>).</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>dontPatchELF</varname></term>
<listitem><para>If set, the <command>patchelf</command> command is
not used to remove unnecessary <literal>RPATH</literal> entries.
Only applies to Linux.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>dontPatchShebangs</varname></term>
<listitem><para>If set, scripts starting with
<literal>#!</literal> do not have their interpreter paths
rewritten to paths in the Nix store.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>forceShare</varname></term>
<listitem><para>The list of directories that must be moved from
<filename>$out</filename> to <filename>$out/share</filename>.
Defaults to <literal>man doc info</literal>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>setupHook</varname></term>
<listitem><para>A package can export a <link
linkend="ssec-setup-hooks">setup hook</link> by setting this
variable. The setup hook, if defined, is copied to
<filename>$out/nix-support/setup-hook</filename>. Environment
variables are then substituted in it using <function
linkend="fun-substituteAll">substituteAll</function>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>preFixup</varname></term>
<listitem><para>Hook executed at the start of the fixup
phase.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>postFixup</varname></term>
<listitem><para>Hook executed at the end of the fixup
phase.</para></listitem>
</varlistentry>
<varlistentry xml:id="stdenv-separateDebugInfo">
<term><varname>separateDebugInfo</varname></term>
<listitem><para>If set to <literal>true</literal>, the standard
environment will enable debug information in C/C++ builds. After
installation, the debug information will be separated from the
executables and stored in the output named
<literal>debug</literal>. (This output is enabled automatically;
you dont need to set the <varname>outputs</varname> attribute
explicitly.) To be precise, the debug information is stored in
<filename><replaceable>debug</replaceable>/lib/debug/.build-id/<replaceable>XX</replaceable>/<replaceable>YYYY…</replaceable></filename>,
where <replaceable>XXYYYY…</replaceable> is the <replaceable>build
ID</replaceable> of the binary — a SHA-1 hash of the contents of
the binary. Debuggers like GDB use the build ID to look up the
separated debug information.</para>
<para>For example, with GDB, you can add
<programlisting>
set debug-file-directory ~/.nix-profile/lib/debug
</programlisting>
to <filename>~/.gdbinit</filename>. GDB will then be able to find
debug information installed via <literal>nix-env
-i</literal>.</para>
</listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="ssec-installCheck-phase"><title>The installCheck phase</title>
<para>The installCheck phase checks whether the package was installed
correctly by running its test suite against the installed directories.
The default <function>installCheck</function> calls <command>make
installcheck</command>.</para>
<variablelist>
<title>Variables controlling the installCheck phase</title>
<varlistentry>
<term><varname>doInstallCheck</varname></term>
<listitem><para>If set to a non-empty string, the installCheck phase is
executed, otherwise it is skipped (default). Thus you should set
<programlisting>doInstallCheck = true;</programlisting>
in the derivation to enable install checks.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>preInstallCheck</varname></term>
<listitem><para>Hook executed at the start of the installCheck
phase.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>postInstallCheck</varname></term>
<listitem><para>Hook executed at the end of the installCheck
phase.</para></listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="ssec-distribution-phase"><title>The distribution
phase</title>
<para>The distribution phase is intended to produce a source
distribution of the package. The default
<function>distPhase</function> first calls <command>make
dist</command>, then it copies the resulting source tarballs to
<filename>$out/tarballs/</filename>. This phase is only executed if
the attribute <varname>doDist</varname> is set.</para>
<variablelist>
<title>Variables controlling the distribution phase</title>
<varlistentry>
<term><varname>distTarget</varname></term>
<listitem><para>The make target that produces the distribution.
Defaults to <literal>dist</literal>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>distFlags</varname> / <varname>distFlagsArray</varname></term>
<listitem><para>Additional flags passed to
<command>make</command>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>tarballs</varname></term>
<listitem><para>The names of the source distribution files to be
copied to <filename>$out/tarballs/</filename>. It can contain
shell wildcards. The default is
<filename>*.tar.gz</filename>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>dontCopyDist</varname></term>
<listitem><para>If set, no files are copied to
<filename>$out/tarballs/</filename>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>preDist</varname></term>
<listitem><para>Hook executed at the start of the distribution
phase.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>postDist</varname></term>
<listitem><para>Hook executed at the end of the distribution
phase.</para></listitem>
</varlistentry>
</variablelist>
</section>
</section>
<section xml:id="ssec-stdenv-functions"><title>Shell functions</title>
<para>The standard environment provides a number of useful
functions.</para>
<variablelist>
<varlistentry xml:id='fun-makeWrapper'>
<term><function>makeWrapper</function>
<replaceable>executable</replaceable>
<replaceable>wrapperfile</replaceable>
<replaceable>args</replaceable></term>
<listitem><para>Constructs a wrapper for a program with various
possible arguments. For example:
<programlisting>
# adds `FOOBAR=baz` to `$out/bin/foo`s environment
makeWrapper $out/bin/foo $wrapperfile --set FOOBAR baz
# prefixes the binary paths of `hello` and `git`
# Be advised that paths often should be patched in directly
# (via string replacements or in `configurePhase`).
makeWrapper $out/bin/foo $wrapperfile --prefix PATH : ${lib.makeBinPath [ hello git ]}
</programlisting>
Theres many more kinds of arguments, they are documented in
<literal>nixpkgs/pkgs/build-support/setup-hooks/make-wrapper.sh</literal>.</para>
<para><literal>wrapProgram</literal> is a convenience function you probably
want to use most of the time.</para>
</listitem>
</varlistentry>
<varlistentry xml:id='fun-substitute'>
<term><function>substitute</function>
<replaceable>infile</replaceable>
<replaceable>outfile</replaceable>
<replaceable>subs</replaceable></term>
<listitem>
<para>Performs string substitution on the contents of
<replaceable>infile</replaceable>, writing the result to
<replaceable>outfile</replaceable>. The substitutions in
<replaceable>subs</replaceable> are of the following form:
<variablelist>
<varlistentry>
<term><option>--replace</option>
<replaceable>s1</replaceable>
<replaceable>s2</replaceable></term>
<listitem><para>Replace every occurrence of the string
<replaceable>s1</replaceable> by
<replaceable>s2</replaceable>.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>--subst-var</option>
<replaceable>varName</replaceable></term>
<listitem><para>Replace every occurrence of
<literal>@<replaceable>varName</replaceable>@</literal> by
the contents of the environment variable
<replaceable>varName</replaceable>. This is useful for
generating files from templates, using
<literal>@<replaceable>...</replaceable>@</literal> in the
template as placeholders.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>--subst-var-by</option>
<replaceable>varName</replaceable>
<replaceable>s</replaceable></term>
<listitem><para>Replace every occurrence of
<literal>@<replaceable>varName</replaceable>@</literal> by
the string <replaceable>s</replaceable>.</para></listitem>
</varlistentry>
</variablelist>
</para>
<para>Example:
<programlisting>
substitute ./foo.in ./foo.out \
--replace /usr/bin/bar $bar/bin/bar \
--replace "a string containing spaces" "some other text" \
--subst-var someVar
</programlisting>
</para>
<para><function>substitute</function> is implemented using the
<command
xlink:href="http://replace.richardlloyd.org.uk/">replace</command>
command. Unlike with the <command>sed</command> command, you
dont have to worry about escaping special characters. It
supports performing substitutions on binary files (such as
executables), though there youll probably want to make sure
that the replacement string is as long as the replaced
string.</para>
</listitem>
</varlistentry>
<varlistentry xml:id='fun-substituteInPlace'>
<term><function>substituteInPlace</function>
<replaceable>file</replaceable>
<replaceable>subs</replaceable></term>
<listitem><para>Like <function>substitute</function>, but performs
the substitutions in place on the file
<replaceable>file</replaceable>.</para></listitem>
</varlistentry>
<varlistentry xml:id='fun-substituteAll'>
<term><function>substituteAll</function>
<replaceable>infile</replaceable>
<replaceable>outfile</replaceable></term>
<listitem><para>Replaces every occurrence of
<literal>@<replaceable>varName</replaceable>@</literal>, where
<replaceable>varName</replaceable> is any environment variable, in
<replaceable>infile</replaceable>, writing the result to
<replaceable>outfile</replaceable>. For instance, if
<replaceable>infile</replaceable> has the contents
<programlisting>
#! @bash@/bin/sh
PATH=@coreutils@/bin
echo @foo@
</programlisting>
and the environment contains
<literal>bash=/nix/store/bmwp0q28cf21...-bash-3.2-p39</literal>
and
<literal>coreutils=/nix/store/68afga4khv0w...-coreutils-6.12</literal>,
but does not contain the variable <varname>foo</varname>, then the
output will be
<programlisting>
#! /nix/store/bmwp0q28cf21...-bash-3.2-p39/bin/sh
PATH=/nix/store/68afga4khv0w...-coreutils-6.12/bin
echo @foo@
</programlisting>
That is, no substitution is performed for undefined variables.</para>
<para>Environment variables that start with an uppercase letter or an
underscore are filtered out,
to prevent global variables (like <literal>HOME</literal>) or private
variables (like <literal>__ETC_PROFILE_DONE</literal>) from accidentally
getting substituted.
The variables also have to be valid bash “names”, as
defined in the bash manpage (alphanumeric or <literal>_</literal>,
must not start with a number).</para>
</listitem>
</varlistentry>
<varlistentry xml:id='fun-substituteAllInPlace'>
<term><function>substituteAllInPlace</function>
<replaceable>file</replaceable></term>
<listitem><para>Like <function>substituteAll</function>, but performs
the substitutions in place on the file
<replaceable>file</replaceable>.</para></listitem>
</varlistentry>
<varlistentry xml:id='fun-stripHash'>
<term><function>stripHash</function>
<replaceable>path</replaceable></term>
<listitem><para>Strips the directory and hash part of a store
path, outputting the name part to <literal>stdout</literal>.
For example:
<programlisting>
# prints coreutils-8.24
stripHash "/nix/store/9s9r019176g7cvn2nvcw41gsp862y6b4-coreutils-8.24"
</programlisting>
If you wish to store the result in another variable, then the
following idiom may be useful:
<programlisting>
name="/nix/store/9s9r019176g7cvn2nvcw41gsp862y6b4-coreutils-8.24"
someVar=$(stripHash $name)
</programlisting>
</para></listitem>
</varlistentry>
<varlistentry xml:id='fun-wrapProgram'>
<term><function>wrapProgram</function>
<replaceable>executable</replaceable>
<replaceable>makeWrapperArgs</replaceable></term>
<listitem><para>Convenience function for <literal>makeWrapper</literal>
that automatically creates a sane wrapper file
It takes all the same arguments as <literal>makeWrapper</literal>,
except for <literal>--argv0</literal>.</para>
<para>It cannot be applied multiple times, since it will overwrite the wrapper
file.</para>
</listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="ssec-setup-hooks"><title>Package setup hooks</title>
<para>The following packages provide a setup hook:
<variablelist>
<varlistentry>
<term>CC Wrapper</term>
<listitem>
<para>
CC Wrapper wraps a C toolchain for a bunch of miscellaneous purposes.
Specifically, a C compiler (GCC or Clang), Binutils (or the CCTools + binutils mashup when targetting Darwin), and a C standard library (glibc or Darwin's libSystem) are all fed in, and dependency finding, hardening (see below), and purity checks for each are handled by CC Wrapper.
Packages typically depend on only CC Wrapper, instead of those 3 inputs directly.
</para>
<para>
Dependency finding is undoubtedly the main task of CC wrapper.
It is currently accomplished by collecting directories of host-platform dependencies (i.e. <varname>buildInputs</varname> and <varname>nativeBuildInputs</varname>) in environment variables.
CC wrapper's setup hook causes any <filename>include</filename> subdirectory of such a dependency to be added to <envar>NIX_CFLAGS_COMPILE</envar>, and any <filename>lib</filename> and <filename>lib64</filename> subdirectories to <envar>NIX_LDFLAGS</envar>.
The setup hook itself contains some lengthy comments describing the exact convoluted mechanism by which this is accomplished.
</para>
<para>
A final task of the setup hook is defining a number of standard environment variables to tell build systems which executables full-fill which purpose.
They are defined to just be the base name of the tools, under the assumption that CC Wrapper's binaries will be on the path.
Firstly, this helps poorly-written packages, e.g. ones that look for just <command>gcc</command> when <envar>CC</envar> isn't defined yet <command>clang</command> is to be used.
Secondly, this helps packages not get confused when cross-compiling, in which case multiple CC wrappers may be simultaneous in use (targeting different platforms).
<envar>BUILD_</envar>- and <envar>TARGET_</envar>-prefixed versions of the normal environment variable are defined for the additional CC Wrappers, properly disambiguating them.
</para>
<para>
A problem with this final task is that CC Wrapper is honest and defines <envar>LD</envar> as <command>ld</command>.
Most packages, however, firstly use the C compiler for linking, secondly use <envar>LD</envar> anyways, defining it as the C compiler, and thirdly, only so define <envar>LD</envar> when it is undefined as a fallback.
This triple-threat means CC Wrapper will break those packages, as LD is already defined as the actually linker which the package won't override yet doesn't want to use.
The workaround is to define, just for the problematic package, <envar>LD</envar> as the C compiler.
A good way to do this would be <command>preConfigure = "LD=$CC"</command>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Perl</term>
<listitem><para>Adds the <filename>lib/site_perl</filename> subdirectory
of each build input to the <envar>PERL5LIB</envar>
environment variable.</para></listitem>
</varlistentry>
<varlistentry>
<term>Python</term>
<listitem><para>Adds the
<filename>lib/${python.libPrefix}/site-packages</filename> subdirectory of
each build input to the <envar>PYTHONPATH</envar> environment
variable.</para></listitem>
</varlistentry>
<varlistentry>
<term>pkg-config</term>
<listitem><para>Adds the <filename>lib/pkgconfig</filename> and
<filename>share/pkgconfig</filename> subdirectories of each
build input to the <envar>PKG_CONFIG_PATH</envar> environment
variable.</para></listitem>
</varlistentry>
<varlistentry>
<term>Automake</term>
<listitem><para>Adds the <filename>share/aclocal</filename>
subdirectory of each build input to the <envar>ACLOCAL_PATH</envar>
environment variable.</para></listitem>
</varlistentry>
<varlistentry>
<term>Autoconf</term>
<listitem><para>The <varname>autoreconfHook</varname> derivation adds
<varname>autoreconfPhase</varname>, which runs autoreconf, libtoolize and
automake, essentially preparing the configure script in autotools-based
builds.</para></listitem>
</varlistentry>
<varlistentry>
<term>libxml2</term>
<listitem><para>Adds every file named
<filename>catalog.xml</filename> found under the
<filename>xml/dtd</filename> and <filename>xml/xsl</filename>
subdirectories of each build input to the
<envar>XML_CATALOG_FILES</envar> environment
variable.</para></listitem>
</varlistentry>
<varlistentry>
<term>teTeX / TeX Live</term>
<listitem><para>Adds the <filename>share/texmf-nix</filename>
subdirectory of each build input to the <envar>TEXINPUTS</envar>
environment variable.</para></listitem>
</varlistentry>
<varlistentry>
<term>Qt 4</term>
<listitem><para>Sets the <envar>QTDIR</envar> environment variable
to Qts path.</para></listitem>
</varlistentry>
<varlistentry>
<term>gdk-pixbuf</term>
<listitem><para>Exports <envar>GDK_PIXBUF_MODULE_FILE</envar>
environment variable the the builder. Add librsvg package
to <varname>buildInputs</varname> to get svg support.</para></listitem>
</varlistentry>
<varlistentry>
<term>GHC</term>
<listitem><para>Creates a temporary package database and registers
every Haskell build input in it (TODO: how?).</para></listitem>
</varlistentry>
<varlistentry>
<term>GStreamer</term>
<listitem><para>Adds the
GStreamer plugins subdirectory of
each build input to the <envar>GST_PLUGIN_SYSTEM_PATH_1_0</envar> or
<envar>GST_PLUGIN_SYSTEM_PATH</envar> environment variable.</para></listitem>
</varlistentry>
<varlistentry>
<term>paxctl</term>
<listitem><para>Defines the <varname>paxmark</varname> helper for
setting per-executable PaX flags on Linux (where it is available by
default; on all other platforms, <varname>paxmark</varname> is a no-op).
For example, to disable secure memory protections on the executable
<replaceable>foo</replaceable>:
<programlisting>
postFixup = ''
paxmark m $out/bin/<replaceable>foo</replaceable>
'';
</programlisting>
The <literal>m</literal> flag is the most common flag and is typically
required for applications that employ JIT compilation or otherwise need to
execute code generated at run-time. Disabling PaX protections should be
considered a last resort: if possible, problematic features should be
disabled or patched to work with PaX.</para></listitem>
</varlistentry>
</variablelist>
</para>
</section>
<section xml:id="sec-purity-in-nixpkgs"><title>Purity in Nixpkgs</title>
<para>[measures taken to prevent dependencies on packages outside the
store, and what you can do to prevent them]</para>
<para>GCC doesn't search in locations such as
<filename>/usr/include</filename>. In fact, attempts to add such
directories through the <option>-I</option> flag are filtered out.
Likewise, the linker (from GNU binutils) doesn't search in standard
locations such as <filename>/usr/lib</filename>. Programs built on
Linux are linked against a GNU C Library that likewise doesn't search
in the default system locations.</para>
</section>
<section xml:id="sec-hardening-in-nixpkgs"><title>Hardening in Nixpkgs</title>
<para>There are flags available to harden packages at compile or link-time.
These can be toggled using the <varname>stdenv.mkDerivation</varname> parameters
<varname>hardeningDisable</varname> and <varname>hardeningEnable</varname>.
</para>
<para>
Both parameters take a list of flags as strings. The special
<varname>"all"</varname> flag can be passed to <varname>hardeningDisable</varname>
to turn off all hardening. These flags can also be used as environment variables
for testing or development purposes.
</para>
<para>The following flags are enabled by default and might require disabling with
<varname>hardeningDisable</varname> if the program to package is incompatible.
</para>
<variablelist>
<varlistentry>
<term><varname>format</varname></term>
<listitem><para>Adds the <option>-Wformat -Wformat-security
-Werror=format-security</option> compiler options. At present,
this warns about calls to <varname>printf</varname> and
<varname>scanf</varname> functions where the format string is
not a string literal and there are no format arguments, as in
<literal>printf(foo);</literal>. This may be a security hole
if the format string came from untrusted input and contains
<literal>%n</literal>.</para>
<para>This needs to be turned off or fixed for errors similar to:</para>
<programlisting>
/tmp/nix-build-zynaddsubfx-2.5.2.drv-0/zynaddsubfx-2.5.2/src/UI/guimain.cpp:571:28: error: format not a string literal and no format arguments [-Werror=format-security]
printf(help_message);
^
cc1plus: some warnings being treated as errors
</programlisting></listitem>
</varlistentry>
<varlistentry>
<term><varname>stackprotector</varname></term>
<listitem>
<para>Adds the <option>-fstack-protector-strong
--param ssp-buffer-size=4</option>
compiler options. This adds safety checks against stack overwrites
rendering many potential code injection attacks into aborting situations.
In the best case this turns code injection vulnerabilities into denial
of service or into non-issues (depending on the application).</para>
<para>This needs to be turned off or fixed for errors similar to:</para>
<programlisting>
bin/blib.a(bios_console.o): In function `bios_handle_cup':
/tmp/nix-build-ipxe-20141124-5cbdc41.drv-0/ipxe-5cbdc41/src/arch/i386/firmware/pcbios/bios_console.c:86: undefined reference to `__stack_chk_fail'
</programlisting></listitem>
</varlistentry>
<varlistentry>
<term><varname>fortify</varname></term>
<listitem>
<para>Adds the <option>-O2 -D_FORTIFY_SOURCE=2</option> compiler
options. During code generation the compiler knows a great deal of
information about buffer sizes (where possible), and attempts to replace
insecure unlimited length buffer function calls with length-limited ones.
This is especially useful for old, crufty code. Additionally, format
strings in writable memory that contain '%n' are blocked. If an application
depends on such a format string, it will need to be worked around.
</para>
<para>Additionally, some warnings are enabled which might trigger build
failures if compiler warnings are treated as errors in the package build.
In this case, set <option>NIX_CFLAGS_COMPILE</option> to
<option>-Wno-error=warning-type</option>.</para>
<para>This needs to be turned off or fixed for errors similar to:</para>
<programlisting>
malloc.c:404:15: error: return type is an incomplete type
malloc.c:410:19: error: storage size of 'ms' isn't known
</programlisting>
<programlisting>
strdup.h:22:1: error: expected identifier or '(' before '__extension__'
</programlisting>
<programlisting>
strsep.c:65:23: error: register name not specified for 'delim'
</programlisting>
<programlisting>
installwatch.c:3751:5: error: conflicting types for '__open_2'
</programlisting>
<programlisting>
fcntl2.h:50:4: error: call to '__open_missing_mode' declared with attribute error: open with O_CREAT or O_TMPFILE in second argument needs 3 arguments
</programlisting>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>pic</varname></term>
<listitem>
<para>Adds the <option>-fPIC</option> compiler options. This options adds
support for position independent code in shared libraries and thus making
ASLR possible.</para>
<para>Most notably, the Linux kernel, kernel modules and other code
not running in an operating system environment like boot loaders won't
build with PIC enabled. The compiler will is most cases complain that
PIC is not supported for a specific build.
</para>
<para>This needs to be turned off or fixed for assembler errors similar to:</para>
<programlisting>
ccbLfRgg.s: Assembler messages:
ccbLfRgg.s:33: Error: missing or invalid displacement expression `private_key_len@GOTOFF'
</programlisting>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>strictoverflow</varname></term>
<listitem>
<para>Signed integer overflow is undefined behaviour according to the C
standard. If it happens, it is an error in the program as it should check
for overflow before it can happen, not afterwards. GCC provides built-in
functions to perform arithmetic with overflow checking, which are correct
and faster than any custom implementation. As a workaround, the option
<option>-fno-strict-overflow</option> makes gcc behave as if signed
integer overflows were defined.
</para>
<para>This flag should not trigger any build or runtime errors.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>relro</varname></term>
<listitem>
<para>Adds the <option>-z relro</option> linker option. During program
load, several ELF memory sections need to be written to by the linker,
but can be turned read-only before turning over control to the program.
This prevents some GOT (and .dtors) overwrite attacks, but at least the
part of the GOT used by the dynamic linker (.got.plt) is still vulnerable.
</para>
<para>This flag can break dynamic shared object loading. For instance, the
module systems of Xorg and OpenCV are incompatible with this flag. In almost
all cases the <varname>bindnow</varname> flag must also be disabled and
incompatible programs typically fail with similar errors at runtime.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>bindnow</varname></term>
<listitem>
<para>Adds the <option>-z bindnow</option> linker option. During program
load, all dynamic symbols are resolved, allowing for the complete GOT to
be marked read-only (due to <varname>relro</varname>). This prevents GOT
overwrite attacks. For very large applications, this can incur some
performance loss during initial load while symbols are resolved, but this
shouldn't be an issue for daemons.
</para>
<para>This flag can break dynamic shared object loading. For instance, the
module systems of Xorg and PHP are incompatible with this flag. Programs
incompatible with this flag often fail at runtime due to missing symbols,
like:</para>
<programlisting>
intel_drv.so: undefined symbol: vgaHWFreeHWRec
</programlisting>
</listitem>
</varlistentry>
</variablelist>
<para>The following flags are disabled by default and should be enabled
with <varname>hardeningEnable</varname> for packages that take untrusted
input like network services.
</para>
<variablelist>
<varlistentry>
<term><varname>pie</varname></term>
<listitem>
<para>Adds the <option>-fPIE</option> compiler and <option>-pie</option>
linker options. Position Independent Executables are needed to take
advantage of Address Space Layout Randomization, supported by modern
kernel versions. While ASLR can already be enforced for data areas in
the stack and heap (brk and mmap), the code areas must be compiled as
position-independent. Shared libraries already do this with the
<varname>pic</varname> flag, so they gain ASLR automatically, but binary
.text regions need to be build with <varname>pie</varname> to gain ASLR.
When this happens, ROP attacks are much harder since there are no static
locations to bounce off of during a memory corruption attack.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>For more in-depth information on these hardening flags and hardening in
general, refer to the
<link xlink:href="https://wiki.debian.org/Hardening">Debian Wiki</link>,
<link xlink:href="https://wiki.ubuntu.com/Security/Features">Ubuntu Wiki</link>,
<link xlink:href="https://wiki.gentoo.org/wiki/Project:Hardened">Gentoo Wiki</link>,
and the <link xlink:href="https://wiki.archlinux.org/index.php/DeveloperWiki:Security">
Arch Wiki</link>.
</para>
</section>
</chapter>