doc: add instructions for static linking

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>

doc/developer/building.rst

@@ -7,6 +7,7 @@ Building FRR
 .. toctree::
    :maxdepth: 2
 
+   static-linking
    building-frr-for-alpine
    building-frr-for-centos6
    building-frr-for-centos7

doc/developer/static-linking.rst

@@ -0,0 +1,98 @@
+.. _static-linking:
+
+Static Linking
+==============
+
+This document describes how to build FRR without hard dependencies on shared
+libraries. Note that it's not possible to build FRR *completely* statically.
+This document just covers how to statically link the dependencies that aren't
+likely to be present on a given platform - libfrr and libyang. The resultant
+binaries should still be fairly portable. For example, here is the DSO
+dependency list for `bgpd` after using these steps:
+
+.. code-block::
+
+   $ ldd bgpd
+        linux-vdso.so.1 (0x00007ffe3a989000)
+        libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f9dc10c0000)
+        libcap.so.2 => /lib/x86_64-linux-gnu/libcap.so.2 (0x00007f9dc0eba000)
+        libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f9dc0b1c000)
+        libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f9dc0918000)
+        libcrypt.so.1 => /lib/x86_64-linux-gnu/libcrypt.so.1 (0x00007f9dc06e0000)
+        libjson-c.so.3 => /lib/x86_64-linux-gnu/libjson-c.so.3 (0x00007f9dc04d5000)
+        librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007f9dc02cd000)
+        libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f9dc00ae000)
+        libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f9dbfe96000)
+        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f9dbfaa5000)
+        /lib64/ld-linux-x86-64.so.2 (0x00007f9dc1449000)
+
+Procedure
+---------
+Note that these steps have only been tested with LLVM 9 / clang.
+
+Today, libfrr can already be statically linked by passing these configure
+options::
+
+   --enable-static --enable-static-bin --enable-shared
+
+libyang is more complicated. You must build and install libyang as a static
+library. To do this, follow the usual libyang build procedure as listed in the
+FRR developer docs, but set the ``ENABLE_STATIC`` option in your cmake
+invocation. You also need to build with PIC enabled, which today is disabled
+when building libyang statically.
+
+The resultant cmake command is::
+
+   cmake -DENABLE_STATIC=ON -DENABLE_LYD_PRIV=ON \
+         -DCMAKE_INSTALL_PREFIX:PATH=/usr \
+         -DCMAKE_POSITION_INDEPENDENT_CODE=TRUE \
+         -DCMAKE_BUILD_TYPE:String="Release" ..
+
+This produces a bunch of ``.a`` static archives that need to ultimately be linked
+into FRR. However, not only is it 6 archives rather than the usual ``libyang.so``,
+you will now also need to link FRR with ``libpcre.a``. Ubuntu's ``libpcre3-dev``
+package provides this, but it hasn't been built with PIC enabled, so it's not
+usable for our purposes. So download ``libpcre`` from
+`SourceForge <https://sourceforge.net/projects/pcre/>`_, and build it
+like this:
+
+.. code-block::
+
+   ./configure --with-pic
+   make
+
+Hopefully you get a nice, usable, PIC ``libpcre.a``.
+
+So now we have to link all these static libraries into FRR. Rather than modify
+FRR to accomodate this, the best option is to create an archive with all of
+libyang's dependencies. Then to avoid making any changes to FRR build foo,
+rename this ``libyang.a`` and copy it over the usual static library location.
+Ugly but it works. To do this, go into your libyang build directory, which
+should have a bunch of ``.a`` files.  Copy ``libpcre.a`` into this directory.
+Write the following into a shell script and run it:
+
+.. code-block:: shell
+
+   #!/bin/bash
+   ar -M <<EOM
+     CREATE libyang_fat.a
+     ADDLIB libyang.a
+     ADDLIB libyangdata.a
+     ADDLIB libmetadata.a
+     ADDLIB libnacm.a
+     ADDLIB libuser_inet_types.a
+     ADDLIB libuser_yang_types.a
+     ADDLIB libpcre.a
+     SAVE
+     END
+   EOM
+   ranlib libyang_fat.a
+
+``libyang_fat.a`` is your archive. Now copy this over your install
+``libyang.a``, which on my machine is located at
+``/usr/lib/x86_64-linux-gnu/libyang.a`` (try ``locate libyang.a`` if not).
+
+Now when you build FRR with the static options enabled as above, clang should
+pick up the static libyang and link it, leaving you with FRR binaries that have
+no hard DSO dependencies beyond common system libraries. To verify, run ``ldd``
+over the resultant binaries.

doc/developer/subdir.am

@@ -44,6 +44,7 @@ dev_RSTFILES = \
 	doc/developer/packaging-redhat.rst \
 	doc/developer/packaging.rst \
 	doc/developer/rcu.rst \
+	doc/developer/static-linking.rst \
 	doc/developer/testing.rst \
 	doc/developer/topotests-snippets.rst \
 	doc/developer/topotests.rst \