Updates to build flags

Signed-off-by: Allan McRae <allan@archlinux.org>

rfcs/0003-buildflags.rst

+======================
+Updates to build flags
+======================
+
+- Date proposed: 2020-03-04
+- RFC MR: https://gitlab.archlinux.org/archlinux/rfcs/-/merge_requests/0003
+
+Summary
+-------
+
+Updating our buildflags will improve our packages security
+
+Motivation
+----------
+
+Compiler security features have advanced since we last updated our buildflags.
+While we have enabled some of these by default in our compilers, there is
+further improvements to be made.
+
+This RFC puts forward a set of compiler flags that have seen real world usage
+in other distributions. e.g. [1] [2]
+
+[1] https://src.fedoraproject.org/rpms/redhat-rpm-config/blob/master/f/buildflags.md.
+[2] https://wiki.ubuntu.com/ToolChain/CompilerFlags
+
+Specification
+-------------
+
+We will change the distributed makepkg.conf to the following:
+::
+
+ #CPPFLAGS=""
+ CFLAGS="-march=x86-64 -mtune=generic -O2 -pipe -fno-plt -fexceptions \
+         -Wp,-D_FORTIFY_SOURCE=2,-D_GLIBCXX_ASSERTIONS -Werror=format-security"
+ CXXFLAGS="$CFLAGS"
+ LDFLAGS="-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now"
+ DEBUG_CFLAGS="-g -fvar-tracking-assignments"
+
+Details of new additions:
+
+* Move ``-D_FORTIFY_SOURCE=2`` from CPPFLAGS to CFLAGS using -Wp
+
+Unfortunately, there are still build systems which use CFLAGS but not CPPFLAGS.
+Or configure tests that use CPPFLAGS and not CFLAGS which creates errors due to
+fortify source needing optimisation. Ultimately, we can cover more code with
+this workaround.
+
+* Add ``-fexceptions``
+
+Provide exception unwinding support for C programs. This also hardens cancellation
+handling in C programs, and makes it possible to unwind the stack (using C++ throw
+or Rust panics) from C callback functions
+
+* Add ``-Wp,-D_GLIBCXX_ASSERTIONS``
+
+Enable lightweight assertions in the C++ standard library. This flag is added to
+both CFLAGS and CXXFLAGS; C compilations will simply ignore it.
+
+* Add ``-Werror=format-security``
+
+Turn on format string warnings and treat them as errors. This catches a set of
+readily exploitable bugs. This can occasionally result in compilation errors,
+but this should be minimal as it is widely used by other distributions.
+
+
+Alternatives Considered
+-----------------------
+
+* Add ``-grecord-gcc-switches`` to our debug builds
+
+This is the default in GCC.
+
+* Add ``-fasynchronous-unwind-tables`` to our debug builds
+
+This is the default in GCC, at least for x86_64.
+
+
+Drawbacks
+---------
+
+Moving ``-D_FORTIFY_SOURCE=2`` is zero cost.
+
+Adding ``-Werror=format-security`` may cause limited build issues, but patches are
+readily available.
+
+There is a minimal performance overhead of adding ``-Wp,-D_GLIBCXX_ASSERTIONS``,
+though many of the added checks are optimised away by the compiler.
+
+Adding ``-fexceptions`` can produce some data size overhead in C programs, though
+does not affect execution. GCC enables it by default for C++.
+
+Unresolved Questions
+--------------------
+
+None.