Enable _FORTIFY_SOURCE=3 by default

Make the _FORTIFY_SOURCE flags configurable so that the command line is
not cluttered with _FORTIFY_SOURCE definitions and undefines.  Introduce
a %_fortify_level variable that a package may override by either
undefining or defining to a specific value.

Also bump the default value to 3, to implement the systemwide proposal
for Fedora 38:

https://fedoraproject.org/wiki/Changes/Add_FORTIFY_SOURCE%3D3_to_distribution_build_flags

buildflags.md

@@ -172,6 +172,30 @@ This turns off certain hardening features, as described in detail
 below.  The main difference is that executables will be
 position-dependent (no full ASLR) and use lazy binding.
 
+### Source Fortification
+
+By default, the build flags include `-Wp,-D_FORTIFY_SOURCE=3`: Source
+fortification activates various hardening features in glibc:
+
+* String functions such as `memcpy` attempt to detect buffer lengths
+  and terminate the process if a buffer overflow is detected.
+* `printf` format strings may only contain the `%n` format specifier
+  if the format string resides in read-only memory.
+* `open` and `openat` flags are checked for consistency with the
+  presence of a *mode* argument.
+* Plus other minor hardening changes.
+
+These changes can, on rare occasions, break valid programs. The source
+fortification level can be overridden by adding this in the RPM spec file:
+
+    %define _fortify_level 2
+
+to reduce source fortification level to 2 or:
+
+    %undefine _fortify_level
+
+to disable fortification altogether.
+
 ### Annotated builds/watermarking
 
 By default, the build flags cause a special output section to be
@@ -377,16 +401,9 @@ The general (architecture-independent) build flags are:
   This can occasionally result in compilation errors. In that case,
   the best option is to rewrite the source code so that only constant
   format strings (string literals) are used.
-* `-Wp,-D_FORTIFY_SOURCE=2`: Source fortification activates various
-  hardening features in glibc:
-    * String functions such as `memcpy` attempt to detect buffer lengths
-      and terminate the process if a buffer overflow is detected.
-    * `printf` format strings may only contain the `%n` format specifier
-      if the format string resides in read-only memory.
-    * `open` and `openat` flags are checked for consistency with the
-      presence of a *mode* argument.
-    * Plus other minor hardening changes.
-  (These changes can occasionally break valid programs.)
+* `-U_FORTIFY_SOURCE, -Wp,-U_FORTIFY_SOURCE -Wp,-D_FORTIFY_SOURCE=3`:
+  See the Source Fortification section above and the `%_fortify_level`
+  override.
 * `-fexceptions`: Provide exception unwinding support for C programs.
   See the [`-fexceptions` option in the GCC
   manual](https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#index-fexceptions)

macros

@@ -362,13 +362,19 @@ print(result)
 %_clang_lto_cflags	-flto
 %_lto_cflags		%{expand:%%{_%{toolchain}_lto_cflags}}
 
+# Default fortification level.
+#   "%define _fortify_level 2" to downgrade and
+#   "%define _fortify_level 0" or "%undefine _fortify_level" to disable
+%_fortify_level		3
+%_fortify_level_flags	%[ 0%{?_fortify_level} > 0 ? "-U_FORTIFY_SOURCE -Wp,-U_FORTIFY_SOURCE -Wp,-D_FORTIFY_SOURCE=%{_fortify_level}" : "" ]
+
 # Some linkers default to a build-id algoritim that is not supported by rpmbuild,
 # so we need to specify the right algorithm to use.
 %_build_id_flags	-Wl,--build-id=sha1
 
 %_general_options       -O2 %{?_lto_cflags} -fexceptions -g -grecord-gcc-switches -pipe
 %_warning_options       -Wall -Werror=format-security
-%_preprocessor_defines  -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS
+%_preprocessor_defines  %{_fortify_level_flags} -Wp,-D_GLIBCXX_ASSERTIONS
 
 # Common variables are no longer generated by default by gcc and clang
 # If they are needed then add "%define _legacy_common_support 1" to the spec file.

redhat-rpm-config.spec

@@ -4,7 +4,7 @@
 # 2) When making changes, increment the version (in baserelease) by 1.
 #    rpmdev-bumpspec and other tools update the macro below, which is used
 #    in Version: to get the desired effect.
-%global baserelease 236
+%global baserelease 237
 
 Summary: Red Hat specific rpm configuration files
 Name: redhat-rpm-config
@@ -221,6 +221,9 @@ install -p -m 644 -t %{buildroot}%{_rpmluadir}/fedora/srpm forge.lua
 %doc buildflags.md
 
 %changelog
+* Tue Jan  3 2023 Siddhesh Poyarekar <siddhesh@redhat.com> - 237-1
+- Make _FORTIFY_SOURCE configurable and bump default to 3.
+
 * Wed Dec 28 2022 Davide Cavalca <dcavalca@fedoraproject.org> - 236-1
 - Add conditional support for always including frame pointers