gcc-plugins: Explicitly document purpose and deprecation schedule

GCC plugins should only exist when some compiler feature needs to be
proven but does not exist in either GCC nor Clang. For example, if a
desired feature is already in Clang, it should be added to GCC upstream.
Document this explicitly.

Additionally, mark the plugins with matching upstream GCC features as
removable past their respective GCC versions.

Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: linux-hardening@vger.kernel.org
Cc: linux-kbuild@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-security-module@vger.kernel.org
Cc: llvm@lists.linux.dev
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20211020173554.38122-2-keescook@chromium.org

Documentation/kbuild/gcc-plugins.rst

@@ -32,6 +32,32 @@ This infrastructure was ported from grsecurity [6]_ and PaX [7]_.
 .. [7] https://pax.grsecurity.net/
 
 
+Purpose
+=======
+
+GCC plugins are designed to provide a place to experiment with potential
+compiler features that are neither in GCC nor Clang upstream. Once
+their utility is proven, the goal is to upstream the feature into GCC
+(and Clang), and then to finally remove them from the kernel once the
+feature is available in all supported versions of GCC.
+
+Specifically, new plugins should implement only features that have no
+upstream compiler support (in either GCC or Clang).
+
+When a feature exists in Clang but not GCC, effort should be made to
+bring the feature to upstream GCC (rather than just as a kernel-specific
+GCC plugin), so the entire ecosystem can benefit from it.
+
+Similarly, even if a feature provided by a GCC plugin does *not* exist
+in Clang, but the feature is proven to be useful, effort should be spent
+to upstream the feature to GCC (and Clang).
+
+After a feature is available in upstream GCC, the plugin will be made
+unbuildable for the corresponding GCC version (and later). Once all
+kernel-supported versions of GCC provide the feature, the plugin will
+be removed from the kernel.
+
+
 Files
 =====
 

scripts/gcc-plugins/Kconfig

@@ -37,6 +37,8 @@ config GCC_PLUGIN_CYC_COMPLEXITY
 
 config GCC_PLUGIN_SANCOV
 	bool
+	# Plugin can be removed once the kernel only supports GCC 6+
+	depends on !CC_HAS_SANCOV_TRACE_PC
 	help
 	  This plugin inserts a __sanitizer_cov_trace_pc() call at the start of
 	  basic blocks. It supports all gcc versions with plugin support (from
@@ -83,8 +85,6 @@ config GCC_PLUGIN_RANDSTRUCT
 	  the existing seed and will be removed by a make mrproper or
 	  make distclean.
 
-	  Note that the implementation requires gcc 4.7 or newer.
-
 	  This plugin was ported from grsecurity/PaX. More information at:
 	   * https://grsecurity.net/
 	   * https://pax.grsecurity.net/

security/Kconfig.hardening

@@ -56,7 +56,8 @@ choice
 
 	config GCC_PLUGIN_STRUCTLEAK_USER
 		bool "zero-init structs marked for userspace (weak)"
-		depends on GCC_PLUGINS
+		# Plugin can be removed once the kernel only supports GCC 12+
+		depends on GCC_PLUGINS && !CC_HAS_AUTO_VAR_INIT_ZERO
 		select GCC_PLUGIN_STRUCTLEAK
 		help
 		  Zero-initialize any structures on the stack containing
@@ -67,7 +68,8 @@ choice
 
 	config GCC_PLUGIN_STRUCTLEAK_BYREF
 		bool "zero-init structs passed by reference (strong)"
-		depends on GCC_PLUGINS
+		# Plugin can be removed once the kernel only supports GCC 12+
+		depends on GCC_PLUGINS && !CC_HAS_AUTO_VAR_INIT_ZERO
 		depends on !(KASAN && KASAN_STACK)
 		select GCC_PLUGIN_STRUCTLEAK
 		help
@@ -85,7 +87,8 @@ choice
 
 	config GCC_PLUGIN_STRUCTLEAK_BYREF_ALL
 		bool "zero-init everything passed by reference (very strong)"
-		depends on GCC_PLUGINS
+		# Plugin can be removed once the kernel only supports GCC 12+
+		depends on GCC_PLUGINS && !CC_HAS_AUTO_VAR_INIT_ZERO
 		depends on !(KASAN && KASAN_STACK)
 		select GCC_PLUGIN_STRUCTLEAK
 		help