gcc-config: store gcc backup into /lib/gcc-backup, not /lib

Writing (and removing) libraries to /lib outside package manager
was a surprise to users in bug #667020# where libunwind.so was
clobbered by gcc-config runs.

This change isolates all logic that handles file copies
outside package manager into /lib/gcc-backup directory.

To make new library still resolveable we install env.d entry
with contents of
    LDPATH="/lib/gcc-backup"

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

4 files changed, 79 insertions, 4 deletions

diff --git a/Makefile b/Makefile
index 2b3b235..087000b 100644
--- a/Makefile
+++ b/Makefile
@@ -3,6 +3,7 @@ EPREFIX ?=
 PN = gcc-config
 PV = git
 P = $(PN)-$(PV)
+BACKUPDIR = gcc-backup
 
 PREFIX = $(EPREFIX)/usr
 BINDIR = $(PREFIX)/bin
@@ -11,15 +12,18 @@ ESELECTDIR = $(PREFIX)/share/eselect/modules
 
 SUBLIBDIR = lib
 LIBDIR = $(PREFIX)/$(SUBLIBDIR)
+LIBGCC_BACKUPDIR = $(EPREFIX)/$(SUBLIBDIR)/$(BACKUPDIR)
+
+ENVD = $(EPREFIX)/etc/env.d
 
 MKDIR_P = mkdir -p -m 755
 INSTALL_EXE = install -m 755
 INSTALL_DATA = install -m 644
 
-all: .gcc-config
+all: .gcc-config .envd-gcc-backup
 
 clean:
-	rm -f .gcc-config
+	rm -f .gcc-config .envd-gcc-backup
 
 .gcc-config: gcc-config
 	sed \
@@ -27,14 +31,26 @@ clean:
 		-e 's:@GENTOO_EPREFIX@:$(EPREFIX):g' \
 		-e 's:@GENTOO_LIBDIR@:$(SUBLIBDIR):g' \
 		-e 's:@PV@:$(PV):g' \
+		-e 's:@GENTOO_GCC_BACKUP_DIR@:$(BACKUPDIR):g' \
 		$< > $@
 	chmod a+rx $@
 
+.envd-gcc-backup: envd-gcc-backup
+	sed \
+		-e 's:@LIBGCC_BACKUPDIR@:$(LIBGCC_BACKUPDIR):g' \
+		$< > $@
+
 install: all
-	$(MKDIR_P) $(DESTDIR)$(BINDIR) $(DESTDIR)$(ESELECTDIR) $(DESTDIR)$(DOCDIR)
+	$(MKDIR_P) $(DESTDIR)$(BINDIR)
 	$(INSTALL_EXE) .gcc-config $(DESTDIR)$(BINDIR)/gcc-config
+	$(MKDIR_P) $(DESTDIR)$(ESELECTDIR)
 	$(INSTALL_DATA) gcc.eselect $(DESTDIR)$(ESELECTDIR)
+	$(MKDIR_P) $(DESTDIR)$(DOCDIR)
 	$(INSTALL_DATA) README $(DESTDIR)$(DOCDIR)
+	$(MKDIR_P) $(DESTDIR)$(LIBGCC_BACKUPDIR)
+	$(INSTALL_DATA) gcc-backup/README $(DESTDIR)$(LIBGCC_BACKUPDIR)
+	$(MKDIR_P) $(DESTDIR)$(ENVD)
+	$(INSTALL_DATA) .envd-gcc-backup $(DESTDIR)$(ENVD)/99gcc-backup
 
 test check: .gcc-config
 	cd tests && ./run_tests
diff --git a/envd-gcc-backup b/envd-gcc-backup
new file mode 100644
index 0000000..4367b62
--- /dev/null
+++ b/envd-gcc-backup
@@ -0,0 +1,3 @@
+# A fallback for libgcc_s.so.* and dependencies.
+# See @LIBGCC_BACKUPDIR@/README for details
+LDPATH="@LIBGCC_BACKUPDIR@"
diff --git a/gcc-backup/README b/gcc-backup/README
new file mode 100644
index 0000000..fdac223
--- /dev/null
+++ b/gcc-backup/README
@@ -0,0 +1,41 @@
+What is gcc-backup
+==================
+
+gcc-backup is a directory to backup critical runtime
+libraries used by gcc to link in code too big to inline it
+from code generator.
+
+See gcc/doc/libgcc.texi doc in gcc source tree.
+
+gcc-config is responsible to back up libgcc_s.so.* and it's
+runtime dependencies.
+
+Which systems needed this backup
+================================
+
+During normal system operation these libraries should not be
+needed (unless gcc installation is somehow completely broken).
+
+The main use case for libgcc-backup is booting of a system
+with the following properties:
+- '/usr' mount is separate from '/' mount
+- AND binaries in /bin (say, /bin/bash) are executed before
+  /usr is mounted
+
+Which systems link libgcc_s.so in
+=================================
+
+By default gcc attempts to link libgcc_s.so in into every
+binary and shared library. If code generator happens to
+generate the code that does not refer libgcc_s.so then
+-Wl,--as-needed linker option omits the dependency.
+
+Thus any system might have libgcc_s.so linked into any binary
+or shared library.
+
+What goes into this directory
+=============================
+
+- libgcc_s.so.* itself: gcc driver attempts to link it in as-is
+- libunwind.so.*: ia64-*-linux only, a runtime dependency of
+                  libgcc_so.so.*
diff --git a/gcc-config b/gcc-config
index 01e2b90..65b30bc 100755
--- a/gcc-config
+++ b/gcc-config
@@ -42,6 +42,9 @@ PV="@PV@"
 GENTOO_LIBDIR="@GENTOO_LIBDIR@"
 [[ ${GENTOO_LIBDIR} == @*@ ]] && GENTOO_LIBDIR="lib"
 
+GENTOO_GCC_BACKUP_DIR="@GENTOO_GCC_BACKUP_DIR@"
+[[ ${GENTOO_GCC_BACKUP_DIR} == @*@ ]] && GENTOO_GCC_BACKUP_DIR="gcc-backup"
+
 usage() {
 	cat <<-EOF
 	Usage: gcc-config [options] [CC Profile]
@@ -316,11 +319,23 @@ handle_split_usr() {
 	if is_same_mountpoint "${EROOT}/lib" "${ROOT}/${LDPATH}" ; then
 		local lib old_libs=0 saved_nullglob=$(shopt -p nullglob)
 		shopt -s nullglob
+
+		# We relied on these copies until Sept 2019.
+		# Can be removed in 2021.
 		for lib in "${EROOT}"/lib*/libgcc_s{.so*,*dylib} "${EROOT}"/lib*/libunwind.so.7* ; do
 			# If we previously had stuff in /, make sure ldconfig gets re-run.
 			rm -f "${lib}"
 			old_libs=1
 		done
+
+		for lib in \
+			"${EROOT}"/lib*/"${GENTOO_GCC_BACKUP_DIR}"/libgcc_s.so.* \
+			"${EROOT}"/lib*/"${GENTOO_GCC_BACKUP_DIR}"/libunwind.so.* ; do
+			# If we previously had stuff in /, make sure ldconfig gets re-run.
+			rm -f "${lib}"
+			old_libs=1
+		done
+
 		${saved_nullglob}
 		return ${old_libs}
 	fi
@@ -328,7 +343,7 @@ handle_split_usr() {
 	# Only bother with this stuff for the native ABI.  We assume the user
 	# doesn't have critical binaries for non-native ABIs which is fair.
 	local gcclib
-	local libdir="${EROOT}${GENTOO_LIBDIR}"
+	local libdir="${EROOT}/${GENTOO_LIBDIR}/${GENTOO_GCC_BACKUP_DIR}"
 	mkdir -p "${libdir}"/.gcc.config.new || return 0 # !?!?!
 	for gcclib in gcc_s unwind ; do
 		# This assumes that we always have the .so symlink,