[PATCH 3/3] Build libc-start with stack protector for SHARED

[Siddhesh Poyarekar <siddhesh@sourceware.org>]

This does not change the emitted code since __libc_start_main does not
return, but is important for formal flags compliance.

This also cleans up the cosmetic inconsistency in the stack protector
flags in csu, especially the incorrect value of STACK_PROTECTOR_LEVEL.
---
 Makeconfig   |  8 ++++++++
 csu/Makefile | 22 ++++++++++++----------
 elf/Makefile |  4 ----
 3 files changed, 20 insertions(+), 14 deletions(-)

diff --git a/Makeconfig b/Makeconfig
index 0a4811b5e5..01f8638c2e 100644
--- a/Makeconfig
+++ b/Makeconfig
@@ -856,6 +856,14 @@ ifneq ($(stack-protector),)
 +stack-protector=$(stack-protector)
 endif
 
+# Some routines are unsafe to build with stack-protection since they're called
+# before the stack check guard is set up.  Provide a way to disable stack
+# protector.  The first argument is the extension (.o, .os, .oS) and the second
+# is a list of routines that this path should be applied to.
+define elide-stack-protector
+$(if $(filter $(@F),$(patsubst %,%$(1),$(2))), $(no-stack-protector))
+endef
+
 # This is the program that generates makefile dependencies from C source files.
 # The -MP flag tells GCC >= 3.2 (which we now require) to produce dummy
 # targets for headers so that removed headers don't break the build.
diff --git a/csu/Makefile b/csu/Makefile
index e587434be8..3054329cea 100644
--- a/csu/Makefile
+++ b/csu/Makefile
@@ -45,18 +45,20 @@ install-lib = $(start-installed-name) g$(start-installed-name) $(csu-dummies)
 # code is compiled with special flags.
 tests =
 
-CFLAGS-.o += $(no-stack-protector)
-CFLAGS-.op += $(no-stack-protector)
-CFLAGS-.os += $(no-stack-protector)
-
-# Dummy object not actually used for anything.  It is linked into
-# crt1.o nevertheless, which in turn is statically linked into
+# static-reloc.os is a dummy object not actually used for anything.  It is
+# linked into crt1.o nevertheless, which in turn is statically linked into
 # applications, so that build flags matter.
 # See <https://sourceware.org/ml/libc-alpha/2018-07/msg00101.html>.
-# NB: Using $(stack-protector) in this way causes a wrong definition
-# STACK_PROTECTOR_LEVEL due to the preceding $(no-stack-protector),
-# but it does not matter for this source file.
-CFLAGS-static-reloc.os += $(stack-protector)
+#
+# libc-start.os is safe to be built with stack protector since
+# __libc_start_main is called after stack canary setup is done.
+ssp-safe.os = static-reloc libc-start
+
+CFLAGS-.o += $(call elide-stack-protector,.o,$(routines))
+CFLAGS-.op += $(call elide-stack-protector,.op,$(routines))
+CFLAGS-.oS += $(call elide-stack-protector,.oS,$(routines))
+CFLAGS-.os += $(call elide-stack-protector,.os,$(filter-out \
+						 $(ssp-safe.os),$(routines)))
 
 ifeq (yes,$(build-shared))
 extra-objs += S$(start-installed-name) gmon-start.os
diff --git a/elf/Makefile b/elf/Makefile
index b06bf6ca20..285d9f2f3c 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -83,10 +83,6 @@ endif
 # Also compile all routines in the static library that are elided from
 # the shared libc because they are in libc.a in the same way.
 
-define elide-stack-protector
-$(if $(filter $(@F),$(patsubst %,%$(1),$(2))), $(no-stack-protector))
-endef
-
 CFLAGS-.o += $(call elide-stack-protector,.o,$(elide-routines.os))
 CFLAGS-.op += $(call elide-stack-protector,.op,$(elide-routines.os))
 CFLAGS-.os += $(call elide-stack-protector,.os,$(all-rtld-routines))
-- 
2.29.2