From 1332429b305044aa75163399ae960c6535828ce6 Mon Sep 17 00:00:00 2001
From: Kees Cook <keescook@chromium.org>
Date: Wed, 6 Aug 2014 16:03:31 -0700
Subject: [PATCH] ./Makefile: explain stack-protector-strong CONFIG logic
This adds a hopefully helpful comment above the (seemingly weird) compiler
flag selection logic.
Signed-off-by: Kees Cook <keescook@chromium.org>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
Makefile | 16 ++++++++++++++++
1 file changed, 16 insertions(+)
diff --git a/Makefile b/Makefile
index d0901b46b4bfd..af2f2fcfe01c1 100644
--- a/Makefile
+++ b/Makefile
@@ -636,6 +636,22 @@ KBUILD_CFLAGS += $(call cc-option,-Wframe-larger-than=${CONFIG_FRAME_WARN})
endif
# Handle stack protector mode.
+#
+# Since kbuild can potentially perform two passes (first with the old
+# .config values and then with updated .config values), we cannot error out
+# if a desired compiler option is unsupported. If we were to error, kbuild
+# could never get to the second pass and actually notice that we changed
+# the option to something that was supported.
+#
+# Additionally, we don't want to fallback and/or silently change which compiler
+# flags will be used, since that leads to producing kernels with different
+# security feature characteristics depending on the compiler used. ("But I
+# selected CC_STACKPROTECTOR_STRONG! Why did it build with _REGULAR?!")
+#
+# The middle ground is to warn here so that the failed option is obvious, but
+# to let the build fail with bad compiler flags so that we can't produce a
+# kernel when there is a CONFIG and compiler mismatch.
+#
ifdef CONFIG_CC_STACKPROTECTOR_REGULAR
stackp-flag := -fstack-protector
ifeq ($(call cc-option, $(stackp-flag)),)
--
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