linux-kernel - [PATCH] kbuild: explain stack-protector-strong CONFIG logic

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Date:	Wed, 2 Jul 2014 18:10:59 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Andi Kleen <ak@...ux.intel.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	linux-next@...r.kernel.org, sfr@...b.auug.org.au, mhocko@...e.cz,
	linux-kernel@...r.kernel.org, Michal Marek <mmarek@...e.cz>,
	linux-kbuild@...r.kernel.org
Subject: [PATCH] kbuild: explain stack-protector-strong CONFIG logic

This adds a hopefully helpful comment above the (seemingly weird)
compiler flag selection logic.

Suggested-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 Makefile |   16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/Makefile b/Makefile
index 13175632137f..ea88e68d121e 100644
--- a/Makefile
+++ b/Makefile
@@ -630,6 +630,22 @@ 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)),)
-- 
1.7.9.5


-- 
Kees Cook
Chrome OS Security
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