kbuild: explain stack-protector-strong CONFIG logic

Commit Message

Kees Cook July 3, 2014, 1:10 a.m. UTC

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

Suggested-by: Andrew Morton <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
---
 Makefile |   16 ++++++++++++++++
 1 file changed, 16 insertions(+)

Patch

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)),)