linux-kernel - [PATCH v4 10/14] kbuild: Introduce FRAME_WARN_BUMP

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Message-Id: <20180711203619.1020-11-keescook@chromium.org>
Date:   Wed, 11 Jul 2018 13:36:15 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Kees Cook <keescook@...omium.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Arnd Bergmann <arnd@...db.de>,
        Eric Biggers <ebiggers@...gle.com>,
        Alasdair Kergon <agk@...hat.com>,
        Giovanni Cabiddu <giovanni.cabiddu@...el.com>,
        Lars Persson <larper@...s.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Rabin Vincent <rabinv@...s.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        "David S. Miller" <davem@...emloft.net>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        linux-crypto@...r.kernel.org, qat-linux@...el.com,
        dm-devel@...hat.com, linux-kernel@...r.kernel.org
Subject: [PATCH v4 10/14] kbuild: Introduce FRAME_WARN_BUMP_FLAG

When building with CONFIG_FRAME_WARN, it is sometimes useful to
give certain compilation units small additional headroom. This is
normally useful when VLA removal exposes a large stack allocation,
especially with FRAME_WARN at 1024 or smaller. This adds 20%.

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 Makefile | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/Makefile b/Makefile
index c9132594860b..06fe93edd052 100644
--- a/Makefile
+++ b/Makefile
@@ -684,7 +684,13 @@ endif
 
 ifneq ($(CONFIG_FRAME_WARN),0)
 KBUILD_CFLAGS += $(call cc-option,-Wframe-larger-than=${CONFIG_FRAME_WARN})
+# Small code (mostly exposed by VLA removal) needs some additional
+# headroom, especially for a FRAME_WARN of 1024. This adds 20% which
+# can be used by the CFLAG_FRAME_WARN_BUMP option.
+FRAME_WARN_BUMP_SIZE := $(shell expr $(CONFIG_FRAME_WARN) / 5 + $(CONFIG_FRAME_WARN))
+FRAME_WARN_BUMP_FLAG := $(call cc-option,-Wframe-larger-than=$(FRAME_WARN_BUMP_SIZE))
 endif
+export FRAME_WARN_BUMP_FLAG
 
 stackp-flags-$(CONFIG_CC_HAS_STACKPROTECTOR_NONE) := -fno-stack-protector
 stackp-flags-$(CONFIG_STACKPROTECTOR)             := -fstack-protector
-- 
2.17.1