From a3ca86aea507904148870946d599e07a340b39bf Mon Sep 17 00:00:00 2001 From: Eugene Teo Date: Wed, 15 Jul 2009 14:59:10 +0800 Subject: Add '-fno-delete-null-pointer-checks' to gcc CFLAGS From: Eugene Teo commit a3ca86aea507904148870946d599e07a340b39bf upstream. Turning on this flag could prevent the compiler from optimising away some "useless" checks for null pointers. Such bugs can sometimes become exploitable at compile time because of the -O2 optimisation. See http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Optimize-Options.html An example that clearly shows this 'problem' is commit 6bf67672. static void __devexit agnx_pci_remove(struct pci_dev *pdev) { struct ieee80211_hw *dev = pci_get_drvdata(pdev); - struct agnx_priv *priv = dev->priv; + struct agnx_priv *priv; AGNX_TRACE; if (!dev) return; + priv = dev->priv; By reverting this patch, and compile it with and without -fno-delete-null-pointer-checks flag, we can see that the check for dev is compiled away. call printk # - testq %r12, %r12 # dev - je .L94 #, movq %r12, %rdi # dev, Clearly the 'fix' is to stop using dev before it is tested, but building with -fno-delete-null-pointer-checks flag at least makes it harder to abuse. Signed-off-by: Eugene Teo Acked-by: Eric Paris Acked-by: Wang Cong Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- Makefile | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) --- a/Makefile +++ b/Makefile @@ -340,7 +340,8 @@ KBUILD_CPPFLAGS := -D__KERNEL__ $(LINUXI KBUILD_CFLAGS := -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \ -fno-strict-aliasing -fno-common \ - -Werror-implicit-function-declaration + -Werror-implicit-function-declaration \ + -fno-delete-null-pointer-checks KBUILD_AFLAGS := -D__ASSEMBLY__ # Read KERNELRELEASE from include/config/kernel.release (if it exists)