Commit a3ca86ae authored by Eugene Teo's avatar Eugene Teo Committed by Linus Torvalds

Add '-fno-delete-null-pointer-checks' to gcc CFLAGS

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: default avatarEugene Teo <eugeneteo@kernel.sg>
Acked-by: default avatarEric Paris <eparis@redhat.com>
Acked-by: default avatarWang Cong <amwang@redhat.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 4a21b8cb
......@@ -343,7 +343,8 @@ KBUILD_CPPFLAGS := -D__KERNEL__
KBUILD_CFLAGS := -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
-fno-strict-aliasing -fno-common \
-Werror-implicit-function-declaration \
-Wno-format-security
-Wno-format-security \
-fno-delete-null-pointer-checks
KBUILD_AFLAGS := -D__ASSEMBLY__
# Read KERNELRELEASE from include/config/kernel.release (if it exists)
......
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