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

[backported from 2.6 commit a3ca86aea507904148870946d599e07a340b39bf]

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 <eugeneteo@kernel.sg>
Acked-by: Eric Paris <eparis@redhat.com>
Acked-by: Wang Cong <amwang@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[wt: make this flag conditional on 2.4 as older gccs don't have it]
Signed-off-by: Willy Tarreau <w@1wt.eu>

1 files changed, 3 insertions, 0 deletions

diff --git a/Makefile b/Makefile
index e59f11117edfef..83d413a72f8050 100644
--- a/Makefile
+++ b/Makefile
@@ -105,6 +105,9 @@ if_gcc4 = $(shell if echo __GNUC__ | $(CC) -E -xc - | grep -q '^4$$' > /dev/null
 # disable pointer signedness warnings in gcc 4.0
 CFLAGS += $(call check_gcc,-Wno-pointer-sign,)
 
+# disable stupid and dangerous "optimization" in gcc 3.2+
+CFLAGS += $(call check_gcc,-fno-delete-null-pointer-checks,)
+
 #
 # ROOT_DEV specifies the default root-device when making the image.
 # This can be either FLOPPY, CURRENT, /dev/xxxx or empty, in which case