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Date: Thu, 10 Oct 2019 16:01:51 +0530
From: Shyam Saini <>
	Shyam Saini <>,
	Matthew Wilcox <>,
	Christopher Lameter <>,
	Kees Cook <>
Subject: [PATCH] slab: Redefine ZERO_SIZE_PTR to include ERR_PTR range

Currently kfree does not accept ERR_PTR range so redefine ZERO_SIZE_PTR
to include this and also change ZERO_OR_NULL_PTR macro to check this new
range. With this change kfree will skip and behave as no-ops when ERR_PTR
is passed.

This will help error related to ERR_PTR stand out better.

After this, we don't need to reset any ERR_PTR variable to NULL before
being passed to any kfree or related wrappers calls, as everything would
be handled by ZERO_SIZE_PTR itself.

This patch is verbatim from Brad Spengler/PaX Team's code in the last
public patch of grsecurity/PaX based on my understanding of the code.
Changes or omissions from the original code are mine and don't reflect the
original grsecurity/PaX code.

Cc: Matthew Wilcox <>
Cc: Christopher Lameter <>
Cc: Kees Cook <>
Signed-off-by: Shyam Saini <>
 include/linux/slab.h | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 877a95c6a2d2..8ffdabd218f8 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -127,11 +127,16 @@
  * ZERO_SIZE_PTR can be passed to kfree though in the same way that NULL can.
  * Both make kfree a no-op.
+ * Note: ZERO_SIZE_PTR also cover ERR_PTR Range.
-#define ZERO_SIZE_PTR ((void *)16)
-#define ZERO_OR_NULL_PTR(x) ((unsigned long)(x) <= \
-				(unsigned long)ZERO_SIZE_PTR)
+#define ZERO_SIZE_PTR				\
+({						\
+	(void *)(-MAX_ERRNO-1L);		\
+#define ZERO_OR_NULL_PTR(x) ((unsigned long)(x) - 1 >= \
+		(unsigned long)ZERO_SIZE_PTR - 1)
 #include <linux/kasan.h>

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