[PATCH v1 5/5] uaccess: CONFIG_DEBUG_PAGEFAULT_COUNT to debug pagefault_count

David Hildenbrand
2014-12-05T11:18:26Z

This patch introduces CONFIG_DEBUG_PAGEFAULT_COUNT, to detect over-/underflows
in the pagefault_count resulting from a wrong usage of pagefault_enable() and
pagefault_disable().

Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
---
include/linux/uaccess.h | 8 +++++++-
lib/Kconfig.debug | 9 +++++++++
mm/maccess.c | 11 +++++++++++
3 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 1dfc678..6ffb90b 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -20,11 +20,17 @@ static __always_inline void pagefault_count_inc(void)
(*pagefault_count_ptr())++;
}

-static __always_inline void pagefault_count_dec(void)
+static __always_inline void __pagefault_count_dec(void)
{
(*pagefault_count_ptr())--;
}

+#ifdef CONFIG_DEBUG_PAGEFAULT_COUNT
+extern void pagefault_count_dec(void);
+#else
+#define pagefault_count_dec() __pagefault_count_dec()
+#endif
+
/*
* These routines enable/disable the pagefault handler. If disabled, it will
* not take any locks and go straight to the fixup table.
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 4e35a5d..529e9d4 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -860,6 +860,15 @@ config DEBUG_PREEMPT
if kernel code uses it in a preemption-unsafe way. Also, the kernel
will detect preemption count underflows.

+config DEBUG_PAGEFAULT_COUNT
+ bool "Debug pagefault_disable / pagefault_enable"
+ depends on DEBUG_KERNEL
+ default n
+ help
+ If you say Y here then the kernel will detect pagefault count
+ over-/underflows and therefore non-matching pagefault_enable() and
+ pagefault_disable() calls.
+
menu "Lock Debugging (spinlocks, mutexes, etc...)"

config DEBUG_RT_MUTEXES
diff --git a/mm/maccess.c b/mm/maccess.c
index d53adf9..4b72aa1 100644
--- a/mm/maccess.c
+++ b/mm/maccess.c
@@ -5,6 +5,17 @@
#include <linux/mm.h>
#include <linux/uaccess.h>

+#ifdef CONFIG_DEBUG_PAGEFAULT_COUNT
+void pagefault_count_dec(void)
+{
+ __pagefault_count_dec();
+
+ /* underflow / previous overflow ? */
+ WARN_ON(pagefault_count() < 0);
+}
+EXPORT_SYMBOL(pagefault_count_dec);
+#endif
+
/**
* probe_kernel_read(): safely attempt to read from a location
* @dst: pointer to the buffer that shall take the data
--
1.8.5.5