[PATCH v2 1/2] Provide READ_ONCE_NOCHECK()

From: Andrey Ryabinin
Date: Tue Oct 13 2015 - 08:38:30 EST


Some code may perform racy by design memory reads. This could be harmless,
yet such code may produce KASAN warnings.

To hide such accesses from KASAN this patch introduces READ_ONCE_NOCHECK()
macro. KASAN will not check the memory accessed by READ_ONCE_NOCHECK().

This patch creates __read_once_size_nocheck() a clone of
__read_once_size_check() (renamed __read_once_size()).
The only difference between them is 'no_sanitized_address' attribute
appended to '*_nocheck' function. This attribute tells compiler to not
instrumentation of memory accesses should not be applied to that function.
We declare it as static '__maybe_unsed' because GCC is not capable to
inline such function: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67368

With KASAN=n READ_ONCE_NOCHECK() is just a clone of READ_ONCE().

Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
---
include/linux/compiler-gcc.h | 13 ++++++++++++
include/linux/compiler.h | 49 ++++++++++++++++++++++++++++++++------------
2 files changed, 49 insertions(+), 13 deletions(-)

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index dfaa7b3..7ab09de 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -237,12 +237,25 @@
#define KASAN_ABI_VERSION 3
#endif

+#if GCC_VERSION >= 40902
+/*
+ * Tell the compiler that address safety instrumentation (e.g. KASAN)
+ * should not be applied to that function.
+ * Confilcts with inlining: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67368
+ */
+#define __no_sanitize_address __attribute__((no_sanitize_address))
+#endif
+
#endif /* gcc version >= 40000 specific checks */

#if !defined(__noclone)
#define __noclone /* not needed */
#endif

+#if !defined(__no_sanitize_address)
+#define __no_sanitize_address
+#endif
+
/*
* A trick to suppress uninitialized variable warning without generating any
* code
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index c836eb2..2153fd8 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -198,20 +198,37 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);

#include <uapi/linux/types.h>

-static __always_inline void __read_once_size(const volatile void *p, void *res, int size)
+#define __READ_ONCE_SIZE \
+({ \
+ switch (size) { \
+ case 1: *(__u8 *)res = *(volatile __u8 *)p; break; \
+ case 2: *(__u16 *)res = *(volatile __u16 *)p; break; \
+ case 4: *(__u32 *)res = *(volatile __u32 *)p; break; \
+ case 8: *(__u64 *)res = *(volatile __u64 *)p; break; \
+ default: \
+ barrier(); \
+ __builtin_memcpy((void *)res, (const void *)p, size); \
+ barrier(); \
+ } \
+})
+
+static __always_inline
+void __read_once_size_check(const volatile void *p, void *res, int size)
{
- switch (size) {
- case 1: *(__u8 *)res = *(volatile __u8 *)p; break;
- case 2: *(__u16 *)res = *(volatile __u16 *)p; break;
- case 4: *(__u32 *)res = *(volatile __u32 *)p; break;
- case 8: *(__u64 *)res = *(volatile __u64 *)p; break;
- default:
- barrier();
- __builtin_memcpy((void *)res, (const void *)p, size);
- barrier();
- }
+ __READ_ONCE_SIZE;
}

+#ifdef CONFIG_KASAN
+static __no_sanitize_address __maybe_unused
+void __read_once_size_nocheck(const volatile void *p, void *res, int size)
+{
+ __READ_ONCE_SIZE;
+}
+#else
+static __always_inline __alias(__read_once_size_check)
+void __read_once_size_nocheck(const volatile void *p, void *res, int size);
+#endif
+
static __always_inline void __write_once_size(volatile void *p, void *res, int size)
{
switch (size) {
@@ -248,8 +265,14 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
* required ordering.
*/

-#define READ_ONCE(x) \
- ({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
+#define __READ_ONCE(x, check) \
+({ \
+ union { typeof(x) __val; char __c[1]; } __u; \
+ __read_once_size##check(&(x), __u.__c, sizeof(x)); \
+ __u.__val; \
+})
+#define READ_ONCE(x) __READ_ONCE(x, _check)
+#define READ_ONCE_NOCHECK(x) __READ_ONCE(x, _nocheck)

#define WRITE_ONCE(x, val) \
({ \
--
2.4.9

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