compiler, atomics, kasan: Provide READ_ONCE_NOCHECK()

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(). The KernelThreadSanitizer
(KTSAN) is going to ignore it as well.

This patch creates __read_once_size_nocheck() a clone of
__read_once_size(). The only difference between them is
'no_sanitized_address' attribute appended to '*_nocheck'
function. This attribute tells the compiler that 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@virtuozzo.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Kostya Serebryany <kcc@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wolfram Gloger <wmglo@dent.med.uni-muenchen.de>
Cc: kasan-dev <kasan-dev@googlegroups.com>
Link: http://lkml.kernel.org/r/1445243838-17763-2-git-send-email-aryabinin@virtuozzo.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index dfaa7b3e9ae900676b61dc7c3f693c78de97e8cd..8efb40e61d6e48021d68f93635eea8d3ab3e8c0b 100644 (file)
--- 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 (KASAN)
+ * should not be applied to that function.
+ * Conflicts 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 c836eb2dc44d5b3a4d5e98dfbbd274c4448daa90..3d7810341b579795398e36cb15597feb344bb912 100644 (file)
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -198,19 +198,45 @@ 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(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
+/*
+ * This function is not 'inline' because __no_sanitize_address confilcts
+ * with inlining. Attempt to inline it may cause a build failure.
+ *     https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67368
+ * '__maybe_unused' allows us to avoid defined-but-not-used warnings.
+ */
+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
+void __read_once_size_nocheck(const volatile void *p, void *res, int size)
+{
+       __READ_ONCE_SIZE;
 }
+#endif
 
 static __always_inline void __write_once_size(volatile void *p, void *res, int size)
 {
@@ -248,8 +274,22 @@ 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;                    \
+       if (check)                                                      \
+               __read_once_size(&(x), __u.__c, sizeof(x));             \
+       else                                                            \
+               __read_once_size_nocheck(&(x), __u.__c, sizeof(x));     \
+       __u.__val;                                                      \
+})
+#define READ_ONCE(x) __READ_ONCE(x, 1)
+
+/*
+ * Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need
+ * to hide memory access from KASAN.
+ */
+#define READ_ONCE_NOCHECK(x) __READ_ONCE(x, 0)
 
 #define WRITE_ONCE(x, val) \
 ({                                                     \