[Devel] [PATCH RHEL7 COMMIT] ms/compiler, atomics, kasan: Provide READ_ONCE_NOCHECK()

Konstantin Khorenko khorenko at virtuozzo.com
Wed Nov 18 01:23:16 PST 2015


The commit is pushed to "branch-rh7-3.10.0-229.7.2.vz7.9.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-229.7.2.vz7.9.10
------>
commit b3e5741aa9311d79ad1bc55a79af596c8d7cc7bc
Author: Andrey Ryabinin <aryabinin at virtuozzo.com>
Date:   Wed Nov 18 13:23:16 2015 +0400

    ms/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 at virtuozzo.com>
    Cc: Alexander Potapenko <glider at google.com>
    Cc: Andrew Morton <akpm at linux-foundation.org>
    Cc: Andrey Konovalov <andreyknvl at google.com>
    Cc: Andy Lutomirski <luto at amacapital.net>
    Cc: Borislav Petkov <bp at alien8.de>
    Cc: Denys Vlasenko <dvlasenk at redhat.com>
    Cc: Dmitry Vyukov <dvyukov at google.com>
    Cc: Kostya Serebryany <kcc at google.com>
    Cc: Linus Torvalds <torvalds at linux-foundation.org>
    Cc: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
    Cc: Peter Zijlstra <peterz at infradead.org>
    Cc: Sasha Levin <sasha.levin at oracle.com>
    Cc: Thomas Gleixner <tglx at linutronix.de>
    Cc: Wolfram Gloger <wmglo at dent.med.uni-muenchen.de>
    Cc: kasan-dev <kasan-dev at googlegroups.com>
    Link: http://lkml.kernel.org/r/1445243838-17763-2-git-send-email-aryabinin@virtuozzo.com
    Signed-off-by: Ingo Molnar <mingo at kernel.org>
    (cherry picked from commit d976441f44bc5d48635d081d277aa76556ffbf8b)
    Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
    
    Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 include/linux/compiler-gcc.h | 13 +++++++++
 include/linux/compiler.h     | 66 +++++++++++++++++++++++++++++++++++---------
 2 files changed, 66 insertions(+), 13 deletions(-)

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 637aeaf..b6a10cc 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -213,12 +213,25 @@
 #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 2c251f2..ae6d069 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -177,20 +177,46 @@ 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 __assign_once_size(volatile void *p, void *res, int size)
 {
 	switch (size) {
@@ -227,8 +253,22 @@ static __always_inline void __assign_once_size(volatile void *p, void *res, int
  * 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 ASSIGN_ONCE(val, x) \
 	({ typeof(x) __val; __val = val; __assign_once_size(&x, &__val, sizeof(__val)); __val; })


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