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From: "tip-bot2 for Will Deacon" <tip-bot2@linutronix.de>
To: linux-tip-commits@vger.kernel.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Will Deacon <will@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Arnd Bergmann <arnd@arndb.de>, x86 <x86@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: [tip: locking/kcsan] READ_ONCE: Enforce atomicity for {READ,WRITE}_ONCE() memory accesses
Date: Tue, 12 May 2020 14:36:56 -0000	[thread overview]
Message-ID: <158929421627.390.5707699650139698296.tip-bot2@tip-bot2> (raw)
In-Reply-To: <20200511204150.27858-12-will@kernel.org>

The following commit has been merged into the locking/kcsan branch of tip:

Commit-ID:     2ab3a0a02905d9994746dc4692c010d47b2beb74
Gitweb:        https://git.kernel.org/tip/2ab3a0a02905d9994746dc4692c010d47b2beb74
Author:        Will Deacon <will@kernel.org>
AuthorDate:    Mon, 11 May 2020 21:41:43 +01:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Tue, 12 May 2020 11:04:14 +02:00

READ_ONCE: Enforce atomicity for {READ,WRITE}_ONCE() memory accesses

{READ,WRITE}_ONCE() cannot guarantee atomicity for arbitrary data sizes.
This can be surprising to callers that might incorrectly be expecting
atomicity for accesses to aggregate structures, although there are other
callers where tearing is actually permissable (e.g. if they are using
something akin to sequence locking to protect the access).

Linus sayeth:

  | We could also look at being stricter for the normal READ/WRITE_ONCE(),
  | and require that they are
  |
  | (a) regular integer types
  |
  | (b) fit in an atomic word
  |
  | We actually did (b) for a while, until we noticed that we do it on
  | loff_t's etc and relaxed the rules. But maybe we could have a
  | "non-atomic" version of READ/WRITE_ONCE() that is used for the
  | questionable cases?

The slight snag is that we also have to support 64-bit accesses on 32-bit
architectures, as these appear to be widespread and tend to work out ok
if either the architecture supports atomic 64-bit accesses (armv7) or if
the variable being accessed represents a virtual address and therefore
only requires 32-bit atomicity in practice.

Take a step in that direction by introducing a variant of
'compiletime_assert_atomic_type()' and use it to check the pointer
argument to {READ,WRITE}_ONCE(). Expose __{READ,WRITE}_ONCE() variants
which are allowed to tear and convert the one broken caller over to the
new macros.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Arnd Bergmann <arnd@arndb.de>
Link: https://lkml.kernel.org/r/20200511204150.27858-12-will@kernel.org

---
 drivers/xen/time.c       |  2 +-
 include/linux/compiler.h | 40 +++++++++++++++++++++++++++++++++++----
 2 files changed, 37 insertions(+), 5 deletions(-)

diff --git a/drivers/xen/time.c b/drivers/xen/time.c
index 0968859..108edbc 100644
--- a/drivers/xen/time.c
+++ b/drivers/xen/time.c
@@ -64,7 +64,7 @@ static void xen_get_runstate_snapshot_cpu_delta(
 	do {
 		state_time = get64(&state->state_entry_time);
 		rmb();	/* Hypervisor might update data. */
-		*res = READ_ONCE(*state);
+		*res = __READ_ONCE(*state);
 		rmb();	/* Hypervisor might update data. */
 	} while (get64(&state->state_entry_time) != state_time ||
 		 (state_time & XEN_RUNSTATE_UPDATE));
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 1b4e64d..733605f 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -199,9 +199,14 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 #include <linux/kasan-checks.h>
 #include <linux/kcsan-checks.h>
 
-#define __READ_ONCE(x)	(*(volatile typeof(x) *)&(x))
+/*
+ * Use __READ_ONCE() instead of READ_ONCE() if you do not require any
+ * atomicity or dependency ordering guarantees. Note that this may result
+ * in tears!
+ */
+#define __READ_ONCE(x)	(*(const volatile typeof(x) *)&(x))
 
-#define READ_ONCE(x)							\
+#define __READ_ONCE_SCALAR(x)						\
 ({									\
 	typeof(x) *__xp = &(x);						\
 	kcsan_check_atomic_read(__xp, sizeof(*__xp));			\
@@ -214,15 +219,32 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 	});								\
 })
 
-#define WRITE_ONCE(x, val)						\
+#define READ_ONCE(x)							\
+({									\
+	compiletime_assert_rwonce_type(x);				\
+	__READ_ONCE_SCALAR(x);						\
+})
+
+#define __WRITE_ONCE(x, val)						\
+do {									\
+	*(volatile typeof(x) *)&(x) = (val);				\
+} while (0)
+
+#define __WRITE_ONCE_SCALAR(x, val)					\
 do {									\
 	typeof(x) *__xp = &(x);						\
 	kcsan_check_atomic_write(__xp, sizeof(*__xp));			\
 	__kcsan_disable_current();					\
-	*(volatile typeof(x) *)__xp = (val);				\
+	__WRITE_ONCE(*__xp, val);					\
 	__kcsan_enable_current();					\
 } while (0)
 
+#define WRITE_ONCE(x, val)						\
+do {									\
+	compiletime_assert_rwonce_type(x);				\
+	__WRITE_ONCE_SCALAR(x, val);					\
+} while (0)
+
 #ifdef CONFIG_KASAN
 /*
  * We can't declare function 'inline' because __no_sanitize_address conflicts
@@ -365,6 +387,16 @@ static inline void *offset_to_ptr(const int *off)
 	compiletime_assert(__native_word(t),				\
 		"Need native word sized stores/loads for atomicity.")
 
+/*
+ * Yes, this permits 64-bit accesses on 32-bit architectures. These will
+ * actually be atomic in many cases (namely x86), but for others we rely on
+ * the access being split into 2x32-bit accesses for a 32-bit quantity (e.g.
+ * a virtual address) and a strong prevailing wind.
+ */
+#define compiletime_assert_rwonce_type(t)					\
+	compiletime_assert(__native_word(t) || sizeof(t) == sizeof(long long),	\
+		"Unsupported access size for {READ,WRITE}_ONCE().")
+
 /* &a[0] degrades to a pointer: a different type from an array */
 #define __must_be_array(a)	BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))
 

  reply	other threads:[~2020-05-12 14:37 UTC|newest]

Thread overview: 127+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-11 20:41 [PATCH v5 00/18] Rework READ_ONCE() to improve codegen Will Deacon
2020-05-11 20:41 ` [PATCH v5 01/18] sparc32: mm: Fix argument checking in __srmmu_get_nocache() Will Deacon
2020-05-12 14:37   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 02/18] sparc32: mm: Restructure sparc32 MMU page-table layout Will Deacon
2020-05-12 14:37   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 03/18] sparc32: mm: Change pgtable_t type to pte_t * instead of struct page * Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 04/18] sparc32: mm: Reduce allocation size for PMD and PTE tables Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-17  0:00   ` [PATCH v5 04/18] " Guenter Roeck
2020-05-17  0:07     ` Guenter Roeck
2020-05-18  8:37       ` Will Deacon
2020-05-18  9:18         ` Mike Rapoport
2020-05-18  9:48         ` Guenter Roeck
2020-05-18 14:23           ` Mike Rapoport
2020-05-18 16:08             ` Guenter Roeck
2020-05-18 18:11               ` Ira Weiny
2020-05-18 18:14               ` Ira Weiny
2020-05-18 18:09             ` Guenter Roeck
2020-05-18 18:21               ` Ira Weiny
2020-05-18 19:15               ` Mike Rapoport
2020-05-19 16:40                 ` Guenter Roeck
2020-05-20 17:03         ` Mike Rapoport
2020-05-20 19:03           ` Guenter Roeck
2020-05-20 19:51             ` Mike Rapoport
2020-05-21 23:02               ` Guenter Roeck
2020-05-24 12:32                 ` Mike Rapoport
2020-05-24 14:01                   ` Guenter Roeck
2020-05-26 13:26                   ` Will Deacon
2020-05-26 14:01                     ` Will Deacon
2020-05-26 15:21                       ` Mike Rapoport
2020-05-26 16:18                       ` Guenter Roeck
2020-05-26 16:29                         ` Mike Rapoport
2020-05-26 17:15                           ` Guenter Roeck
2020-05-11 20:41 ` [PATCH v5 05/18] compiler/gcc: Raise minimum GCC version for kernel builds to 4.8 Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 06/18] netfilter: Avoid assigning 'const' pointer to non-const pointer Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 07/18] net: tls: " Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 08/18] fault_inject: Don't rely on "return value" from WRITE_ONCE() Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 09/18] arm64: csum: Disable KASAN for do_csum() Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 10/18] READ_ONCE: Simplify implementations of {READ,WRITE}_ONCE() Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 11/18] READ_ONCE: Enforce atomicity for {READ,WRITE}_ONCE() memory accesses Will Deacon
2020-05-12 14:36   ` tip-bot2 for Will Deacon [this message]
2020-05-11 20:41 ` [PATCH v5 12/18] READ_ONCE: Drop pointer qualifiers when reading from scalar types Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 13/18] locking/barriers: Use '__unqual_scalar_typeof' for load-acquire macros Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 14/18] arm64: barrier: Use '__unqual_scalar_typeof' for acquire/release macros Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 15/18] gcov: Remove old GCC 3.4 support Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 16/18] kcsan: Rework data_race() so that it can be used by READ_ONCE() Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 17/18] READ_ONCE: Use data_race() to avoid KCSAN instrumentation Will Deacon
2020-05-12  8:23   ` Peter Zijlstra
2020-05-12  9:49     ` Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-20 22:17     ` Borislav Petkov
2020-05-20 22:30       ` Marco Elver
2020-05-21  7:25         ` Borislav Petkov
2020-05-21  9:37           ` Marco Elver
2020-05-21  3:30       ` Nathan Chancellor
2020-05-22 16:08       ` [tip: locking/kcsan] compiler.h: Avoid nested statement expression in data_race() tip-bot2 for Marco Elver
2020-05-11 20:41 ` [PATCH v5 18/18] linux/compiler.h: Remove redundant '#else' Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-12  8:18 ` [PATCH v5 00/18] Rework READ_ONCE() to improve codegen Peter Zijlstra
2020-05-12 17:53   ` Marco Elver
2020-05-12 18:55     ` Marco Elver
2020-05-12 19:07     ` Peter Zijlstra
2020-05-12 20:31       ` Marco Elver
2020-05-13 11:10         ` Peter Zijlstra
2020-05-13 11:14           ` Peter Zijlstra
2020-05-13 11:48           ` Marco Elver
2020-05-13 12:32             ` Peter Zijlstra
2020-05-13 12:40               ` Will Deacon
2020-05-13 13:15                 ` Marco Elver
2020-05-13 13:24                   ` Peter Zijlstra
2020-05-13 13:58                     ` Marco Elver
2020-05-14 11:21                       ` Peter Zijlstra
2020-05-14 11:24                         ` Peter Zijlstra
2020-05-14 11:35                         ` Peter Zijlstra
2020-05-14 12:01                         ` Will Deacon
2020-05-14 12:27                           ` Peter Zijlstra
2020-05-14 13:07                             ` Marco Elver
2020-05-14 13:14                               ` Peter Zijlstra
2020-05-14 12:20                         ` Peter Zijlstra
2020-05-14 14:13                       ` Peter Zijlstra
2020-05-14 14:20                         ` Marco Elver
2020-05-15  9:20                           ` Peter Zijlstra
2020-05-13 16:50                   ` Will Deacon
2020-05-13 17:32                     ` Marco Elver
2020-05-13 17:47                       ` Will Deacon
2020-05-13 18:54                         ` Marco Elver
2020-05-13 21:25                           ` Will Deacon
2020-05-14  7:31                             ` Marco Elver
2020-05-14 11:05                               ` Will Deacon
2020-05-14 13:35                                 ` Marco Elver
2020-05-14 13:47                                   ` Peter Zijlstra
2020-05-14 13:50                                   ` Peter Zijlstra
2020-05-14 13:56                                   ` Peter Zijlstra
2020-05-14 14:24                                   ` Peter Zijlstra
2020-05-14 15:09                                     ` Thomas Gleixner
2020-05-14 15:29                                       ` Marco Elver
2020-05-14 19:37                                         ` Thomas Gleixner
2020-05-15 13:55                                     ` David Laight
2020-05-15 14:04                                       ` Marco Elver
2020-05-15 14:07                                       ` Peter Zijlstra
2020-05-14 15:38                                   ` Paul E. McKenney
2020-05-22 16:08                                   ` [tip: locking/kcsan] kcsan: Restrict supported compilers tip-bot2 for Marco Elver
2020-06-03 18:52                                 ` [PATCH v5 00/18] Rework READ_ONCE() to improve codegen Borislav Petkov
2020-06-03 19:23                                   ` Marco Elver
2020-06-03 22:05                                     ` Borislav Petkov
2020-06-08 17:32                                     ` Martin Liška
2020-06-08 19:56                                       ` Marco Elver
2020-06-09 11:55                                         ` Martin Liška
2020-06-09 12:36                                           ` Martin Liška
2020-06-09 13:45                                             ` Marco Elver
2020-05-22 16:08                           ` [tip: locking/kcsan] kcsan: Remove 'noinline' from __no_kcsan_or_inline tip-bot2 for Marco Elver
2020-05-13 13:21                 ` [PATCH v5 00/18] Rework READ_ONCE() to improve codegen David Laight
2020-05-13 16:32                   ` Thomas Gleixner
2020-05-12 21:14       ` Will Deacon
2020-05-12 22:00         ` Marco Elver

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