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SubjectRe: READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops))
On Thu, Dec 12, 2019 at 10:07:56AM +0000, Will Deacon wrote:

> > So your proposed change _should_ be fine. Will, I'm assuming you never
> > saw this on your ARGH64 builds when you did this code ?
>
> I did see it, but (a) looking at the code out-of-line makes it look a lot
> worse than it actually is (so the ext4 example is really helpful -- thanks
> Michael!) and (b) I chalked it up to a crappy compiler.
>
> However, see this comment from Arnd on my READ_ONCE series from the other
> day:
>
> https://lore.kernel.org/lkml/CAK8P3a0f=WvSQSBQ4t0FmEkcFE_mC3oARxaeTviTSkSa-D2qhg@mail.gmail.com
>
> In which case, I'm thinking that we should be doing better in READ_ONCE()
> for non-buggy compilers which would also keep the KCSAN folks happy for this
> code (and would help with [1] too).

So something like this then? Although I suppose that should be moved
into compiler-gcc.h and then guarded by #ifndef READ_ONCE or so.

---

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index ad8c76144a3c..8326e2cf28b4 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -179,20 +179,8 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,

#include <uapi/linux/types.h>
#include <linux/kcsan-checks.h>
-
-#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(); \
- } \
-})
+#include <asm/barrier.h>
+#include <linux/kasan-checks.h>

#ifdef CONFIG_KASAN
/*
@@ -222,6 +210,22 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
#define __no_sanitize_or_inline __always_inline
#endif

+#ifdef GCC_VERSION < 40800
+
+#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 __no_kcsan_or_inline
void __read_once_size(const volatile void *p, void *res, int size)
{
@@ -274,9 +278,6 @@ void __write_once_size(volatile void *p, void *res, int size)
* with an explicit memory barrier or atomic instruction that provides the
* required ordering.
*/
-#include <asm/barrier.h>
-#include <linux/kasan-checks.h>
-
#define __READ_ONCE(x, check) \
({ \
union { typeof(x) __val; char __c[1]; } __u; \
@@ -295,6 +296,23 @@ void __write_once_size(volatile void *p, void *res, int size)
*/
#define READ_ONCE_NOCHECK(x) __READ_ONCE(x, 0)

+#else /* GCC_VERSION < 40800 */
+
+#define READ_ONCE_NOCHECK(x) \
+({ \
+ typeof(x) __x = *(volatile typeof(x))&(x); \
+ smp_read_barrier_depends(); \
+ __x;
+})
+
+#define READ_ONCE(x) \
+({ \
+ kcsan_check_atomic_read(&(x), sizeof(x)); \
+ READ_ONCE_NOCHECK(x); \
+})
+
+#endif /* GCC_VERSION < 40800 */
+
static __no_kasan_or_inline
unsigned long read_word_at_a_time(const void *addr)
{
\
 
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