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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 Fri, Dec 13, 2019 at 11:47:06AM +0100, Luc Van Oostenryck wrote:
> On Thu, Dec 12, 2019 at 09:53:38PM +0100, Peter Zijlstra wrote:
> > Now, looking at the current GCC source:
> >
> > https://github.com/gcc-mirror/gcc/blob/97d7270f894395e513667a031a0c309d1819d05e/gcc/c/c-parser.c#L3707
> >
> > it seems that __typeof__() is supposed to strip all qualifiers from
> > _Atomic types. That lead me to try:
> >
> > typeof(_Atomic typeof(p)) __p = (p);
> >
> > But alas, I still get the same junk you got for ool_store_release() :/
>
> I was checking this to see if Sparse was ready to support this.
> I was a bit surprised because at first sigth GCC was doing as
> it claims (typeof striping const & volatile on _Atomic types)
> but your exampe wasn't working. But it's working if an
> intermediate var is used:
> _Atomic typeof(p) tmp;
> typeof(tmp) __p = (p);
> or, uglier but probably more practical:
> typeof(({_Atomic typeof(p) tmp; })) __p = (p);
>
> Go figure!

Excellent! I had to change it to something like:

#define unqual_typeof(x) typeof(({_Atomic typeof(x) ___x __maybe_unused; ___x; }))

but that does indeed work!

Now I suppose we should wrap that in a symbol that indicates our
compiler does indeed support _Atomic, otherwise things will come apart.

That is, my gcc-4.6 doesn't seem to have it, while gcc-4.8 does, which
is exactly the range that needs the daft READ_ONCE() construct, how
convenient :/

Something a little like this perhaps?

---

diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index 7d9cc5ec4971..c389af602da8 100644
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -75,9 +75,9 @@ static inline unsigned long array_index_mask_nospec(unsigned long idx,

#define __smp_store_release(p, v) \
do { \
- typeof(p) __p = (p); \
- union { typeof(*p) __val; char __c[1]; } __u = \
- { .__val = (__force typeof(*p)) (v) }; \
+ unqual_typeof(p) __p = (p); \
+ union { unqual_typeof(*p) __val; char __c[1]; } __u = \
+ { .__val = (__force unqual_typeof(*p)) (v) }; \
compiletime_assert_atomic_type(*p); \
kasan_check_write(__p, sizeof(*p)); \
switch (sizeof(*p)) { \
@@ -110,8 +110,8 @@ do { \

#define __smp_load_acquire(p) \
({ \
- union { typeof(*p) __val; char __c[1]; } __u; \
- typeof(p) __p = (p); \
+ union { unqual_typeof(*p) __val; char __c[1]; } __u; \
+ unqual_typeof(p) __p = (p); \
compiletime_assert_atomic_type(*p); \
kasan_check_read(__p, sizeof(*p)); \
switch (sizeof(*p)) { \
@@ -141,8 +141,8 @@ do { \

#define smp_cond_load_relaxed(ptr, cond_expr) \
({ \
- typeof(ptr) __PTR = (ptr); \
- typeof(*ptr) VAL; \
+ unqual_typeof(ptr) __PTR = (ptr); \
+ unqual_typeof(*ptr) VAL; \
for (;;) { \
VAL = READ_ONCE(*__PTR); \
if (cond_expr) \
@@ -154,8 +154,8 @@ do { \

#define smp_cond_load_acquire(ptr, cond_expr) \
({ \
- typeof(ptr) __PTR = (ptr); \
- typeof(*ptr) VAL; \
+ unqual_typeof(ptr) __PTR = (ptr); \
+ unqual_typeof(*ptr) VAL; \
for (;;) { \
VAL = smp_load_acquire(__PTR); \
if (cond_expr) \
diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index 85b28eb80b11..dd5bb055f5ab 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -228,8 +228,8 @@ do { \
*/
#ifndef smp_cond_load_relaxed
#define smp_cond_load_relaxed(ptr, cond_expr) ({ \
- typeof(ptr) __PTR = (ptr); \
- typeof(*ptr) VAL; \
+ unqual_typeof(ptr) __PTR = (ptr); \
+ unqual_typeof(*ptr) VAL; \
for (;;) { \
VAL = READ_ONCE(*__PTR); \
if (cond_expr) \
@@ -250,7 +250,7 @@ do { \
*/
#ifndef smp_cond_load_acquire
#define smp_cond_load_acquire(ptr, cond_expr) ({ \
- typeof(*ptr) _val; \
+ unqual_typeof(*ptr) _val; \
_val = smp_cond_load_relaxed(ptr, cond_expr); \
smp_acquire__after_ctrl_dep(); \
_val; \
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 0eb2a1cc411d..15fd7ea3882a 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -179,3 +179,10 @@
#endif

#define __no_fgcse __attribute__((optimize("-fno-gcse")))
+
+#if GCC_VERSION < 40800
+/*
+ * GCC-4.6 doesn't support _Atomic, which is required to strip qualifiers.
+ */
+#define unqual_typeof(x) typeof(x)
+#endif
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index ad8c76144a3c..9736993f2ba1 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -279,7 +279,7 @@ void __write_once_size(volatile void *p, void *res, int size)

#define __READ_ONCE(x, check) \
({ \
- union { typeof(x) __val; char __c[1]; } __u; \
+ union { unqual_typeof(x) __val; char __c[1]; } __u; \
if (check) \
__read_once_size(&(x), __u.__c, sizeof(x)); \
else \
@@ -302,12 +302,12 @@ unsigned long read_word_at_a_time(const void *addr)
return *(unsigned long *)addr;
}

-#define WRITE_ONCE(x, val) \
-({ \
- union { typeof(x) __val; char __c[1]; } __u = \
- { .__val = (__force typeof(x)) (val) }; \
- __write_once_size(&(x), __u.__c, sizeof(x)); \
- __u.__val; \
+#define WRITE_ONCE(x, val) \
+({ \
+ union { unqual_typeof(x) __val; char __c[1]; } __u = \
+ { .__val = (__force unqual_typeof(x)) (val) }; \
+ __write_once_size(&(x), __u.__c, sizeof(x)); \
+ __u.__val; \
})

#include <linux/kcsan.h>
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 72393a8c1a6c..fe8012c54251 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -243,4 +243,11 @@ struct ftrace_likely_data {
#define __diag_error(compiler, version, option, comment) \
__diag_ ## compiler(version, error, option)

+#ifndef unqual_typeof
+/*
+ * GCC __typeof__() strips all qualifiers from _Atomic types.
+ */
+#define unqual_typeof(x) typeof(({_Atomic typeof(x) ___x __maybe_unused; ___x; }))
+#endif
+
#endif /* __LINUX_COMPILER_TYPES_H */
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