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(). This patch creates __read_once_size_nocheck() a clone of __read_once_size_check() (renamed __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 <aryabi...@virtuozzo.com> --- include/linux/compiler-gcc.h | 13 ++++++++++ include/linux/compiler.h | 60 ++++++++++++++++++++++++++++++++++---------- 2 files changed, 60 insertions(+), 13 deletions(-) diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h index dfaa7b3..f2a9aec 100644 --- 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. + * Confilcts 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 c836eb2..aa2ae4c 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -198,19 +198,42 @@ 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_check(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 __alias(__read_once_size_check) +void __read_once_size_nocheck(const volatile void *p, void *res, int size); +#endif static __always_inline void __write_once_size(volatile void *p, void *res, int size) { @@ -248,8 +271,19 @@ 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; \ + __read_once_size##check(&(x), __u.__c, sizeof(x)); \ + __u.__val; \ +}) +#define READ_ONCE(x) __READ_ONCE(x, _check) + +/* + * 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, _nocheck) #define WRITE_ONCE(x, val) \ ({ \ -- 2.4.9 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
