Re: [tip:locking/urgent] compiler, atomics: Provide READ_ONCE_NOCHECK ()

[Paul E. McKenney]

On Wed, Oct 14, 2015 at 08:28:43AM -0700, tip-bot for Andrey Ryabinin wrote:
> Commit-ID:  4115ffdf4d6f8986a7abe1dd522c163f599bc0e6
> Gitweb:     http://git.kernel.org/tip/4115ffdf4d6f8986a7abe1dd522c163f599bc0e6
> Author:     Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> AuthorDate: Tue, 13 Oct 2015 18:28:07 +0300
> Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
> CommitDate: Wed, 14 Oct 2015 16:44:06 +0200
> 
> compiler, atomics: 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().
> 
> 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().

So I add READ_ONCE_NOCHECK() for accesses for which the compiler cannot
prove safe address for KASAN's benefit, but READ_ONCE() suffices for
the data-race-detection logic in KTSAN, correct?

							Thanx, Paul

> Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Kostya Serebryany <kcc@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Sasha Levin <sasha.levin@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Wolfram Gloger <wmglo@xxxxxxxxxxxxxxxxxxxxxxxx>
> Cc: kasan-dev <kasan-dev@xxxxxxxxxxxxxxxx>
> Link: http://lkml.kernel.org/r/1444750088-24444-2-git-send-email-aryabinin@xxxxxxxxxxxxx
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> ---
>  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) \
>  ({							\
> 

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