On 5/27/2021 12:44 PM, Marco Elver wrote:
> Until now no compiler supported an attribute to disable coverage
> instrumentation as used by KCOV.
>
> To work around this limitation on x86, noinstr functions have their
> coverage instrumentation turned into nops by objtool. However, this
> solution doesn't scale automatically to other architectures, such as
> arm64, which are migrating to use the generic entry code.
>
> Clang [1] and GCC [2] have added support for the attribute recently.
> [1] https://github.com/llvm/llvm-project/commit/280333021e9550d80f5c1152a34e33e81df1e178
> [2] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=cec4d4a6782c9bd8d071839c50a239c49caca689
> The changes will appear in Clang 13 and GCC 12.
>
> Add __no_sanitize_coverage for both compilers, and add it to noinstr.
>
> Note: In the Clang case, __has_feature(coverage_sanitizer) is only true
> if the feature is enabled, and therefore we do not require an additional
> defined(CONFIG_KCOV) (like in the GCC case where __has_attribute(..) is
> always true) to avoid adding redundant attributes to functions if KCOV
> is off. That being said, compilers that support the attribute will not
> generate errors/warnings if the attribute is redundantly used; however,
> where possible let's avoid it as it reduces preprocessed code size and
> associated compile-time overheads.
>
> Signed-off-by: Marco Elver <[email protected]>
> Acked-by: Peter Zijlstra (Intel) <[email protected]>
> Reviewed-by: Miguel Ojeda <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
> ---
> v3:
> * Add comment explaining __has_feature() in Clang.
> * Add Miguel's Reviewed-by.
>
> v2:
> * Implement __has_feature(coverage_sanitizer) in Clang
> (https://reviews.llvm.org/D103159) and use instead of version check.
> * Add Peter's Ack.
> ---
> include/linux/compiler-clang.h | 17 +++++++++++++++++
> include/linux/compiler-gcc.h | 6 ++++++
> include/linux/compiler_types.h | 2 +-
> 3 files changed, 24 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> index adbe76b203e2..49b0ac8b6fd3 100644
> --- a/include/linux/compiler-clang.h
> +++ b/include/linux/compiler-clang.h
> @@ -13,6 +13,12 @@
> /* all clang versions usable with the kernel support KASAN ABI version 5 */
> #define KASAN_ABI_VERSION 5
>
> +/*
> + * Note: Checking __has_feature(*_sanitizer) is only true if the feature is
> + * enabled. Therefore it is not required to additionally check defined(CONFIG_*)
> + * to avoid adding redundant attributes in other configurations.
> + */
> +
> #if __has_feature(address_sanitizer) || __has_feature(hwaddress_sanitizer)
> /* Emulate GCC's __SANITIZE_ADDRESS__ flag */
> #define __SANITIZE_ADDRESS__
> @@ -45,6 +51,17 @@
> #define __no_sanitize_undefined
> #endif
>
> +/*
> + * Support for __has_feature(coverage_sanitizer) was added in Clang 13 together
> + * with no_sanitize("coverage"). Prior versions of Clang support coverage
> + * instrumentation, but cannot be queried for support by the preprocessor.
> + */
> +#if __has_feature(coverage_sanitizer)
> +#define __no_sanitize_coverage __attribute__((no_sanitize("coverage")))
> +#else
> +#define __no_sanitize_coverage
> +#endif
> +
> /*
> * Not all versions of clang implement the type-generic versions
> * of the builtin overflow checkers. Fortunately, clang implements
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> index 5d97ef738a57..cb9217fc60af 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -122,6 +122,12 @@
> #define __no_sanitize_undefined
> #endif
>
> +#if defined(CONFIG_KCOV) && __has_attribute(__no_sanitize_coverage__)
> +#define __no_sanitize_coverage __attribute__((no_sanitize_coverage))
> +#else
> +#define __no_sanitize_coverage
> +#endif
> +
> #if GCC_VERSION >= 50100
> #define COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW 1
> #endif
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index d29bda7f6ebd..cc2bee7f0977 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -210,7 +210,7 @@ struct ftrace_likely_data {
> /* Section for code which can't be instrumented at all */
> #define noinstr \
> noinline notrace __attribute((__section__(".noinstr.text"))) \
> - __no_kcsan __no_sanitize_address
> + __no_kcsan __no_sanitize_address __no_sanitize_coverage
>
> #endif /* __KERNEL__ */
>
>