Re: [PATCH v2] kcov: add __no_sanitize_coverage to fix noinstr for all architectures

[Nick Desaulniers <ndesaulniers@google.com>]

On Thu, May 27, 2021 at 9:27 AM Marco Elver <elver@google.com> 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 <elver@google.com>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> 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 | 11 +++++++++++
>  include/linux/compiler-gcc.h   |  6 ++++++
>  include/linux/compiler_types.h |  2 +-
>  3 files changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> index adbe76b203e2..e15eebfa8e5d 100644
> --- a/include/linux/compiler-clang.h
> +++ b/include/linux/compiler-clang.h
> @@ -45,6 +45,17 @@
>  #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.

I'm not against a version check for supporting older releases (in
addition to the cleaner feature check, since the feature check was
non-existent); we can clean it up someday when clang-13 is the
minimally supported version.  Would having an additional version check
help support existing/older releases here?

> + */
> +#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__ */
>
> --
> 2.31.1.818.g46aad6cb9e-goog
>


-- 
Thanks,
~Nick Desaulniers