Re: [PATCH 09/43] kmsan: introduce __no_sanitize_memory and __no_kmsan_checks

[Alexander Potapenko <glider@xxxxxxxxxx>]

On Wed, Dec 15, 2021 at 2:27 PM Mark Rutland <mark.rutland@xxxxxxx> wrote:

On Tue, Dec 14, 2021 at 05:20:16PM +0100, Alexander Potapenko wrote:
> __no_sanitize_memory is a function attribute that instructs KMSAN to
> skip a function during instrumentation. This is needed to e.g. implement
> the noinstr functions.
>
> __no_kmsan_checks is a function attribute that makes KMSAN
> ignore the uninitialized values coming from the function's
> inputs, and initialize the function's outputs.
>
> Functions marked with this attribute can't be inlined into functions
> not marked with it, and vice versa.

Just to check, I assume an unmarked __always_inline() function can be inlined
into a marked function? Otherwise this is going to be really painful to manage
for low-level helper functions.

Yes, as a matter of fact __always_inline prevails over the requirement of having the same attribute.

I'll probably need to clarify this in the description.

Thanks,
Mark.

>
> __SANITIZE_MEMORY__ is a macro that's defined iff the file is
> instrumented with KMSAN. This is not the same as CONFIG_KMSAN, which is
> defined for every file.
>
> Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
> ---
> Link: https://linux-review.googlesource.com/id/I004ff0360c918d3cd8b18767ddd1381c6d3281be
> ---
>  include/linux/compiler-clang.h | 23 +++++++++++++++++++++++
>  include/linux/compiler-gcc.h   |  6 ++++++
>  2 files changed, 29 insertions(+)
>
> diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> index 3c4de9b6c6e3e..5f11a6f269e28 100644
> --- a/include/linux/compiler-clang.h
> +++ b/include/linux/compiler-clang.h
> @@ -51,6 +51,29 @@
>  #define __no_sanitize_undefined
>  #endif
> 
> +#if __has_feature(memory_sanitizer)
> +#define __SANITIZE_MEMORY__
> +/*
> + * Unlike other sanitizers, KMSAN still inserts code into functions marked with
> + * no_sanitize("kernel-memory"). Using disable_sanitizer_instrumentation
> + * provides the behavior consistent with other __no_sanitize_ attributes,
> + * guaranteeing that __no_sanitize_memory functions remain uninstrumented.
> + */
> +#define __no_sanitize_memory __disable_sanitizer_instrumentation
> +
> +/*
> + * The __no_kmsan_checks attribute ensures that a function does not produce
> + * false positive reports by:
> + *  - initializing all local variables and memory stores in this function;
> + *  - skipping all shadow checks;
> + *  - passing initialized arguments to this function's callees.
> + */
> +#define __no_kmsan_checks __attribute__((no_sanitize("kernel-memory")))
> +#else
> +#define __no_sanitize_memory
> +#define __no_kmsan_checks
> +#endif
> +
>  /*
>   * Support for __has_feature(coverage_sanitizer) was added in Clang 13 together
>   * with no_sanitize("coverage"). Prior versions of Clang support coverage
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> index ccbbd31b3aae5..f6e69387aad05 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -129,6 +129,12 @@
>  #define __SANITIZE_ADDRESS__
>  #endif
> 
> +/*
> + * GCC does not support KMSAN.
> + */
> +#define __no_sanitize_memory
> +#define __no_kmsan_checks
> +
>  /*
>   * Turn individual warnings and errors on and off locally, depending
>   * on version.
> --
> 2.34.1.173.g76aa8bc2d0-goog
>

--

Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Straße, 33
80636 München

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