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Date:   Wed, 13 Jul 2022 12:04:46 +0200
From:   Marco Elver <elver@google.com>
To:     Alexander Potapenko <glider@google.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Alexei Starovoitov <ast@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Dmitry Vyukov <dvyukov@google.com>,
        Eric Dumazet <edumazet@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Ilya Leoshkevich <iii@linux.ibm.com>,
        Ingo Molnar <mingo@redhat.com>, Jens Axboe <axboe@kernel.dk>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
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        Vegard Nossum <vegard.nossum@oracle.com>,
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        linux-mm@kvack.org, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 11/45] kmsan: add KMSAN runtime core
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References: <20220701142310.2188015-1-glider@google.com>
 <20220701142310.2188015-12-glider@google.com>
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On Fri, Jul 01, 2022 at 04:22PM +0200, 'Alexander Potapenko' via kasan-dev wrote:
[...]
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 2e24db4bff192..59819e6fa5865 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -963,6 +963,7 @@ config DEBUG_STACKOVERFLOW
>  
>  source "lib/Kconfig.kasan"
>  source "lib/Kconfig.kfence"
> +source "lib/Kconfig.kmsan"
>  
>  endmenu # "Memory Debugging"
>  
> diff --git a/lib/Kconfig.kmsan b/lib/Kconfig.kmsan
> new file mode 100644
> index 0000000000000..8f768d4034e3c
> --- /dev/null
> +++ b/lib/Kconfig.kmsan
> @@ -0,0 +1,50 @@
> +# SPDX-License-Identifier: GPL-2.0-only
> +config HAVE_ARCH_KMSAN
> +	bool
> +
> +config HAVE_KMSAN_COMPILER
> +	# Clang versions <14.0.0 also support -fsanitize=kernel-memory, but not
> +	# all the features necessary to build the kernel with KMSAN.
> +	depends on CC_IS_CLANG && CLANG_VERSION >= 140000
> +	def_bool $(cc-option,-fsanitize=kernel-memory -mllvm -msan-disable-checks=1)
> +
> +config HAVE_KMSAN_PARAM_RETVAL
> +	# Separate check for -fsanitize-memory-param-retval support.

This comment doesn't add much value, maybe instead say that "Supported
only by Clang >= 15."

> +	depends on CC_IS_CLANG && CLANG_VERSION >= 140000

Why not just "depends on HAVE_KMSAN_COMPILER"? (All
fsanitize-memory-param-retval supporting compilers must also be KMSAN
compilers.)

> +	def_bool $(cc-option,-fsanitize=kernel-memory -fsanitize-memory-param-retval)
> +
> +

HAVE_KMSAN_PARAM_RETVAL should be moved under "if KMSAN" so that this
isn't unnecessarily evaluated in every kernel build (saving 1 shelling
out to clang in most builds).

> +config KMSAN
> +	bool "KMSAN: detector of uninitialized values use"
> +	depends on HAVE_ARCH_KMSAN && HAVE_KMSAN_COMPILER
> +	depends on SLUB && DEBUG_KERNEL && !KASAN && !KCSAN
> +	select STACKDEPOT
> +	select STACKDEPOT_ALWAYS_INIT
> +	help
> +	  KernelMemorySanitizer (KMSAN) is a dynamic detector of uses of
> +	  uninitialized values in the kernel. It is based on compiler
> +	  instrumentation provided by Clang and thus requires Clang to build.
> +
> +	  An important note is that KMSAN is not intended for production use,
> +	  because it drastically increases kernel memory footprint and slows
> +	  the whole system down.
> +
> +	  See <file:Documentation/dev-tools/kmsan.rst> for more details.
> +
> +if KMSAN
> +
> +config KMSAN_CHECK_PARAM_RETVAL
> +	bool "Check for uninitialized values passed to and returned from functions"
> +	default HAVE_KMSAN_PARAM_RETVAL

This can be enabled even if !HAVE_KMSAN_PARAM_RETVAL. Should this be:

	default y
	depends on HAVE_KMSAN_PARAM_RETVAL

instead?

> +	help
> +	  If the compiler supports -fsanitize-memory-param-retval, KMSAN will
> +	  eagerly check every function parameter passed by value and every
> +	  function return value.
> +
> +	  Disabling KMSAN_CHECK_PARAM_RETVAL will result in tracking shadow for
> +	  function parameters and return values across function borders. This
> +	  is a more relaxed mode, but it generates more instrumentation code and
> +	  may potentially report errors in corner cases when non-instrumented
> +	  functions call instrumented ones.
> +