linux-kernel - Re: [PATCH] kcov: rust: add flags for KCOV with Rust

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Message-ID: <CANp29Y4o8o6gz6GbM6NhP9sJUi94q29=aa+tLc1aCk0UVpgj0w@mail.gmail.com>
Date: Wed, 30 Apr 2025 13:55:06 +0200
From: Aleksandr Nogikh <nogikh@...gle.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Dmitry Vyukov <dvyukov@...gle.com>, Andrey Konovalov <andreyknvl@...il.com>, 
	Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
	Miguel Ojeda <ojeda@...nel.org>, Nicolas Schier <nicolas.schier@...ux.dev>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, 
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, kasan-dev@...glegroups.com, 
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, llvm@...ts.linux.dev, 
	Matthew Maurer <mmaurer@...gle.com>
Subject: Re: [PATCH] kcov: rust: add flags for KCOV with Rust

On Wed, Apr 30, 2025 at 10:04 AM Alice Ryhl <aliceryhl@...gle.com> wrote:
>
> Rust code is currently not instrumented properly when KCOV is enabled.
> Thus, add the relevant flags to perform instrumentation correctly. This
> is necessary for efficient fuzzing of Rust code.
>
> The sanitizer-coverage features of LLVM have existed for long enough
> that they are available on any LLVM version supported by rustc, so we do
> not need any Kconfig feature detection.
>
> The coverage level is set to 3, as that is the level needed by trace-pc.
>
> Co-developed-by: Matthew Maurer <mmaurer@...gle.com>
> Signed-off-by: Matthew Maurer <mmaurer@...gle.com>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>

Thanks!

I've run syzkaller against a kernel built with the patch applied and
the tool was able to successfully obtain coverage feedback from the
Rust code, so
Tested-by: Aleksandr Nogikh <nogikh@...gle.com>

As a side note, in the resulting code coverage I also see a lot of PCs
from rustlib, which isn't the primary target when fuzzing the kernel.
Do you find it reasonable not to instrument rustlib with coverage
callbacks? For C code, there do exist some exceptions for KCOV, see
e.g. lib/Makefile.

> ---
>  scripts/Makefile.kcov | 6 ++++++
>  scripts/Makefile.lib  | 3 +++
>  2 files changed, 9 insertions(+)
>
> diff --git a/scripts/Makefile.kcov b/scripts/Makefile.kcov
> index 67e8cfe3474b7dcf7552e675cffe356788e6c3a2..ddcc3c6dc513e1988aeaf07b8efa106e8dffa640 100644
> --- a/scripts/Makefile.kcov
> +++ b/scripts/Makefile.kcov
> @@ -3,4 +3,10 @@ kcov-flags-$(CONFIG_CC_HAS_SANCOV_TRACE_PC)    += -fsanitize-coverage=trace-pc
>  kcov-flags-$(CONFIG_KCOV_ENABLE_COMPARISONS)   += -fsanitize-coverage=trace-cmp
>  kcov-flags-$(CONFIG_GCC_PLUGIN_SANCOV)         += -fplugin=$(objtree)/scripts/gcc-plugins/sancov_plugin.so
>
> +kcov-rflags-y                                  += -Cpasses=sancov-module
> +kcov-rflags-y                                  += -Cllvm-args=-sanitizer-coverage-level=3
> +kcov-rflags-y                                  += -Cllvm-args=-sanitizer-coverage-trace-pc
> +kcov-rflags-$(CONFIG_KCOV_ENABLE_COMPARISONS)  += -Cllvm-args=-sanitizer-coverage-trace-compares
> +
>  export CFLAGS_KCOV := $(kcov-flags-y)
> +export RUSTFLAGS_KCOV := $(kcov-rflags-y)
> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> index 2fe73cda0bddb9dcf709d0a9ae541318d54754d2..520905f19a9b19631394cfb5e129effb8846d5b8 100644
> --- a/scripts/Makefile.lib
> +++ b/scripts/Makefile.lib
> @@ -169,6 +169,9 @@ ifeq ($(CONFIG_KCOV),y)
>  _c_flags += $(if $(patsubst n%,, \
>         $(KCOV_INSTRUMENT_$(target-stem).o)$(KCOV_INSTRUMENT)$(if $(is-kernel-object),$(CONFIG_KCOV_INSTRUMENT_ALL))), \
>         $(CFLAGS_KCOV))
> +_rust_flags += $(if $(patsubst n%,, \
> +       $(KCOV_INSTRUMENT_$(target-stem).o)$(KCOV_INSTRUMENT)$(if $(is-kernel-object),$(CONFIG_KCOV_INSTRUMENT_ALL))), \
> +       $(RUSTFLAGS_KCOV))
>  endif
>
>  #
>
> ---
> base-commit: 9c32cda43eb78f78c73aee4aa344b777714e259b
> change-id: 20250430-rust-kcov-6c74fd0f1f06
>
> Best regards,
> --
> Alice Ryhl <aliceryhl@...gle.com>
>