kcov: rust: add flags for KCOV with Rust

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.

We do not instrument `core` since when we fuzz the kernel, we are
looking for bugs in the kernel, not the Rust stdlib.

Link: https://lkml.kernel.org/r/20250501-rust-kcov-v2-1-b71e83e9779f@google.com
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Co-developed-by: Matthew Maurer <mmaurer@google.com>
Signed-off-by: Matthew Maurer <mmaurer@google.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Tested-by: Aleksandr Nogikh <nogikh@google.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Cc: Andreas Hindborg <a.hindborg@kernel.org>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Benno Lossin <benno.lossin@proton.me>
Cc: Bill Wendling <morbo@google.com>
Cc: Björn Roy Baron <bjorn3_gh@protonmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Cc: Gary Guo <gary@garyguo.net>
Cc: Justin Stitt <justinstitt@google.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Trevor Gross <tmgross@umich.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/rust/Makefile b/rust/Makefile
index 3aca903a7d08cfbf4d4e0f172dab66e9115001e3..80c84749d734842774a3ac2aabbc944a68d02484 100644 (file)
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -492,6 +492,7 @@ $(obj)/core.o: $(RUST_LIB_SRC)/core/src/lib.rs \
 ifneq ($(or $(CONFIG_X86_64),$(CONFIG_X86_32)),)
 $(obj)/core.o: scripts/target.json
 endif
+KCOV_INSTRUMENT_core.o := n
 
 $(obj)/compiler_builtins.o: private skip_gendwarfksyms = 1
 $(obj)/compiler_builtins.o: private rustc_objcopy = -w -W '__*'

diff --git a/scripts/Makefile.kcov b/scripts/Makefile.kcov
index 67e8cfe3474b7dcf7552e675cffe356788e6c3a2..ddcc3c6dc513e1988aeaf07b8efa106e8dffa640 100644 (file)
--- 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 (file)
--- 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
 
 #