[PATCH v2] kcov: rust: add flags for KCOV with Rust

[Alice Ryhl <aliceryhl@google.com>]

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.

Co-developed-by: Matthew Maurer <mmaurer@google.com>
Signed-off-by: Matthew Maurer <mmaurer@google.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
I did not pick up the Tested-by due to the changes. I verified that it
looks right under objdump, but I don't have a syzkaller setup I can try
it with.
---
Changes in v2:
- Ignore `core` in KCOV.
- Link to v1: https://lore.kernel.org/r/20250430-rust-kcov-v1-1-b9ae94148175@google.com
---
 rust/Makefile         | 1 +
 scripts/Makefile.kcov | 6 ++++++
 scripts/Makefile.lib  | 3 +++
 3 files changed, 10 insertions(+)

diff --git a/rust/Makefile b/rust/Makefile
index 3aca903a7d08cfbf4d4e0f172dab66e9115001e3..80c84749d734842774a3ac2aabbc944a68d02484 100644
--- 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
 $(obj)/compiler_builtins.o: private rustc_objcopy = -w -W '__*'
diff --git a/scripts/Makefile.kcov b/scripts/Makefile.kcov
index 67e8cfe3474b7dcf7552e675cffe356788e6c3a2..ddcc3c6dc513e1988aeaf07b8efa106e8dffa640 100644
--- a/scripts/Makefile.kcov
+++ b/scripts/Makefile.kcov
@@ -3,4 +3,10 @@ kcov-flags-$(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@google.com>