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    Subject[PATCH v2 3/8] build, kcsan: Add KCSAN build exceptions
    From
    This blacklists several compilation units from KCSAN. See the respective
    inline comments for the reasoning.

    Signed-off-by: Marco Elver <elver@google.com>
    ---
    kernel/Makefile | 5 +++++
    kernel/sched/Makefile | 6 ++++++
    mm/Makefile | 8 ++++++++
    3 files changed, 19 insertions(+)

    diff --git a/kernel/Makefile b/kernel/Makefile
    index 74ab46e2ebd1..4a597a68b8bc 100644
    --- a/kernel/Makefile
    +++ b/kernel/Makefile
    @@ -23,6 +23,9 @@ endif
    # Prevents flicker of uninteresting __do_softirq()/__local_bh_disable_ip()
    # in coverage traces.
    KCOV_INSTRUMENT_softirq.o := n
    +# Avoid KCSAN instrumentation in softirq ("No shared variables, all the data
    +# are CPU local" => assume no data-races), to reduce overhead in interrupts.
    +KCSAN_SANITIZE_softirq.o = n
    # These are called from save_stack_trace() on slub debug path,
    # and produce insane amounts of uninteresting coverage.
    KCOV_INSTRUMENT_module.o := n
    @@ -30,6 +33,7 @@ KCOV_INSTRUMENT_extable.o := n
    # Don't self-instrument.
    KCOV_INSTRUMENT_kcov.o := n
    KASAN_SANITIZE_kcov.o := n
    +KCSAN_SANITIZE_kcov.o := n
    CFLAGS_kcov.o := $(call cc-option, -fno-conserve-stack -fno-stack-protector)

    # cond_syscall is currently not LTO compatible
    @@ -118,6 +122,7 @@ obj-$(CONFIG_RSEQ) += rseq.o

    obj-$(CONFIG_GCC_PLUGIN_STACKLEAK) += stackleak.o
    KASAN_SANITIZE_stackleak.o := n
    +KCSAN_SANITIZE_stackleak.o := n
    KCOV_INSTRUMENT_stackleak.o := n

    $(obj)/configs.o: $(obj)/config_data.gz
    diff --git a/kernel/sched/Makefile b/kernel/sched/Makefile
    index 21fb5a5662b5..e9307a9c54e7 100644
    --- a/kernel/sched/Makefile
    +++ b/kernel/sched/Makefile
    @@ -7,6 +7,12 @@ endif
    # that is not a function of syscall inputs. E.g. involuntary context switches.
    KCOV_INSTRUMENT := n

    +# There are numerous races here, however, most of them due to plain accesses.
    +# This would make it even harder for syzbot to find reproducers, because these
    +# bugs trigger without specific input. Disable by default, but should re-enable
    +# eventually.
    +KCSAN_SANITIZE := n
    +
    ifneq ($(CONFIG_SCHED_OMIT_FRAME_POINTER),y)
    # According to Alan Modra <alan@linuxcare.com.au>, the -fno-omit-frame-pointer is
    # needed for x86 only. Why this used to be enabled for all architectures is beyond
    diff --git a/mm/Makefile b/mm/Makefile
    index d996846697ef..33ea0154dd2d 100644
    --- a/mm/Makefile
    +++ b/mm/Makefile
    @@ -7,6 +7,14 @@ KASAN_SANITIZE_slab_common.o := n
    KASAN_SANITIZE_slab.o := n
    KASAN_SANITIZE_slub.o := n

    +# These produce frequent data-race reports: most of them are due to races on
    +# the same word but accesses to different bits of that word. Re-enable KCSAN
    +# for these when we have more consensus on what to do about them.
    +KCSAN_SANITIZE_slab_common.o := n
    +KCSAN_SANITIZE_slab.o := n
    +KCSAN_SANITIZE_slub.o := n
    +KCSAN_SANITIZE_page_alloc.o := n
    +
    # These files are disabled because they produce non-interesting and/or
    # flaky coverage that is not a function of syscall inputs. E.g. slab is out of
    # free pages, or a task is migrated between nodes.
    --
    2.23.0.866.gb869b98d4c-goog
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