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    Subject[PATCH -tip v3 02/11] kcsan: Avoid inserting __tsan_func_entry/exit if possible
    From
    To avoid inserting  __tsan_func_{entry,exit}, add option if supported by
    compiler. Currently only Clang can be told to not emit calls to these
    functions. It is safe to not emit these, since KCSAN does not rely on
    them.

    Note that, if we disable __tsan_func_{entry,exit}(), we need to disable
    tail-call optimization in sanitized compilation units, as otherwise we
    may skip frames in the stack trace; in particular when the tail called
    function is one of the KCSAN's runtime functions, and a report is
    generated, might we miss the function where the actual access occurred.
    Since __tsan_func_{entry,exit}() insertion effectively disabled
    tail-call optimization, there should be no observable change. [This was
    caught and confirmed with kcsan-test & UNWINDER_ORC.]

    Acked-by: Will Deacon <will@kernel.org>
    Signed-off-by: Marco Elver <elver@google.com>
    ---
    scripts/Makefile.kcsan | 11 ++++++++++-
    1 file changed, 10 insertions(+), 1 deletion(-)

    diff --git a/scripts/Makefile.kcsan b/scripts/Makefile.kcsan
    index caf1111a28ae..20337a7ecf54 100644
    --- a/scripts/Makefile.kcsan
    +++ b/scripts/Makefile.kcsan
    @@ -1,6 +1,15 @@
    # SPDX-License-Identifier: GPL-2.0
    ifdef CONFIG_KCSAN

    -CFLAGS_KCSAN := -fsanitize=thread
    +# GCC and Clang accept backend options differently. Do not wrap in cc-option,
    +# because Clang accepts "--param" even if it is unused.
    +ifdef CONFIG_CC_IS_CLANG
    +cc-param = -mllvm -$(1)
    +else
    +cc-param = --param -$(1)
    +endif
    +
    +CFLAGS_KCSAN := -fsanitize=thread \
    + $(call cc-option,$(call cc-param,tsan-instrument-func-entry-exit=0) -fno-optimize-sibling-calls)

    endif # CONFIG_KCSAN
    --
    2.26.2.761.g0e0b3e54be-goog
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