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    Subject[PATCH v2 00/23] kcsan: Support detecting a subset of missing memory barriers
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    Detection of some missing memory barriers has been on the KCSAN feature
    wishlist for some time: this series adds support for modeling a subset
    of weak memory as defined by the LKMM, which enables detection of a
    subset of data races due to missing memory barriers.

    KCSAN's approach to detecting missing memory barriers is based on
    modeling access reordering. Each memory access for which a watchpoint is
    set up, is also selected for simulated reordering within the scope of
    its function (at most 1 in-flight access).

    We are limited to modeling the effects of "buffering" (delaying the
    access), since the runtime cannot "prefetch" accesses. Once an access
    has been selected for reordering, it is checked along every other access
    until the end of the function scope. If an appropriate memory barrier is
    encountered, the access will no longer be considered for reordering.

    When the result of a memory operation should be ordered by a barrier,
    KCSAN can then detect data races where the conflict only occurs as a
    result of a missing barrier due to reordering accesses.

    Some more details and an example are captured in the updated
    <Documentation/dev-tools/kcsan.rst>.

    Some light fuzzing with the feature also resulted in a discussion [1]
    around an issue which appears to be allowed, but unlikely in practice.

    [1] https://lkml.kernel.org/r/YRo58c+JGOvec7tc@elver.google.com


    The first half of the series are core KCSAN changes, documentation
    updates, and test changes. The second half adds instrumentation to
    barriers, atomics, bitops, along with enabling barrier instrumentation
    for some currently uninstrumented subsystems. The last two patches are
    objtool changes to add the usual entries to the uaccess whitelist, but
    also instruct objtool to remove memory barrier instrumentation from
    noinstr code (on x86).

    ---

    v2:
    * Rewrite objtool patch after rebase to v5.16-rc1.
    * Note the reason in documentation that address or control dependencies
    do not require special handling.
    * Rename kcsan_atomic_release() to kcsan_atomic_builtin_memorder() to
    avoid confusion.
    * Define kcsan_noinstr as noinline if we rely on objtool nop'ing out
    calls, to avoid things like LTO inlining it.

    v1: https://lore.kernel.org/all/20211005105905.1994700-1-elver@google.com/

    Marco Elver (23):
    kcsan: Refactor reading of instrumented memory
    kcsan: Remove redundant zero-initialization of globals
    kcsan: Avoid checking scoped accesses from nested contexts
    kcsan: Add core support for a subset of weak memory modeling
    kcsan: Add core memory barrier instrumentation functions
    kcsan, kbuild: Add option for barrier instrumentation only
    kcsan: Call scoped accesses reordered in reports
    kcsan: Show location access was reordered to
    kcsan: Document modeling of weak memory
    kcsan: test: Match reordered or normal accesses
    kcsan: test: Add test cases for memory barrier instrumentation
    kcsan: Ignore GCC 11+ warnings about TSan runtime support
    kcsan: selftest: Add test case to check memory barrier instrumentation
    locking/barriers, kcsan: Add instrumentation for barriers
    locking/barriers, kcsan: Support generic instrumentation
    locking/atomics, kcsan: Add instrumentation for barriers
    asm-generic/bitops, kcsan: Add instrumentation for barriers
    x86/barriers, kcsan: Use generic instrumentation for non-smp barriers
    x86/qspinlock, kcsan: Instrument barrier of pv_queued_spin_unlock()
    mm, kcsan: Enable barrier instrumentation
    sched, kcsan: Enable memory barrier instrumentation
    objtool, kcsan: Add memory barrier instrumentation to whitelist
    objtool, kcsan: Remove memory barrier instrumentation from noinstr

    Documentation/dev-tools/kcsan.rst | 76 +++-
    arch/x86/include/asm/barrier.h | 10 +-
    arch/x86/include/asm/qspinlock.h | 1 +
    include/asm-generic/barrier.h | 54 ++-
    .../asm-generic/bitops/instrumented-atomic.h | 3 +
    .../asm-generic/bitops/instrumented-lock.h | 3 +
    include/linux/atomic/atomic-instrumented.h | 135 +++++-
    include/linux/kcsan-checks.h | 51 ++-
    include/linux/kcsan.h | 11 +-
    include/linux/sched.h | 3 +
    include/linux/spinlock.h | 2 +-
    init/init_task.c | 9 +-
    kernel/kcsan/Makefile | 2 +
    kernel/kcsan/core.c | 326 +++++++++++---
    kernel/kcsan/kcsan_test.c | 416 ++++++++++++++++--
    kernel/kcsan/report.c | 51 ++-
    kernel/kcsan/selftest.c | 141 ++++++
    kernel/sched/Makefile | 7 +-
    lib/Kconfig.kcsan | 16 +
    mm/Makefile | 2 +
    scripts/Makefile.kcsan | 15 +-
    scripts/Makefile.lib | 5 +
    scripts/atomic/gen-atomic-instrumented.sh | 41 +-
    tools/objtool/check.c | 41 +-
    tools/objtool/include/objtool/elf.h | 2 +-
    25 files changed, 1248 insertions(+), 175 deletions(-)

    --
    2.34.0.rc2.393.gf8c9666880-goog

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