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    perf: Convert perf_event_context.refcount to refcount_t · 8c94abbb
    Elena Reshetova 提交于
    atomic_t variables are currently used to implement reference
    counters with the following properties:
    
     - counter is initialized to 1 using atomic_set()
     - a resource is freed upon counter reaching zero
     - once counter reaches zero, its further
       increments aren't allowed
     - counter schema uses basic atomic operations
       (set, inc, inc_not_zero, dec_and_test, etc.)
    
    Such atomic variables should be converted to a newly provided
    refcount_t type and API that prevents accidental counter overflows
    and underflows. This is important since overflows and underflows
    can lead to use-after-free situation and be exploitable.
    
    The variable perf_event_context.refcount is used as pure reference counter.
    Convert it to refcount_t and fix up the operations.
    
    ** Important note for maintainers:
    
    Some functions from refcount_t API defined in lib/refcount.c
    have different memory ordering guarantees than their atomic
    counterparts. Please check Documentation/core-api/refcount-vs-atomic.rst
    for more information.
    
    Normally the differences should not matter since refcount_t provides
    enough guarantees to satisfy the refcounting use cases, but in
    some rare cases it might matter.
    Please double check that you don't have some undocumented
    memory guarantees for this variable usage.
    
    For the perf_event_context.refcount it might make a difference
    in following places:
    
     - get_ctx(), perf_event_ctx_lock_nested(), perf_lock_task_context()
       and __perf_event_ctx_lock_double(): increment in
       refcount_inc_not_zero() only guarantees control dependency
       on success vs. fully ordered atomic counterpart
     - put_ctx(): decrement in refcount_dec_and_test() provides
       RELEASE ordering and ACQUIRE ordering + control dependency on success
       vs. fully ordered atomic counterpart
    Suggested-by: NKees Cook <keescook@chromium.org>
    Signed-off-by: NElena Reshetova <elena.reshetova@intel.com>
    Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: NDavid Windsor <dwindsor@gmail.com>
    Reviewed-by: NHans Liljestrand <ishkamiel@gmail.com>
    Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
    Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
    Cc: Jiri Olsa <jolsa@redhat.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: acme@kernel.org
    Cc: namhyung@kernel.org
    Link: https://lkml.kernel.org/r/1548678448-24458-2-git-send-email-elena.reshetova@intel.comSigned-off-by: NIngo Molnar <mingo@kernel.org>
    8c94abbb
perf_event.h 40.2 KB