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authorElena Reshetova <elena.reshetova@intel.com>2019-03-07 16:30:00 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2019-03-07 18:32:02 -0800
commit39e07cb60860e3162fc377380b8a60409315681e (patch)
treec676461716664fd7292605fb07394647aee2e5d7 /scripts/gdb/linux
parentec9672d57670d495404f36ab8b651bfefc0ea10b (diff)
download96b-common-39e07cb60860e3162fc377380b8a60409315681e.tar.gz
kcov: convert kcov.refcount to refcount_t
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 kcov.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. The full comparison can be seen in https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon in state to be merged to the documentation tree. 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 kcov.refcount it might make a difference in following places: - kcov_put(): decrement in refcount_dec_and_test() only provides RELEASE ordering and control dependency on success vs. fully ordered atomic counterpart Link: http://lkml.kernel.org/r/1547634429-772-1-git-send-email-elena.reshetova@intel.com Signed-off-by: Elena Reshetova <elena.reshetova@intel.com> Suggested-by: Kees Cook <keescook@chromium.org> Reviewed-by: David Windsor <dwindsor@gmail.com> Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com> Reviewed-by: Dmitry Vyukov <dvyukov@google.com> Reviewed-by: Andrea Parri <andrea.parri@amarulasolutions.com> Cc: Mark Rutland <mark.rutland@arm.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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