fs, nfs: convert nfs_lock_context.count from atomic_t 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 nfs_lock_context.count is used as pure reference counter. Convert it to refcount_t and fix up the operations. Suggested-by: NKees Cook <keescook@chromium.org> Reviewed-by: NDavid Windsor <dwindsor@gmail.com> Reviewed-by: NHans Liljestrand <ishkamiel@gmail.com> Signed-off-by: NElena Reshetova <elena.reshetova@intel.com> Signed-off-by: NAnna Schumaker <Anna.Schumaker@Netapp.com>
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