Mark Johnston
8f3227f527
When registering a new kevent backed by a file descriptor, we first look up the file description with fget(), then lock the kqueue, then see if a corresponding knote is already registered. If not, and KN_ADD is specified, we add the knote to the kqueue. closefp_impl() interlocks with this process by calling knote_fdclose(), which locks each kqueue and checks to see if the fd is registered with a knote. But, if userspace closes an fd while a different thread is registering it, i.e., after fget() succeeds but before the kqueue is locked, then we may end up with a mismatch in the knote table, where the knote kn_fp field points to a different file description than the knote ident. Fix the problem by double-checking before registering a knote. Add a new fget_noref_unlocked() helper for this purpose. It is a clone of fget_noref(). We could simply use fget_noref(), but I like having an explicit unlocked variant. PR: 293382 Reviewed by: kib MFC after: 2 weeks Differential Revision: https://reviews.freebsd.org/D55852
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