userfaultfd: non-cooperative: closing the uffd without triggering SIGBUS (656710a6) · Commits · 戴 / test

This is an enhancement to avoid a non cooperative userfaultfd manager having to unregister all regions before it can close the uffd after all userfaultfd activity completed. The UFFDIO_UNREGISTER would serialize against the handle_userfault by taking the mmap_sem for writing, but we can simply repeat the page fault if we detect the uffd was closed and so the regular page fault paths should takeover. Link: http://lkml.kernel.org/r/20170823181227.19926-1-aarcange@redhat.com Signed-off-by:

Andrea Arcangeli <aarcange@redhat.com> Acked-by:

Mike Rapoport <rppt@linux.vnet.ibm.com> Cc: Mike Kravetz <mike.kravetz@oracle.com> Cc: Pavel Emelyanov <xemul@virtuozzo.com> Signed-off-by:

Andrew Morton <akpm@linux-foundation.org> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

fs/userfaultfd.c

+19 −1

Original line number	Diff line number	Diff line
		@@ -381,8 +381,26 @@ int handle_userfault(struct vm_fault *vmf, unsigned long reason)
		* in __get_user_pages if userfaultfd_release waits on the
		* caller of handle_userfault to release the mmap_sem.
		*/
		if (unlikely(ACCESS_ONCE(ctx->released)))
		if (unlikely(ACCESS_ONCE(ctx->released))) {
		/*
		* Don't return VM_FAULT_SIGBUS in this case, so a non
		* cooperative manager can close the uffd after the
		* last UFFDIO_COPY, without risking to trigger an
		* involuntary SIGBUS if the process was starting the
		* userfaultfd while the userfaultfd was still armed
		* (but after the last UFFDIO_COPY). If the uffd
		* wasn't already closed when the userfault reached
		* this point, that would normally be solved by
		* userfaultfd_must_wait returning 'false'.
		*
		* If we were to return VM_FAULT_SIGBUS here, the non
		* cooperative manager would be instead forced to
		* always call UFFDIO_UNREGISTER before it can safely
		* close the uffd.
		*/
		ret = VM_FAULT_NOPAGE;
		goto out;
		}

		/*
		* Check that we can return VM_FAULT_RETRY.

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