From: Andrea Arcangeli <aarcange@xxxxxxxxxx> Subject: userfaultfd: non-cooperative: closing the uffd without triggering SIGBUS 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@xxxxxxxxxx Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx> Acked-by: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx> Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- fs/userfaultfd.c | 20 +++++++++++++++++++- 1 file changed, 19 insertions(+), 1 deletion(-) diff -puN fs/userfaultfd.c~userfaultfd-non-cooperative-closing-the-uffd-without-triggering-sigbus fs/userfaultfd.c --- a/fs/userfaultfd.c~userfaultfd-non-cooperative-closing-the-uffd-without-triggering-sigbus +++ a/fs/userfaultfd.c @@ -381,8 +381,26 @@ int handle_userfault(struct vm_fault *vm * 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. _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
