The patch titled
Subject: userfaultfd: non-cooperative: closing the uffd without triggering SIGBUS
has been added to the -mm tree. Its filename is
userfaultfd-non-cooperative-closing-the-uffd-without-triggering-sigbus.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/userfaultfd-non-cooperative-closing-the-uffd-without-triggering-sigbus.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/userfaultfd-non-cooperative-closing-the-uffd-without-triggering-sigbus.patch
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*** Remember to use Documentation/SubmitChecklist when testing your code ***
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------------------------------------------------------
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
@@ -384,8 +384,26 @@ 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.
_
Patches currently in -mm which might be from aarcange@xxxxxxxxxx are
userfaultfd-selftest-exercise-uffdio_copy-zeropage-eexist.patch
userfaultfd-selftest-explicit-failure-if-the-sigbus-test-failed.patch
userfaultfd-call-userfaultfd_unmap_prep-only-if-__split_vma-succeeds.patch
userfaultfd-provide-pid-in-userfault-msg-add-feat-union.patch
mm-oom-let-oom_reap_task-and-exit_mmap-to-run-concurrently.patch
mm-oom-let-oom_reap_task-and-exit_mmap-to-run-concurrently-fix.patch
mm-oom-let-oom_reap_task-and-exit_mmap-to-run-concurrently-fix-3.patch
userfaultfd-non-cooperative-closing-the-uffd-without-triggering-sigbus.patch
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