[Bug 1909502] [NEW] FUSE fails to unmount without privilege in kernel v5.9+

[Sargun]

Public bug reported:

In kernel v5.9+ there was a refactor to the unmount code path that
results in libfuse no longer being able to detect UMOUNT_NOFOLLOW. This
resulted in an alternate code path that relied on being able to call
unshare and some other work.

The patch that caused the bug upstream:
https://lore.kernel.org/linux-fsdevel/20200726071356.287160-3-hch@lst.de/

The fix upstream (not in tree):
https://lore.kernel.org/linux-fsdevel/20201223102604.2078-1-sargun@sargun.me/

The fix in libfuse:
https://github.com/libfuse/libfuse/commit/b96e4758041ffac1b8d8c39f4897edd6182a46e9

** Affects: fuse (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1909502

Title:
  FUSE fails to unmount without privilege in kernel v5.9+

Status in fuse package in Ubuntu:
  New

Bug description:
  In kernel v5.9+ there was a refactor to the unmount code path that
  results in libfuse no longer being able to detect UMOUNT_NOFOLLOW.
  This resulted in an alternate code path that relied on being able to
  call unshare and some other work.

  The patch that caused the bug upstream:
  https://lore.kernel.org/linux-fsdevel/20200726071356.287160-3-hch@lst.de/

  The fix upstream (not in tree):
  https://lore.kernel.org/linux-fsdevel/20201223102604.2078-1-sargun@sargun.me/

  The fix in libfuse:
  https://github.com/libfuse/libfuse/commit/b96e4758041ffac1b8d8c39f4897edd6182a46e9

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