Re: [PATCH 00/22][RFC] Unionfs: Stackable Namespace Unification Filesystem

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>I agree that unionfs shouldn't oops, it should handle that situation in
>a more graceful manner, but once the "backing store" is modified
>underneath it, all bets are off for either unionfs or ext2/3 behaving
>"correctly" (where "correctly" doesn't just mean handle the error
>But are you also 100% sure that messing with the underlying backing
>store wouldn't be considered an admin bug as opposed to an administrator
>bug? I mean there's nothing that we can do to prevent an administrator
>from FUBAR'ing their system by 
>dd if=/dev/random of=/dev/kmem.
>where does one draw the line?  I agree that stackable file systems make
>this a more pressing issue, as the "backing store" can be visible within
>the file system namespace as a regular file system that people are
>generally accustomed to interacting with.

So here's an idea. When a branch is added, mount an empty space onto the
branch. (From within the kernel, so it appears as a side-effect of mount(2))

  mount -t unionfs -o dirs=/a=rw:/b=ro none /union

should imply something like

  mount --bind /var/lib/empty /a
  mount --bind /var/lib/empty /b

Or better, yet, make them read-only:

  mount --rbind -o ro /a /a
  mount --rbind -o ro /b /b
(hope this works as intended?)

So that no one can tinker with /a and /b while the union is mounted.

Jan Engelhardt
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