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31 lines
1.1 KiB
Markdown
31 lines
1.1 KiB
Markdown
## shiftfs
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Shiftfs is a virtual filesystem for mapping mountpoints across user namespaces.
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The idea is that it would be useful for dockerds spawning containers: they can
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keep filesystems on the host disk in terms of real root, but mount the
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container roots via shiftfs, allowing containers to share a particular
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filesystem with different uid maps, while not having to uidshift every file on
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disk (and thus destroying some of the sharing properties).
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The version included here is the v2 version of shiftfs, using the superblock's
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user namespace instead of mountopts to figure out mappings. Thus, an extra step
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of "marking" mounts is needed. For example:
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# mkdir source
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# touch source/foo # a root owned file
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# mount -t shiftfs -o mark source source
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# chmod 777 source
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Now, let's make a user namespace:
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# setuid 1000 unshare -rm
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# cat /proc/self/uidmap
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0 1000 1
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# mkdir dest
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# mount -t shiftfs source dest
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# stat dest/foo | grep Uid
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Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
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And thanks to the magic of shiftfs, the file is root owned in the user
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namespace.
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