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authorJamie Liu <jamieliu@google.com>2020-10-27 12:08:20 -0700
committergVisor bot <gvisor-bot@google.com>2020-10-27 12:10:24 -0700
commit6d50185e7c1b551168f9318d0f26a25ee37d6ef9 (patch)
treecc3d3700b5ead48340e107d9bf4ac0ba16405440 /pkg/ilist
parent59e2c9f16a9a4cce2ecf8b6449a47316fdf76ca2 (diff)
Assign VFS2 overlay device numbers based on layer device numbers.
In VFS1's overlayfs, files use the device and inode number of the lower layer inode if one exists, and the upper layer inode otherwise. The former behavior is inefficient (requiring lower layer lookups even if the file exists and is otherwise wholly determined by the upper layer), and somewhat dangerous if the lower layer is also observable (since both the overlay and lower layer file will have the same device and inode numbers and thus appear to be the same file, despite being behaviorally different). VFS2 overlayfs imitates Linux overlayfs (in its default configuration) instead; it always uses the inode number from the originating layer, but synthesizes a unique device number for directories and another device number for non-directory files that have not been copied-up. As it turns out, the latter is insufficient (in VFS2, and possibly Linux as well), because a given layer may include files with different device numbers. If two distinct files on such a layer have device number X and Y respectively, but share inode number Z, then the overlay will map both files to some private device number X' and inode number Z, potentially confusing applications. Fix this by assigning synthetic device numbers based on the lower layer's device number, rather than the lower layer's vfs.Filesystem. PiperOrigin-RevId: 339300341
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