btrfs-progs: remove loopback device resolution #940
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[BUG] mkfs.btrfs has a built-in loopback device resolution, to avoid the same file being added to the same fs, using loopback device and the file itself. But it has one big bug: - It doesn't detect partition on loopback devices correctly The function is_loop_device() only utilize major number to detect a loopback device. But partitions on loopback devices doesn't use the same major number as the loopback device: NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINTS loop0 7:0 0 5G 0 loop |-loop0p1 259:3 0 128M 0 part `-loop0p2 259:4 0 4.9G 0 part Thus `/dev/loop0p1` will not be treated as a loopback device, thus it will not even resolve the source file. And this can not even be fixed, as if we do extra "/dev/loop*" based file lookup, `/dev/loop0p1` and `/dev/loop0p2` will resolve to the same source file, and refuse to mkfs on two different partitions. [FIX] The loopback file detection is the baby sitting that no one asks for. Just as I explained, it only brings new bugs, and we will never fix all ways that an experienced user can come up with. And I didn't see any other mkfs tool doing such baby sitting. So remove the loopback file resolution, just regular is_same_blk_file() is good enough. Signed-off-by: Qu Wenruo <[email protected]>
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[BUG]
mkfs.btrfs has a built-in loopback device resolution, to avoid the same file being added to the same fs, using loopback device and the file itself.
But it has one big bug:
It doesn't detect partition on loopback devices correctly The function is_loop_device() only utilize major number to detect a loopback device. But partitions on loopback devices doesn't use the same major number as the loopback device:
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINTS
loop0 7:0 0 5G 0 loop
|-loop0p1 259:3 0 128M 0 part
`-loop0p2 259:4 0 4.9G 0 part
Thus
/dev/loop0p1will not be treated as a loopback device, thus it will not even resolve the source file.And this can not even be fixed, as if we do extra "/dev/loop*" based file lookup,
/dev/loop0p1and/dev/loop0p2will resolve to the same source file, and refuse to mkfs on two different partitions.[FIX]
The loopback file detection is the baby sitting that no one asks for.
Just as I explained, it only brings new bugs, and we will never fix all ways that an experienced user can come up with.
And I didn't see any other mkfs tool doing such baby sitting.
So remove the loopback file resolution, just regular is_same_blk_file() is good enough.