Re: [PATCH] btrfs-progs: remove loopback device resolution

[David Sterba <dsterba@suse.cz>]

On Thu, Jan 16, 2025 at 08:13:01PM +1030, Qu Wenruo wrote:
> [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.

The loop device resolution had some bugs in the past and was added for a
reason that's not mentioned in the changelogs.

The commits have some details why it's done, they also mention that
partitioned devices are resolved so it's not that there's no support for
that.

0cf3b78f404b01 ("btrfs-progs: Fix partitioned loop devices resolving")
abdb0ced0123d4 ("Btrfs-progs: fix resolving of loop devices")
09559bfe7bcd43 ("multidevice support for check_mounted")

and some fixup commits.

So before removing the code completely I'd like to see if there's a use
case that can be broken, but I don't have anything in particular.
There's only mkfs-tests/006-partitioned-loopdev that's quite simple and
does not try to do any tricks with symlinks or partitions on the
devices.