There seem to be some bugs in handling the combination of resizing inodes and specifying an undo file with the -z option. Before each run, i have created a test image with the command:

$ mke2fs -F -o Linux -I 128 test.img 512
mke2fs 1.43.5-WIP (17-Feb-2017)
test.img contains a ext2 file system
        created on Fri Feb 24 07:54:45 2017
Creating filesystem with 512 1k blocks and 64 inodes

Allocating group tables: done                            
Writing inode tables: done                            
Writing superblocks and filesystem accounting information: done


Then, when running 

$ E2FSPROGS_UNDO_DIR=. tune2fs -f -I 256 -z my.e2undo test.img

I get the following output

tune2fs 1.43.5-WIP (17-Feb-2017)
Resizing inodes could take some time.
Proceed anyway (or wait 5 seconds) ? (y,N) <proceeding>
Overwriting existing filesystem; this can be undone using the command:
    e2undo ./tune2fs-test.img.e2undo test.img

Resizing inodes could take some time.
Proceed anyway (or wait 5 seconds) ? (y,N) n

./tune2fs-test.img.e2undo: while force-closing undo file


It asked two times. I let the first question timeout, but the second time it waited forever. Also, the last line looks a bit confusing, apparently it got an error but does not tell which one.


Next, i tried
$ E2FSPROGS_UNDO_DIR=/nonexisting tune2fs -f -I 256 -z my.e2undo test.img

tune2fs 1.43.5-WIP (17-Feb-2017)
Resizing inodes could take some time.
Proceed anyway (or wait 5 seconds) ? (y,N) y
Setting inode size 256

i.e. it happily proceeded without ever creating an undo file.


And last, when i run
$ E2FSPROGS_UNDO_DIR=. tune2fs -f -I 256 -z my.e2undo test.img

I get

tune2fs 1.43.5-WIP (17-Feb-2017)
Resizing inodes could take some time.
Proceed anyway (or wait 5 seconds) ? (y,N) y
Overwriting existing filesystem; this can be undone using the command:
    e2undo ./tune2fs-test.img.e2undo test.img

Resizing inodes could take some time.
Proceed anyway (or wait 5 seconds) ? (y,N) y
Setting inode size 256

It created an undo file, but this is the default, ignoring the commandline option.