Created attachment 386198 [details]
Copy of the wrongly detected superblock

Description of problem:
Today, blkid detected my (perfectly valid) ext4 root filesystem also as minix filesystem, thus throwing an error and not determining the filesystem type correctly. As a result, mount could not mount my root filesystem type (complaining that I need to specify the filesystem type), leaving me with an unbootable system.

I will attach a copy of the superblock in the broken as well as in the correct state. One can observe that in the broken case, a minix fs signature (2 bytes only) was detected at offset 0x410, the position of the least two significant bytes of the ext4 free inode count.

A workaround was specifying rootfstype=ext4 on the grub kernel command line, which allowed booting. After that, fortunately a few additional inodes were used, thus I needed that workaround only once ;-)

Still, blkid should use better heuristics and/or additional sanity checks to determine the type of the filesystem, because a root filesystem being unmountable by pure coincidence is not a good thing for the average user, who will probably reinstall his whole system.