Do comprehensive UFS/FFS superblock integrity checks when reading a superblock.
Historically only minimal checks were made of a superblock when it was read in as it was assumed that fsck would have been run to correct any errors before attempting to use the filesystem. Recently several bug reports have been submitted reporting kernel panics that can be triggered by deliberately corrupting filesystem superblocks, see Bug 263979 - [meta] UFS / FFS / GEOM crash (panic) tracking which is tracking the reported corruption bugs. This change upgrades the checks that are performed. These additional checks should prevent panics from a corrupted superblock. Although it appears in only one place, the new code will apply to the kernel modules and (through libufs) user applications that read in superblocks. Reported by: Robert Morris and Neeraj Reviewed by: kib Tested by: Peter Holm PR: 263979 MFC after: 1 month Differential Revision: https://reviews.freebsd.org/D35219