Historically only fsck_ffs(8), the UFS filesystem checker had code to track down and use alternate superblocks. Since fsdb(8) used much of the fsck_ffs(8) implementation it had some ability to track down alternate superblocks.
This change extracts the code to track down alternate superblocks from fsck_ffs(8) and puts it into a new function ffs_sbsearch() in sys/ufs/ffs/ffs_subr.c. Like ffs_sbget() and ffs_sbput() also found in ffs_subr.c, these functions can be used directly by the kernel subsystems. Additionally they are exported to the UFS library, libufs(8) so that they can be used by user-level programs. The new functions added to libufs(8) are sbfind(3) that is an alternative to sbread(3) and sbsearch(3) that is an alternative to sbget(3).
The utilities that have been changed to search for superblocks are dumpfs(8), fsdb(8), ffsinfo(8), and fsck_ffs(8). Also, the prtblknos(8) tool found in tools/diag/prtblknos searches for superblocks..
The UFS specific mount code uses the superblock search interface when mounting the root filesystem and when the administrator doing a mount(8) command specifies the force flag (-f). The standalone UFS boot code (found in stand/libsa/ufs.c) uses the superblock search code in the hope of being able to get the system up and running so that fsck_ffs(8) can be used to get the filesystem cleaned up.
The following utilities have not been changed to not search for superblocks: clri(8), tunefs(8), snapinfo(8), fstyp(8), quot(8), dump(8), fsirand(8), growfs(8), quotacheck(8), gjournal(8), and glabel(8). When these utilities fail, they do report the cause of the failure. The one exception is the tasting code used to try and figure what a given disk contains. The tasting code will remain silent so as not to put out a slew of messages as it trying to taste every new mass storage device that shows up.
Reviewers are solicited to provide feedback on the above selection of utilities to use or not use the new search facility. The current criterion is to make the utilities that are mainly used for debugging search for superblocks while those used for administrative tasks to not search. The thinking for the administrative commands to not search is that if a filesystem superblock is inconsistent, then fsck_ffs(8) should be used first to clean it up before attempting the administrative task.