boot0/boot0sio have disk serial number compiled-in by default. It is named "version 2" boot block by boot0cfg(8) and this format places 4 byte Disk Serial Number where "version 1" boot block contains options. Support for disk serial number in boot0/boot0sio is optional, and could be turned off to save some bytes, needed for other options. boot0cfg(8) properly supports old placement of boot block options, properly detect boot block without disk serial number support and properly prepare options for it. Problem is, boot0cfg(8) uses "gpart bootcode" command to wriet boot block with new options back to disk. And gmart_mbr always try to save disk serial number, which kills options if boot block is "version 1": static int g_part_mbr_bootcode(struct g_part_table *basetable, struct g_part_parms *gpp) { struct g_part_mbr_table *table; uint32_t dsn; if (gpp->gpp_codesize != MBRSIZE) return (ENODEV); table = (struct g_part_mbr_table *)basetable; dsn = *(uint32_t *)(table->mbr + DOSDSNOFF); bcopy(gpp->gpp_codeptr, table->mbr, DOSPARTOFF); if (dsn != 0) *(uint32_t *)(table->mbr + DOSDSNOFF) = dsn; return (0); } Sometimes, it leads to complete bogus options written which kill boot0/boot0sio (it have invalid slice mask, for example, and is unable to boot at all) and sometimes it simply didn't give user ability to change boot0/boot0sio options.
https://reviews.freebsd.org/D17386
Committed via rS341067 Nov 27 2018.
^Triage: - Assign to committer that resolved - Correct resolution (FIXED, by way of change/commit) Re-open: - base r341067 was tagged for MFC, but i couldnt not find corresponding merges to either stable/12 or stable/11. Did this end up being merged? Apologies if I missed it.
I have no recollection of this. I don't think it was merged and I'm not sure there's much point in doing so now since there will not be another 11.x or 12.x release.