[bug] fsck refuses to repair damaged UFS using backup superblock

[Rick Macklem]

________________________________________
From: Rick Macklem
Sent: Sunday, November 25, 2018 6:45 PM
To: Kirk McKusick
Cc: soralx at cydem.org; freebsd-fs at freebsd.org; Julian H. Stacey; Konstantin Belousov
Subject: Re: [bug] fsck refuses to repair damaged UFS using backup superblock

Rick Macklem wrote:
[stuff snipped]
>Maybe setting all elements of fs_sparecon32[] to zeros before writing the
>superblock out would minimize these issues for the future and could be MFC'd.
>(I'm not claiming that a new FFS2 should be movable between stable/11 and
> head, but it might be a nice feature?)
Ok, it looks like the FS_METACKHASH flag was meant to handle the old->new
kernel transition. However, if I am reading the code correctly, it needs to be
checked for sooner. Here's the code snippet:
      /* fetch the superblock and summary information */
812        if ((error = ffs_sbget(devvp, &fs, -1, M_UFSMNT, ffs_use_bread)) != 0)
813                goto out;
814        fs->fs_fmod = 0;
815        /* if we ran on a kernel without metadata check hashes, disable them */
816        if ((fs->fs_flags & FS_METACKHASH) == 0)
817                fs->fs_metackhash = 0;
I think ffs_sbget() calls readsuper() which calls calc_sbhash(), so lines 815-817
need to be near the top of readsuper(), I think?

rick
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