Date: Sun, 25 Nov 2018 00:38:33 -0800 From: Kirk McKusick <mckusick@mckusick.com> To: soralx@cydem.org Cc: "Julian H. Stacey" <jhs@berklix.com>, freebsd-fs@freebsd.org Subject: Re: [bug] fsck refuses to repair damaged UFS using backup superblock Message-ID: <201811250838.wAP8cXoj046038@chez.mckusick.com> In-Reply-To: <201811230117.wAN1HKAT037185@fire.js.berklix.net>
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> To: soralx@cydem.org > Subject: Re: [bug] fsck refuses to repair damaged UFS using backup super= block > From: "Julian H. Stacey" <jhs@berklix.com> > Organization: http://berklix.eu BSD Unix Linux Consultants, Munich Germa= ny > Date: Fri, 23 Nov 2018 02:17:20 +0100 > = > Hi soralx@cydem.org, > Added cc: <freebsd-fs@freebsd.org> to ensure file system specialists see= this. > = > Reference: >> From: <soralx@cydem.org> >> Date: Tue, 20 Nov 2018 05:30:00 -0800 > = > soralx@cydem.org wrote: >> = >> Howdy! >> = >> Since send-pr(1) is now gone, I guess the next option is to send a >> message directly to the developers... >> = >> Yesterday, I ran into a bug in fsck_ffs that gave me a little scare. >> = >> Short story: on -CURRENT, fsck refuses to check a FS with a corrupted >> superblock, even when an alternate (backup) SB location is given. >> = >> Long story. I've been testing a newly-built system based on an X399 >> platform with a 2950X CPU and an Optane 905P 480GB U.2 drive. The >> system ran a ~2-day old -CURRENT; when compiling newest world and >> kernel, I found the machine in a locked-up state. After a hard reset, >> boot failed because the root FS became corrupted & was not available: >> kernel: Superblock check-hash failed: recorded check-hash XXX !=3D c= omputed check-hash YYY >> = >> I have not yet figured out why the corruption happened... bad hardware= ? >> bug in the NVMe driver? >> = >> "OK", I thought, "No worries. We'll just boot using another disk, fsck >> the corrupted FS with a backup superblock, and be up in a moment". >> The machine was doing nothing but compiling, so no valuable data loss. >> = >> So I did `dumpfs -m /dev/ada0p3` on the spare disk (which was the >> source for the new disk image, thus had almost identical partitions >> and filesystems) to get the FS details, then did `newfs -N [...] >> /dev/ada0p3` to find locations of superblock backups, then finally >> ran `fsck_ffs -b 192 /dev/nvd0p3` -- only to get the same "check- >> -hash failed" message, plus another strange message: "Can't open >> /dev/nvd0p3: [...]". Then fsck quits. >> Note that `fsck_ffs -b ...` on a FS with good superblock works OK. >> = >> After fiddling with a debugger for a bit, I commented out the line >> "return (0);" in /usr/src/sbin/fsck_ffs/setup.c:136, recompiled fsck, >> and the FS was recovered successfully. >> = >> What was actually happening: fsck's setup.c calls ufs_disk_fillout() >> from libufs' type.c, which in turn calls sbread() from the same >> library, which then calls sbget(disk->d_fd, &fs, -1) [[where '-1' >> is hard-coded to indicate the primary superblock]] that then simply >> invokes ffs_sbget from ffs kernel driver -- and this returns ENOENT, >> which eventually causes fsck to give up before even looking at the >> specified backup superblock. >> = >> I don't know what exactly ufs_disk_fillout() does, but fortunately >> for me, fsck worked without the "sbread(disk)" part of that function >> having much luck on a disk with corrupted superblock. Also, I have a >> feeling that calling a kernel's ffs driver function when using fsck >> to fix a broken filesystem is not the best thing to do... >> = >> Please CC, as I am not subscribed. >> = >> -- = >> [SorAlx] ridin' VN2000 Classic LT > = > Cheers, > Julian Below is a proposed fix for fsck_ffs to properly handle superblock check-hash failures (notably to optionally search for a usable alternate superblock). Let me know if you still have a filesystem on which you can test it, and if so whether it works correctly. Kirk McKusick
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