Date: Tue, 20 Nov 2018 05:30:00 -0800 From: <soralx@cydem.org> To: <freebsd-hackers@freebsd.org> Subject: [bug] fsck refuses to repair damaged UFS using backup superblock Message-ID: <20181120053000.56fbee6b@mscad14>
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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 != computed 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
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