Murphy hits (Kernel 2.6, ext2, "check=strict"): corrupted filesystem

[Ulrich Windl]

Hello everybody,

I'd like to present a little story how to shredder your ext2 filesystem:

I was installing SuSE Linux 9.1 when the kernel froze rather late during 
installation. So I had to reset the PC. There is a minor bug in the forementioned 
distribution that prevents a filesystem check of the root filesystem after such a 
crash it seems. Anyway, the system booted without a fsck being run. I realized 
that and shut down (maybe it would have been better to hit the reset button 
immediately), but during the manual fsck, the system reported (among a lot of 
others) about 1700 inodes that share blocks. You can imagine what that means...

Why I'm writing this: If something can go wrong, eventually it will. For a true 
disaster you always need more than just one problem (1: Kernel freeze, 2: no fsck 
being run, 3: kernel happily mounts unclean filesystem for read-write).

In Kernel 2.2 there was a "check=strict" that caused some extra checks during 
mounting a filesystem. In 2.4 the option was ignored, and a warning was presented 
instead. In 2.6 it seems that "check=strict" prevents the filesystem from being 
mounted read-write (I ended up with an read-only root filesystem when having that 
option).

I'll have to spend a few more days to cleanup the mess in the root filesystem 
(which is everything except /home and /boot), but the lession to learn might be 
this: The kernel should not mount a unclean filesystem read-write. Maybe also if a 
filesystem code results in an attempt to access a partition outside its limits, 
the filesystem should trtigger a panic immediately instead of going on.

I think nobody really wants to read reports where Linux has shreddered a 
filesystem, do we?

Regards,
Ulrich


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