Re: [PATCH 4.14 024/110] btrfs: use proper endianness accessors for super_copy

[David Sterba]

On Fri, Mar 16, 2018 at 01:30:49PM +0100, Greg Kroah-Hartman wrote:
> On Thu, Mar 15, 2018 at 07:55:42PM +0100, Christoph Biedl wrote:
> > Greg Kroah-Hartman wrote...
> > 
> > > 4.14-stable review patch.  If anyone has any objections, please let me know.
> > 
> > > commit 3c181c12c431fe33b669410d663beb9cceefcd1b upstream.
> > (...)
> > 
> > > If the filesystem is always used on a same endian host, this will not
> > > be a problem.
> > 
> > >From my observations I cannot quite subscribe to that.
> > 
> > On big-endian systems, this change intruduces severe corruption,
> > resulting in complete loss of the data on the used block device.
> > 
> > Steps to reproduce (tested on ppc/powerpc and parisc/hppa):
> > 
> > # mkfs.btrfs $DEV
> > # mount $DEV /mnt/tmp/
> > # umount /mnt/tmp/
> > 
> > This simple umount corrupts the file system:
> > 
> > # mount $DEV /mnt/tmp/
> > mount: /mnt/tmp: wrong fs type, bad option, bad superblock on $DEV, missing codepage or helper program, or other error.
> > 
> > # dmesg:
> > BTRFS critical (device <dev>): unable to find logical 4294967296 length 4096
> > BTRFS critical (device <dev>): unable to find logical 4294967296 length 4096
> > BTRFS critical (device <dev>): unable to find logical 18102363734671360 length 16384
> > BTRFS error (device <dev>): failed to read chunk root
> > BTRFS error (device <dev>): open_ctree failed
> > 
> > Also fsck is of no help:
> > 
> > # btrfsck $DEV
> > Couldn't map the block 18102363734671360
> > No mapping for 18102363734671360-18102363734687744
> > Couldn't map the block 18102363734671360
> > bytenr mismatch, want=18102363734671360, have=0
> > ERROR: cannot read chunk root
> > ERROR: cannot open file system
> > 
> > 
> > Trying mount or fsck on a little-endian system does not help either. So
> > I consider the data on that device lost - luckily I use btrfs only for
> > files where a backup exists all the time.
> > 
> > 
> > Reverting that change restored the previous error-free behaviour. I
> > didn't check HEAD, i.e. v4.16-rc5, since the upstream commt was the last
> > that affected these files. Still I could give this a try if anybody
> > wishes so.
> 
> That sucks.  Can you test Linus's tree to verify the problem is there?
> I'll gladly revert this if Linus's tree also gets the revert, I don't
> want you to hit this when you upgrade to a newer kernel.

I'll push a fix for the upcoming rc but I think it would be better to
remove the broken patch from stable kernels ASAP, so I'd recommend to
revert it now.