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

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On 03/16/2018 10:02 PM, Greg Kroah-Hartman wrote:
On Fri, Mar 16, 2018 at 02:22:02PM +0100, David Sterba wrote:
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.

Now reverted, thanks.

 Thanks ! Sorry for the mess.

-Anand

greg k-h
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