[PATCH v2] erofs-utils: fix battach on full buffer block

[Gao Xiang]

Hi Weiwen,

On Fri, Jan 22, 2021 at 12:26:06AM +0800, Hu Weiwen wrote:
> When __erofs_battach() is called on an buffer block of which
> (bb->buffers.off % EROFS_BLKSIZ == 0), `tail_blkaddr' will not be
> updated correctly. This bug can be reproduced by:
> 
> mkdir bug-repo
> head -c 4032 /dev/urandom > bug-repo/1
> head -c 4095 /dev/urandom > bug-repo/2
> head -c 12345 /dev/urandom > bug-repo/3  # arbitrary size
> mkfs.erofs -Eforce-inode-compact bug-repo.erofs.img bug-repo
> 
> Then mount this image and see that file `3' in the image is different
> from `bug-repo/3'.
> 
> This patch fix this by:
> 
> * Don't inline tail-end data in this case, since the tail-end data will
> be in a different block from inode.
> * Correctly handle `battach' in this case.
> 
> Signed-off-by: Hu Weiwen <sehuww at mail.scut.edu.cn>
> ---

Now I get what you mentioned to, and I think this is a valid case and might
influence old versions (even I have no idea how to reproduce it effectively.)
So I tend to apply this patch right now, and thanks for your patch!

Reviewed-by: Gao Xiang <hsiangkao at aol.com>

With updated commit message below:

When the subsequent erofs_battach() is called on an buffer block of
which (bb->buffers.off % EROFS_BLKSIZ == 0), `tail_blkaddr' won't be
updated correctly. This bug can be reproduced by:

mkdir bug-repo
head -c 4032 /dev/urandom > bug-repo/1
head -c 4095 /dev/urandom > bug-repo/2
head -c 12345 /dev/urandom > bug-repo/3  # arbitrary size
mkfs.erofs -Eforce-inode-compact bug-repo.erofs.img bug-repo
Then mount this image and see that file `3' in the image is different
from `bug-repo/3'.

This patch fix this by:
 * Handle `oob' and `tail_blkaddr' for the case above properly;
 * Don't inline tail-packing data for such case, since the tail-packing
   data is actually in a different block from inode itself even kernel
   can handle such cases properly.


Thanks,
Gao Xiang