mkfs.f2fs: write fill chunk in sparse file for zeroed block

mkfs.f2fs: write fill chunk in sparse file for zeroed block

As zhaowuyun reported:

we met one problem of f2fs, and found one issue of make_f2fs, so I write
this email to search for your help to confirm this issue.

The issue was found on one of Android projects. We use f2fs as the filesystem
of userdata, and make sparse userdata.img using following command, which
invoked in script mkf2fsuserimg.sh
make_f2fs -S $SIZE -f -O encrypt -O quota -O verity $MKFS_OPTS $OUTPUT_FILE

use fastboot to flash this userdata.img to device, and it encountered f2fs
problem and leading to the mount fail of data partition.

we can make this issue 100% persent reproduced by making the data partition
dirty before flashing userdata.img.

suspect that issue is caused by the dirty data in the data partition.
so we checked that source code of make_f2fs in f2fs-tool, found that when
making f2fs, it use dev_fill to do some process:

...

we change code to the following, and the issue is gone.

if (c.sparse_mode)
       return dev_write(buf, offset, len);

Chao Yu:
>
> After checking the codes, IIUC, I guess the problem here is, unlike
> img2simg, mkfs.f2fs won't record zeroed block in sparse image, so
> during transforming to normal image, some critical region like
> NAT/SIT/CP.payload area weren't be zeroed correctly, later kernel may
> load obsoleting data from those region.
>
> Also, The way you provide will obviously increase the size of sparse
> file, since with it we need to write all zeroed blocks of
> NAT/SIT/CP.payload to sparse file, it's not needed.
>
> Not sure, maybe we should use sparse_file_add_fill() to record zeroed
> blocks, so that this will make formatted image more like img2simged one.

Jaegeuk:
> We have to call sparse_file_add_fill() for dev_fill().

This patch fixes to support writing fill chunk sparse file for those
zeroed blocks in mkfs.f2fs.

Reported-and-tested-by: zhaowuyun <zhaowuyun@wingtech.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>