Re: [PATCH 1/1] ext4: fix crash on BUG_ON in ext4_alloc_group_tables

[Baokun Li]

On 2024/9/26 19:32, Aleksandr Mikhalitsyn wrote:
> > > Question to you and Jan. Do you guys think that it makes sense to try
> > > to create a minimal reproducer for this problem without Incus/LXD involved?
> > > (only e2fsprogs, lvm tools, etc)
> > >
> > > I guess this test can be put in the xfstests test suite, right?
> > >
> > > Kind regards,
> > > Alex
> > I think it makes sense, and it's good to have more use cases to look
> > around some corners. If you have an idea, let it go.
> Minimal reproducer:
>
> mkdir -p /tmp/ext4_crash/mnt
> EXT4_CRASH_IMG="/tmp/ext4_crash/disk.img"
> rm -f $EXT4_CRASH_IMG
> truncate $EXT4_CRASH_IMG --size 25MiB
> EXT4_CRASH_DEV=$(losetup --find --nooverlap --direct-io=on --show
> $EXT4_CRASH_IMG)
> mkfs.ext4 -E nodiscard,lazy_itable_init=0,lazy_journal_init=0 $EXT4_CRASH_DEV
> mount $EXT4_CRASH_DEV /tmp/ext4_crash/mnt
> truncate $EXT4_CRASH_IMG --size 3GiB
> losetup -c $EXT4_CRASH_DEV
> resize2fs $EXT4_CRASH_DEV
Hi Alex,

This replicator didn't replicate the issue in my VM, so I took a deeper
look. The reproduction of the problem requires the following:

o_group = flexbg_size * 2 * n;
o_size = (o_group + 1) * group_size;
n_group: [o_group + flexbg_size, o_group + flexbg_size * 2)

Take n=1,flexbg_size=16 as an example:
                                                 last:47
|----------------|----------------|o---------------|--------------n-|
                                  old:32 >>>           new:62

Thus the replicator can be simplified as:

img=test.img
truncate -s 600M $img
mkfs.ext4 -F $img -b 1024 -G 16 264M
dev=`losetup -f --show $img`
mkdir -p /tmp/test
mount $dev /tmp/test
resize2fs $dev 504M


--
Cheers,
Baokun