Re: [PATCH] ext4: fix kernel BUG in ext4_free_blocks

[Tadeusz Struk]

On 7/14/22 05:23, Lukas Czerner wrote:
> > This does not seem right. we should never work with block number smaller
> > than s_first_data_block. The first 1024 bytes of the file system are
> > unused and in case we have 1k block size, the entire first block is
> > unused.
> >
> > I guess the image we work here with is corrupted, from the log it seems
> > that it was noticed correctly so the question is why did we still ended
> > up calling ext4_free_blocks() ? Seems like this should have been stopped
> > earlier by ext4_clear_blocks() ?
> >
> > I did notice that in ext4_mb_clear_bb() we call
> > ext4_get_group_no_and_offset() before ext4_inode_block_valid() but
> > again we should have caught this problem earlier.
> >
> > Can you link me the file system image that generated this problem?
> ok, I got the syzkaller C repro to work. The problem is that it's
> bigalloc file system and the 'block' and 'count' to free in
> ext4_free_blocks will get adjusted after the ext4_inode_block_valid().
>
> We should make sure that if this happens we also clear the
> EXT4_FREE_BLOCKS_VALIDATED. Additonally the ext4_inode_block_valid()
> in ext4_mb_clear_bb() should be called*before*  the values are taken for
> granted. I'll prepare a patch to fix this.

Thank you for feedback Lukas. Please CC me on your patch so I could test it.
--
Thanks,
Tadeusz