Re: ext4 regression panic

Jan Kara
2021-01-21T21:11:24Z

On Thu 21-01-21 12:40:56, Theodore Ts'o wrote:
> On Thu, Jan 21, 2021 at 06:15:47PM +0800, Murphy Zhou wrote:
> > Hi Jack,
> >
> > A panic was introduced by this commit. It's easy and reliable to
> > reproduce.
> >
> > commit 2d01ddc86606564fb08c56e3bc93a0693895f710
> > Author: Jan Kara <jack@suse.cz>
> > Date: Wed Dec 16 11:18:40 2020 +0100
> >
> > ext4: save error info to sb through journal if available
>
> Hi Murphy,
>
> Thanks for the bug report. What's happening is that we haven't yet
> initialized mballoc yet --- that happens in line 4943 of
> fs/ext4/super.c, in ext4_fill_super().
>
> But in line 4903 (in the case of the BZ #199275 reproducer), we
> attempt to fetch the root inode, which is fails because it is
> unallocated. That then triggers a call to ext4_error(), which now
> results in a journalled change, since the journal is initialized
> starting in line 4793, and in line 4838, we set up the
> j_commit_callback, which is what ends up calling
> ext4_process_freed_data(), but since the multiblock allocator hasn't
> been set up yet, that causes the NULL pointer dereference.
>
> So what we need to do is to *not* set up the callback until after the
> call to ext4_mb_init().
>
> We should probably create an ext4-specific test in xfstests which
> tries mounting a small, deliberately corrupted file system, to make
> sure we handle this case correctly in the future.
>
> - Ted

Thanks for looking into this. You beat me to my fix (which was slightly
different - I moved ext4_mb_init() somewhat earlier during mount). But this
should work fine as well. So feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza

>
> commit 6c2f9a8247273cf1108ff71c99680b7457f48318
> Author: Theodore Ts'o <tytso@mit.edu>
> Date: Thu Jan 21 12:33:20 2021 -0500
>
> ext4: don't try to processed freed blocks until mballoc is initialized
>
> If we try to make any changes via the journal between when the journal
> is initialized, but before the multi-block allocated is initialized,
> we will end up deferencing a NULL pointer when the journal commit
> callback function calls ext4_process_freed_data().
>
> The proximate cause of this failure was commit 2d01ddc86606 ("ext4:
> save error info to sb through journal if available") since file system
> corruption problems detected before the call to ext4_mb_init() would
> result in a journal commit before we aborted the mount of the file
> system.... and we would then trigger the NULL pointer deref.
>
> Cc: Jan Kara <jack@suse.cz>
> Reported by: Murphy Zhou <jencce.kernel@gmail.com>
> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
>
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 0f0db49031dc..802ef55f0a55 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -4876,7 +4876,6 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
>
> set_task_ioprio(sbi->s_journal->j_task, journal_ioprio);
>
> - sbi->s_journal->j_commit_callback = ext4_journal_commit_callback;
> sbi->s_journal->j_submit_inode_data_buffers =
> ext4_journal_submit_inode_data_buffers;
> sbi->s_journal->j_finish_inode_data_buffers =
> @@ -4993,6 +4992,14 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
> goto failed_mount5;
> }
>
> + /*
> + * We can only set up the journal commit callback once
> + * mballoc is initialized
> + */
> + if (sbi->s_journal)
> + sbi->s_journal->j_commit_callback =
> + ext4_journal_commit_callback;
> +
> block = ext4_count_free_clusters(sb);
> ext4_free_blocks_count_set(sbi->s_es,
> EXT4_C2B(sbi, block));
--
Jan Kara <jack@suse.com>
SUSE Labs, CR