Re: [PATCH] ext4: don't set SB_RDONLY after filesystem errors

[Jan Kara <jack@suse.cz>]

On Mon 05-08-24 22:12:41, Jan Kara wrote:
> When the filesystem is mounted with errors=remount-ro, we were setting
> SB_RDONLY flag to stop all filesystem modifications. We knew this misses
> proper locking (sb->s_umount) and does not go through proper filesystem
> remount procedure but it has been the way this worked since early ext2
> days and it was good enough for catastrophic situation damage
> mitigation. Recently, syzbot has found a way (see link) to trigger
> warnings in filesystem freezing because the code got confused by
> SB_RDONLY changing under its hands. Since these days we set
> EXT4_FLAGS_SHUTDOWN on the superblock which is enough to stop all
> filesystem modifications, modifying SB_RDONLY shouldn't be needed. So
> stop doing that.
> 
> Link: https://lore.kernel.org/all/000000000000b90a8e061e21d12f@google.com
> Reported-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/ext4/super.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> Note that this patch introduces fstests failure with generic/459 test
> because it assumes that either freezing succeeds or 'ro' is among mount
> options. But we fail the freeze with EFSCORRUPTED. This needs fixing in
> the test but at this point I'm not sure how exactly.

OK, I have noticed that recent versions of fstests have the check already
improved and so generic/459 passes with these changes.

								Honza
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index e72145c4ae5a..93c016b186c0 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -735,11 +735,12 @@ static void ext4_handle_error(struct super_block *sb, bool force_ro, int error,
>  
>  	ext4_msg(sb, KERN_CRIT, "Remounting filesystem read-only");
>  	/*
> -	 * Make sure updated value of ->s_mount_flags will be visible before
> -	 * ->s_flags update
> +	 * EXT4_FLAGS_SHUTDOWN was set which stops all filesystem
> +	 * modifications. We don't set SB_RDONLY because that requires
> +	 * sb->s_umount semaphore and setting it without proper remount
> +	 * procedure is confusing code such as freeze_super() leading to
> +	 * deadlocks and other problems.
>  	 */
> -	smp_wmb();
> -	sb->s_flags |= SB_RDONLY;
>  }
>  
>  static void update_super_work(struct work_struct *work)
> -- 
> 2.35.3
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR