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

[Jan Stancek <jstancek@redhat.com>]

On Mon, Aug 05, 2024 at 10:12:41PM +0200, 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.
>
>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;

Hi,

shouldn't the SB_RDONLY still be set (in __ext4_remount()) for the case
when user triggers the abort with mount(.., "abort")? Because now we seem
to always hit the condition that returns EROFS to user-space.

I'm seeing LTP's fanotify22 failing for a about week (roughly since
commit de5cb0dcb74c) on:

   fanotify22.c:59: TINFO: Mounting /dev/loop0 to /tmp/LTP_fanqgL299/test_mnt fstyp=ext4 flags=21
   fanotify22.c:59: TBROK: mount(/dev/loop0, test_mnt, ext4, 33, 0x4211ed) failed: EROFS (30)

	static void trigger_fs_abort(void)
	{
		SAFE_MOUNT(tst_device->dev, MOUNT_PATH, tst_device->fs_type,
			   MS_REMOUNT|MS_RDONLY, "abort");
	}

Thanks,
Jan