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From:   Chung-Chiang Cheng <shepjeng@gmail.com>
Date:   Tue, 6 Dec 2022 10:42:03 +0800
Message-ID: <CAHuHWtnbVsS2pp5EySmZ_72fCrDqKJTAOkssa-D-X5wKoR9uWQ@mail.gmail.com>
Subject: Re: [PATCH] btrfs: refuse to remount read-write with unsupported
 compat-ro features
To:     Filipe Manana <fdmanana@kernel.org>
Cc:     Chung-Chiang Cheng <cccheng@synology.com>, clm@fb.com,
        josef@toxicpanda.com, dsterba@suse.com,
        linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel@cccheng.net, Johnny Chang <johnnyc@synology.com>
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On Mon, Dec 5, 2022 at 6:45 PM Filipe Manana <fdmanana@kernel.org> wrote:
>
> Wasn't this already done by the following commit?
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=81d5d61454c365718655cfc87d8200c84e25d596
>
> Thanks.
>

Wow. I did not notice this commit doing the same job by Qu. But I have
tested the latest linux-6.1 rc-7, and it's still able to mount a unsupported
comat-ro btrfs as read-write via remount.

It's caused by the follow-up commit d7f67ac9a928 ("btrfs: relax
block-group-tree feature dependency checks"). This commit checks read-
only with the current superblock, which will always pass in the situation
remounting from read-only to read-write. It seems `btrfs_check_features()`
cannot cover this scenario.

        if (compat_ro_unsupp && !sb_rdonly(sb)) {
                                ^^^^^^^^^^^^^^

Thanks,
C.C.Cheng