Re: [PATCH 6/6] fs: Make bind mounts work with bdev_allow_write_mounted=n

From: Christoph Hellwig <hch@infradead.org>
To: Jan Kara <jack@suse.cz>
Cc: linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
	Christoph Hellwig <hch@infradead.org>,
	Christian Brauner <brauner@kernel.org>,
	Jens Axboe <axboe@kernel.dk>, Kees Cook <keescook@google.com>,
	Ted Tso <tytso@mit.edu>, syzkaller <syzkaller@googlegroups.com>,
	Alexander Popov <alex.popov@linux.com>,
	Eric Biggers <ebiggers@google.com>,
	linux-xfs@vger.kernel.org, linux-btrfs@vger.kernel.org,
	Dmitry Vyukov <dvyukov@google.com>
Subject: Re: [PATCH 6/6] fs: Make bind mounts work with bdev_allow_write_mounted=n
Date: Thu, 6 Jul 2023 08:55:18 -0700	[thread overview]
Message-ID: <ZKbj5v4VKroW7cFp@infradead.org> (raw)
In-Reply-To: <20230704125702.23180-6-jack@suse.cz>

On Tue, Jul 04, 2023 at 02:56:54PM +0200, Jan Kara wrote:
> When we don't allow opening of mounted block devices for writing, bind
> mounting is broken because the bind mount tries to open the block device
> before finding the superblock for it already exists. Reorganize the
> mounting code to first look whether the superblock for a particular
> device is already mounted and open the block device only if it is not.

Warning: this might be a rathole.

I really hate how mount_bdev / get_tree_bdev try to deal with multiple
mounts.

The idea to just open the device and work from there just feels very
bogus.

There is really no good reason to have the bdev to find a superblock,
the dev_t does just fine (and in fact I have a patch to remove
the bdev based get_super and just use the dev_t based one all the
time).  So I'd really like to actually turn this around and only
open when we need to allocate a new super block.  That probably
means tearning sget_fc apart a bit, so it will turn into a fair
amount of work, but I think it's the right thing to do.