Re: [PATCH 1/2] block: handle BLK_OPEN_RESTRICT_WRITES correctly

[Matthew Wilcox <willy@infradead.org>]

On Sat, Mar 23, 2024 at 05:11:19PM +0100, Christian Brauner wrote:
> Last kernel release we introduce CONFIG_BLK_DEV_WRITE_MOUNTED. By
> default this option is set. When it is set the long-standing behavior
> of being able to write to mounted block devices is enabled.
> 
> But in order to guard against unintended corruption by writing to the
> block device buffer cache CONFIG_BLK_DEV_WRITE_MOUNTED can be turned
> off. In that case it isn't possible to write to mounted block devices
> anymore.
> 
> A filesystem may open its block devices with BLK_OPEN_RESTRICT_WRITES
> which disallows concurrent BLK_OPEN_WRITE access. When we still had the
> bdev handle around we could recognize BLK_OPEN_RESTRICT_WRITES because
> the mode was passed around. Since we managed to get rid of the bdev
> handle we changed that logic to recognize BLK_OPEN_RESTRICT_WRITES based
> on whether the file was opened writable and writes to that block device
> are blocked. That logic doesn't work because we do allow
> BLK_OPEN_RESTRICT_WRITES to be specified without BLK_OPEN_WRITE.
> 
> So fix the detection logic. Use O_EXCL as an indicator that
> BLK_OPEN_RESTRICT_WRITES has been requested. We do the exact same thing
> for pidfds where O_EXCL means that this is a pidfd that refers to a
> thread. For userspace open paths O_EXCL will never be retained but for
> internal opens where we open files that are never installed into a file
> descriptor table this is fine.
> 
> Note that BLK_OPEN_RESTRICT_WRITES is an internal only flag that cannot
> directly be raised by userspace. It is implicitly raised during
> mounting.
> 
> Passes xftests and blktests with CONFIG_BLK_DEV_WRITE_MOUNTED set and
> unset.
> 
> Fixes: 321de651fa56 ("block: don't rely on BLK_OPEN_RESTRICT_WRITES when yielding write access")
> Reported-by: Matthew Wilcox <willy@infradead.org>
> Link: https://lore.kernel.org/r/ZfyyEwu9Uq5Pgb94@casper.infradead.org
> Signed-off-by: Christian Brauner <brauner@kernel.org>

So v1 of this patch works fine.  I just got round to testing v2, and it
does not.  Indeed, applying 2/2 causes root to fail to mount:

/dev/root: Can't open blockdev
List of all bdev filesystems:
 ext3
 ext2
 ext4
 xfs

Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(254,0)

Applying only 1/2 boots but fails to fix the bug.