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

[Christian Brauner <brauner@kernel.org>]

On Mon, Mar 25, 2024 at 07:51:27PM +0800, Yu Kuai wrote:
> Hi,
> 
> 在 2024/03/24 0:11, Christian Brauner 写道:
> > 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.
> 
> I don't get it here, looks like there are no such use case. All users
> passed in BLK_OPEN_RESTRICT_WRITES together with BLK_OPEN_WRITE.

sb_open_mode() does

#define sb_open_mode(flags) \
        (BLK_OPEN_READ | BLK_OPEN_RESTRICT_WRITES | \
         (((flags) & SB_RDONLY) ? 0 : BLK_OPEN_WRITE))