linux-kernel - Re: [PATCH V2 for-6.10/block 1/2] loop: Fix a race between loop detach and loop open

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Date: Thu, 23 May 2024 10:12:07 +0200
From: Christoph Hellwig <hch@....de>
To: Gulam Mohamed <gulam.mohamed@...cle.com>
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	axboe@...nel.dk, shinichiro.kawasaki@....com, chaitanyak@...dia.com,
	hch@....de
Subject: Re: [PATCH V2 for-6.10/block 1/2] loop: Fix a race between loop
 detach and loop open

On Tue, May 21, 2024 at 10:42:48PM +0000, Gulam Mohamed wrote:
> Description
> ===========

That's a weird way to format a patch.  Between this and the odd subject
not matching patch 2 I was tricked into thinking this was just a cover
letter and patch 1 was missing for a while.  Please take a look at other
patches/commit and try to word it similarly.

> V1->V2:
> 	Added a test case, 010, in blktests in tests/loop/

These kind of patch revision changelogs belong after the --- so that they
don't go into git history.  Or even better into the cover letter, which
is missing here.

> Signed-off-by: Gulam Mohamed <gulam.mohamed@...cle.com>
> ---
>  drivers/block/loop.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 28a95fd366fe..9a235d8c062d 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -1717,6 +1717,24 @@ static int lo_compat_ioctl(struct block_device *bdev, blk_mode_t mode,
>  }
>  #endif
>  
> +static int lo_open(struct gendisk *disk, blk_mode_t mode)
> +{
> +        struct loop_device *lo = disk->private_data;
> +        int err;
> +
> +        if (!lo)
> +                return -ENXIO;

->private_data is never cleared, so the NULL check here doesn't
make sense.

> +        err = mutex_lock_killable(&lo->lo_mutex);
> +        if (err)
> +                return err;
> +
> +        if (lo->lo_state == Lo_rundown)
> +                err = -ENXIO;
> +        mutex_unlock(&lo->lo_mutex);

What if we race with setting Lo_rundown and that gets set right
after we unlock here?