linux-kernel - Re: [PATCH] block: Fix the maximum minor value is blk_alloc_ext

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Date:   Mon, 28 Mar 2022 10:20:40 +0200
From:   Jan Kara <jack@...e.cz>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     Jens Axboe <axboe@...nel.dk>, Hannes Reinecke <hare@...e.de>,
        Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org, linux-block@...r.kernel.org,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH] block: Fix the maximum minor value is
 blk_alloc_ext_minor()

On Sat 26-03-22 15:50:46, Christophe JAILLET wrote:
> ida_alloc_range(..., min, max, ...) returns values from min to max,
> inclusive.
> 
> So, NR_EXT_DEVT is a valid idx returned by blk_alloc_ext_minor().
> 
> This is an issue because in device_add_disk(), this value is used in:
>    ddev->devt = MKDEV(disk->major, disk->first_minor);
> and NR_EXT_DEVT is '(1 << MINORBITS)'.
> 
> So, should 'disk->first_minor' be NR_EXT_DEVT, it would overflow.
> 
> Fixes: 22ae8ce8b892 ("block: simplify bdev/disk lookup in blkdev_get")
> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>

Indeed. The patch looks good to me so feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

> ---
> #define MKDEV(ma,mi)	(((ma) << MINORBITS) | (mi))
> 
> This patch is completely speculative, but it seems that idr_alloc() and
> ida_alloc_range() don't have the same semantic regarding the upper bound.
> idr_alloc() looks exclusive, while ida_alloc_range() is inclusive.
> 
> We changed from the first one to the other one in the commit in Fixes:.

Yes, this difference is really a landmine. Matthew, why is the semantics of
max parameter for idr_alloc() different from ida_alloc_range() or say
idr_alloc_u32()? It is really easy to introduce subtle bugs with this...

								Honza

> ---
>  block/genhd.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/genhd.c b/block/genhd.c
> index c9a4fc90d3e9..b8b6759d670f 100644
> --- a/block/genhd.c
> +++ b/block/genhd.c
> @@ -335,7 +335,7 @@ int blk_alloc_ext_minor(void)
>  {
>  	int idx;
>  
> -	idx = ida_alloc_range(&ext_devt_ida, 0, NR_EXT_DEVT, GFP_KERNEL);
> +	idx = ida_alloc_range(&ext_devt_ida, 0, NR_EXT_DEVT - 1, GFP_KERNEL);
>  	if (idx == -ENOSPC)
>  		return -EBUSY;
>  	return idx;
> -- 
> 2.32.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR