linux-kernel - Re: question about bd_inode hashing against device_add() // Re: [PATCH 03/11] block: call bdev_add later in device_add

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <2025103106-proposal-jogging-a076@gregkh>
Date: Fri, 31 Oct 2025 15:34:09 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Gao Xiang <hsiangkao@...ux.alibaba.com>
Cc: Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
	Jan Kara <jack@...e.cz>, Christian Brauner <brauner@...nel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Jason Wang <jasowang@...hat.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Luis Chamberlain <mcgrof@...nel.org>, linux-block@...r.kernel.org,
	Joseph Qi <joseph.qi@...ux.alibaba.com>,
	guanghuifeng@...ux.alibaba.com, zongyong.wzy@...baba-inc.com,
	zyfjeff@...ux.alibaba.com, "Rafael J. Wysocki" <rafael@...nel.org>,
	Danilo Krummrich <dakr@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: question about bd_inode hashing against device_add() // Re:
 [PATCH 03/11] block: call bdev_add later in device_add_disk

On Fri, Oct 31, 2025 at 08:23:32PM +0800, Gao Xiang wrote:
> 
> 
> On 2025/10/31 18:12, Gao Xiang wrote:
> > Hi Greg,
> > 
> > On 2025/10/31 17:58, Greg Kroah-Hartman wrote:
> > > On Fri, Oct 31, 2025 at 05:54:10PM +0800, Gao Xiang wrote:
> > > > 
> > > > 
> > > > On 2025/10/31 17:45, Christoph Hellwig wrote:
> 
> ...
> 
> > > > > But why does the device node
> > > > > get created earlier?  My assumption was that it would only be
> > > > > created by the KOBJ_ADD uevent.  Adding the device model maintainers
> > > > > as my little dig through the core drivers/base/ code doesn't find
> > > > > anything to the contrary, but maybe I don't fully understand it.
> > > > 
> > > > AFAIK, device_add() is used to trigger devtmpfs file
> > > > creation, and it can be observed if frequently
> > > > hotpluging device in the VM and mount.  Currently
> > > > I don't have time slot to build an easy reproducer,
> > > > but I think it's a real issue anyway.
> > > 
> > > As I say above, that's not normal, and you have to be root to do this,
> I just spent time to reproduce with dynamic loop devices and
> actually it's easy if msleep() is located artificiallly,
> the diff as below:
> 
> diff --git a/block/bdev.c b/block/bdev.c
> index 810707cca970..a4273b5ad456 100644
> --- a/block/bdev.c
> +++ b/block/bdev.c
> @@ -821,7 +821,7 @@ struct block_device *blkdev_get_no_open(dev_t dev, bool autoload)
>  	struct inode *inode;
> 
>  	inode = ilookup(blockdev_superblock, dev);
> -	if (!inode && autoload && IS_ENABLED(CONFIG_BLOCK_LEGACY_AUTOLOAD)) {
> +	if (0) {
>  		blk_request_module(dev);
>  		inode = ilookup(blockdev_superblock, dev);
>  		if (inode)
> diff --git a/block/genhd.c b/block/genhd.c
> index 9bbc38d12792..3c9116fdc1ce 100644
> --- a/block/genhd.c
> +++ b/block/genhd.c
> @@ -428,6 +428,8 @@ static void add_disk_final(struct gendisk *disk)
>  	set_bit(GD_ADDED, &disk->state);
>  }
> 
> +#include <linux/delay.h>
> +
>  static int __add_disk(struct device *parent, struct gendisk *disk,
>  		      const struct attribute_group **groups,
>  		      struct fwnode_handle *fwnode)
> @@ -497,6 +499,9 @@ static int __add_disk(struct device *parent, struct gendisk *disk,
>  	if (ret)
>  		goto out_free_ext_minor;
> 
> +	if (disk->major == LOOP_MAJOR)
> +		msleep(2500);           // delay 2.5s for all loops
> +

Yes, so you need to watch for the uevent to happen, THEN it is safe to
access the block device.  Doing it before then isn't a good idea :)

But, if you think this is an issue, do you have a patch that passes your
testing to fix it?

thanks,

greg k-h