linux-kernel - Re: [PATCH 1/2] xen-blkback: Implement diskseq checks

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Date:   Wed, 7 Jun 2023 00:44:22 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Demi Marie Obenour <demi@...isiblethingslab.com>
Cc:     Roger Pau Monné <roger.pau@...rix.com>,
        Jens Axboe <axboe@...nel.dk>,
        Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>, xen-devel@...ts.xenproject.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] xen-blkback: Implement diskseq checks

On Thu, Jun 01, 2023 at 05:48:22PM -0400, Demi Marie Obenour wrote:
> +	if (diskseq) {
> +		struct gendisk *disk = bdev->bd_disk;
> +
> +		if (unlikely(disk == NULL)) {
> +			pr_err("%s: device %08x has no gendisk\n",
> +			       __func__, vbd->pdevice);
> +			xen_vbd_free(vbd);
> +			return -EFAULT;
> +		}

bdev->bd_disk is never NULL.

> +	diskseq_str = xenbus_read(XBT_NIL, dev->nodename, "diskseq", &diskseq_len);

Please avoid the overly long line.

> +	if (IS_ERR(diskseq_str)) {
> +		int err = PTR_ERR(diskseq_str);
> +		diskseq_str = NULL;
> +
> +		/*
> +		 * If this does not exist, it means legacy userspace that does not

.. even more so in comments.

> +		 * support diskseq.
> +		 */
> +		if (unlikely(!XENBUS_EXIST_ERR(err))) {
> +			xenbus_dev_fatal(dev, err, "reading diskseq");
> +			return;
> +		}
> +		diskseq = 0;
> +	} else if (diskseq_len <= 0) {
> +		xenbus_dev_fatal(dev, -EFAULT, "diskseq must not be empty");
> +		goto fail;
> +	} else if (diskseq_len > 16) {

No need for a else after a return.

> +		xenbus_dev_fatal(dev, -ERANGE, "diskseq too long: got %d but limit is 16",
> +				 diskseq_len);
> +		goto fail;
> +	} else if (diskseq_str[0] == '0') {
> +		xenbus_dev_fatal(dev, -ERANGE, "diskseq must not start with '0'");
> +		goto fail;
> +	} else {
> +		char *diskseq_end;
> +		diskseq = simple_strtoull(diskseq_str, &diskseq_end, 16);
> +		if (diskseq_end != diskseq_str + diskseq_len) {
> +			xenbus_dev_fatal(dev, -EINVAL, "invalid diskseq");
> +			goto fail;
> +		}
> +		kfree(diskseq_str);
> +		diskseq_str = NULL;
> +	}

And I suspect the code will be a lot easier to follow if you move
the diskseq validation into a separate helper.