Re: [PATCH v2 13/16] xen-blkback: Implement diskseq checks

Roger Pau Monné
2023-06-06T08:28:50Z

On Tue, May 30, 2023 at 04:31:13PM -0400, Demi Marie Obenour wrote:
> This allows specifying a disk sequence number in XenStore. If it does
> not match the disk sequence number of the underlying device, the device
> will not be exported and a warning will be logged. Userspace can use
> this to eliminate race conditions due to major/minor number reuse.
> Old kernels do not support the new syntax, but a later patch will allow
> userspace to discover that the new syntax is supported.
>
> Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
> ---
> drivers/block/xen-blkback/xenbus.c | 112 +++++++++++++++++++++++------
> 1 file changed, 89 insertions(+), 23 deletions(-)
>
> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> index 4807af1d58059394d7a992335dabaf2bc3901721..9c3eb148fbd802c74e626c3d7bcd69dcb09bd921 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -24,6 +24,7 @@ struct backend_info {
> struct xenbus_watch backend_watch;
> unsigned major;
> unsigned minor;
> + unsigned long long diskseq;

Since diskseq is declared as u64 in gendisk, better use the same type
here too?

> char *mode;
> };
>
> @@ -479,7 +480,7 @@ static void xen_vbd_free(struct xen_vbd *vbd)
>
> static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
> unsigned major, unsigned minor, int readonly,
> - int cdrom)
> + bool cdrom, u64 diskseq)
> {
> struct xen_vbd *vbd;
> struct block_device *bdev;
> @@ -507,6 +508,26 @@ static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
> xen_vbd_free(vbd);
> return -ENOENT;
> }
> +
> + if (diskseq) {
> + struct gendisk *disk = bdev->bd_disk;

const.

> +
> + if (unlikely(disk == NULL)) {
> + pr_err("%s: device %08x has no gendisk\n",
> + __func__, vbd->pdevice);
> + xen_vbd_free(vbd);
> + return -EFAULT;

ENODEV or ENOENT might be more accurate IMO.

> + }
> +
> + if (unlikely(disk->diskseq != diskseq)) {
> + pr_warn("%s: device %08x has incorrect sequence "
> + "number 0x%llx (expected 0x%llx)\n",

I prefer %#llx, and likely pr_err like above. Also I think it's now
preferred to not split printed lines, so that `grep "has incorrect
sequence number" ...` can find the instance.

> + __func__, vbd->pdevice, disk->diskseq, diskseq);
> + xen_vbd_free(vbd);
> + return -ENODEV;
> + }
> + }
> +
> vbd->size = vbd_sz(vbd);
>
> if (cdrom || disk_to_cdi(vbd->bdev->bd_disk))
> @@ -707,6 +728,9 @@ static void backend_changed(struct xenbus_watch *watch,
> int cdrom = 0;
> unsigned long handle;
> char *device_type;
> + char *diskseq_str = NULL;

const, and I think there's no need to init to NULL.

> + int diskseq_len;

unsigned int

> + unsigned long long diskseq;

u64

>
> pr_debug("%s %p %d\n", __func__, dev, dev->otherend_id);
>
> @@ -725,10 +749,46 @@ static void backend_changed(struct xenbus_watch *watch,
> return;
> }
>
> - if (be->major | be->minor) {
> - if (be->major != major || be->minor != minor)
> - pr_warn("changing physical device (from %x:%x to %x:%x) not supported.\n",
> - be->major, be->minor, major, minor);
> + diskseq_str = xenbus_read(XBT_NIL, dev->nodename, "diskseq", &diskseq_len);
> + 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
> + * 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) {
> + 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;
> + }

Won't it be simpler to use xenbus_scanf() with %llx formatter?

Also, we might want to fetch "physical-device" and "diskseq" inside
the same xenstore transaction.

Also, you tie this logic to the "physical-device" watch, which
strictly implies that the "diskseq" node must be written to xenstore
before the "physical-device" node. This seems fragile, but I don't
see much better optiono since the "diskseq" is optional.

The node and its behaviour should be documented in blkif.h.

> + if (be->major | be->minor | be->diskseq) {
> + if (be->major != major || be->minor != minor || be->diskseq != diskseq)
> + pr_warn("changing physical device (from %x:%x:%llx to %x:%x:%llx)"
> + " not supported.\n",
> + be->major, be->minor, be->diskseq, major, minor, diskseq);
> return;

You are leaking diskseq_str here, and in all the error cases between
here and up to the call to xen_vbd_create().

It might be better to simnply free diskseq_str once you are done with
the processing, and have set diskseq.

Otherwise see my suggestion of using xenbus_scanf().

Thanks, Roger.