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

[Demi Marie Obenour]

On Tue, Jun 06, 2023 at 10:25:47AM +0200, Roger Pau Monné wrote:
> 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@xxxxxxxxxxxxxxxxxxxxxx>
> > ---
> >  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?

simple_strtoull() returns an unsigned long long, and C permits unsigned
long long to be larger than 64 bits.

> >  	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.

I will drop it, as this turns out to be unreachable code.

> > +		}
> > +
> > +		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.

Ah, so _that_ is why I got a warning from checkpatch!

> > +				__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?

xenbus_scanf() doesn’t check for overflow and accepts lots of junk it
really should not.  Should this be fixed in xenbus_scanf()?

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

Should the rest of the xenstore reads be included in the same
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.

What about including the diskseq in the "physical-device" node?  Perhaps
use diskseq@major:minor syntax?

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

Indeed so.

> > +	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().

I will fix this by moving the diskseq reading code into its own
function.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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