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

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

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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