[RFC PATCH] Implement diskseq checks in blkback

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

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.
Older kernels will ignore this, so it is safe for userspace to set it
unconditionally.

Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
---
I'm marking this patch as RFC because I have not tested it beyond
compiliation, and because I want to get feedback on the API before I put
more effort into it.

 drivers/block/xen-blkback/xenbus.c | 35 ++++++++++++++++++++++++++++--
 1 file changed, 33 insertions(+), 2 deletions(-)

diff --git a/drivers/block/xen-blkback/xenbus.c 
b/drivers/block/xen-blkback/xenbus.c
index 
97de13b14175eb8ced14d1649a3be461a464ee8a..cb531236b19709c791a82c868bfb4ed9aa284485
 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -480,7 +480,7 @@ MODULE_PARM_DESC(feature_persistent,
 
 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;
@@ -508,6 +508,25 @@ static int xen_vbd_create(struct xen_blkif *blkif, 
blkif_vdev_t handle,
                xen_vbd_free(vbd);
                return -ENOENT;
        }
+
+       if (diskseq != (u64)-1) {
+               struct gendisk *disk = bdev->bd_disk;
+               if (unlikely(disk == NULL)) {
+                       pr_err("xen_vbd_create: device %08x has no gendisk\n",
+                              vbd->pdevice);
+                       xen_vbd_free(vbd);
+                       return -EFAULT;
+               }
+
+               if (unlikely(disk->diskseq != diskseq)) {
+                       pr_warn("xen_vbd_create: device %08x has incorrect 
sequence "
+                               "number 0x%llx (expected 0x%llx)\n",
+                               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))
@@ -712,6 +731,7 @@ static void backend_changed(struct xenbus_watch *watch,
        int cdrom = 0;
        unsigned long handle;
        char *device_type;
+       unsigned long long diskseq;
 
        pr_debug("%s %p %d\n", __func__, dev, dev->otherend_id);
 
@@ -730,6 +750,17 @@ static void backend_changed(struct xenbus_watch *watch,
                return;
        }
 
+       err = xenbus_scanf(XBT_NIL, dev->nodename, "diskseq", "%llx", &diskseq);
+       if (err == -ENOENT) {
+               diskseq = (uint64_t)-1;
+       } else if (err != 1) {
+               xenbus_dev_fatal(dev, err, "reading diskseq");
+               return;
+       } else if (diskseq == (uint64_t)-1) {
+               xenbus_dev_fatal(dev, err, "UINT64_MAX is not a valid diskseq");
+               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",
@@ -763,7 +794,7 @@ static void backend_changed(struct xenbus_watch *watch,
        be->minor = minor;
 
        err = xen_vbd_create(be->blkif, handle, major, minor,
-                            !strchr(be->mode, 'w'), cdrom);
+                            !strchr(be->mode, 'w'), cdrom, diskseq);
 
        if (err)
                xenbus_dev_fatal(dev, err, "creating vbd structure");
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab