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
