MFC r310013 (by cperciva):

Check that blkfront devices have a non-zero number of sectors and a
non-zero sector size.  Such a device would be a virtual disk of zero
bytes; clearly not useful, and not something we should try to attach.

As a fortuitous side effect, checking that these values are non-zero
here results in them not *becoming* zero later on the function.  This
odd behaviour began with r309124 (clang 3.9.0) but is challenging to
debug; making any changes to this function whatsoever seems to affect
the llvm optimizer behaviour enough to make the unexpected zeroing of
the sector_size variable cease.

PR:		215209
Security:	The potential for variables to unexpectedly become zero
		has worrying consequences for security in general, but
		not so much in this particular context.

MFC r310086:

In xbd_connect(), use correct scanf conversion specifiers for the
feature_barrier and feature_flush variables.  Otherwise, adjacent
variables on the stack, such as sector_size, may be overwritten, with
disastrous results.

Note that I did not see a good reason to revert the addition of zero
checks introduced in r310013.  Better safe than sorry.

PR:		215209
Tested by:	royger

1 files changed, 10 insertions, 2 deletions

diff --git a/sys/dev/xen/blkfront/blkfront.c b/sys/dev/xen/blkfront/blkfront.c
index 4de5884..9eca220 100644
--- a/sys/dev/xen/blkfront/blkfront.c
+++ b/sys/dev/xen/blkfront/blkfront.c
@@ -1245,19 +1245,27 @@ xbd_connect(struct xbd_softc *sc)
 		    xenbus_get_otherend_path(dev));
 		return;
 	}
+	if ((sectors == 0) || (sector_size == 0)) {
+		xenbus_dev_fatal(dev, 0,
+		    "invalid parameters from %s:"
+		    " sectors = %lu, sector_size = %lu",
+		    xenbus_get_otherend_path(dev),
+		    sectors, sector_size);
+		return;
+	}
 	err = xs_gather(XST_NIL, xenbus_get_otherend_path(dev),
 	     "physical-sector-size", "%lu", &phys_sector_size,
 	     NULL);
 	if (err || phys_sector_size <= sector_size)
 		phys_sector_size = 0;
 	err = xs_gather(XST_NIL, xenbus_get_otherend_path(dev),
-	     "feature-barrier", "%lu", &feature_barrier,
+	     "feature-barrier", "%d", &feature_barrier,
 	     NULL);
 	if (err == 0 && feature_barrier != 0)
 		sc->xbd_flags |= XBDF_BARRIER;
 
 	err = xs_gather(XST_NIL, xenbus_get_otherend_path(dev),
-	     "feature-flush-cache", "%lu", &feature_flush,
+	     "feature-flush-cache", "%d", &feature_flush,
 	     NULL);
 	if (err == 0 && feature_flush != 0)
 		sc->xbd_flags |= XBDF_FLUSH;