9p/xen: fix check for xenbus_read error in front_probe

If the xen bus exists but does not expose the proper interface, it is
possible to get a non-zero length but still some error, leading to
strcmp failing trying to load invalid memory addresses e.g.
fffffffffffffffe.

There is then no need to check length when there is no error, as the
xenbus driver guarantees that the string is nul-terminated.

Link: http://lkml.kernel.org/r/1534236007-10170-1-git-send-email-asmadeus@codewreck.org
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Latchesar Ionkov <lucho@ionkov.net>

diff --git a/net/9p/trans_xen.c b/net/9p/trans_xen.c
index c2d54ac76bfdbb72fa7ad24d63f7379afdc5433d..843cb823d9b95eb31ada57858944090afad6ee38 100644 (file)
--- a/net/9p/trans_xen.c
+++ b/net/9p/trans_xen.c
@@ -391,8 +391,8 @@ static int xen_9pfs_front_probe(struct xenbus_device *dev,
        unsigned int max_rings, max_ring_order, len = 0;
 
        versions = xenbus_read(XBT_NIL, dev->otherend, "versions", &len);
-       if (!len)
-               return -EINVAL;
+       if (IS_ERR(versions))
+               return PTR_ERR(versions);
        if (strcmp(versions, "1")) {
                kfree(versions);
                return -EINVAL;