[Bug 266862] devctl getpath cause panic invoking with root0

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=266862

            Bug ID: 266862
           Summary: devctl getpath cause panic invoking with root0
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: takawata@FreeBSD.org

Invoking 
% devctl devpath FreeBSD root0
cause kernel panic

This may because parent of "root0" device is NULL and invoking 
BUS_GET_DEVICE_PATH(device_get_parent(dev), dev, locator, sb);
references NULL pointer.
So check before invoking it.

Following code is not tested yet.

diff --git a/sys/kern/subr_bus.c b/sys/kern/subr_bus.c
index 041e77259313..92447f825643 100644
--- a/sys/kern/subr_bus.c
+++ b/sys/kern/subr_bus.c
@@ -5310,9 +5310,13 @@ device_get_path(device_t dev, const char *locator)
        ssize_t len;
        char *rv = NULL;
        int error;
+       device_t parent = device_get_parent(dev);
+
+       if(parent == NULL)
+               return EINVAL;

        sb = sbuf_new(NULL, NULL, 0, SBUF_AUTOEXTEND | SBUF_INCLUDENUL);
-       error = BUS_GET_DEVICE_PATH(device_get_parent(dev), dev, locator, sb);
+       error = BUS_GET_DEVICE_PATH(parent, dev, locator, sb);
        sbuf_finish(sb);        /* Note: errors checked with sbuf_len() below
*/
        if (error != 0)
                goto out;

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