commit b999e2038dbc54e2c8b1c390f8b8fe50d0f1d10a
Author: John Baldwin <jhb@FreeBSD.org>
Date:   Tue Jan 9 13:35:17 2018 -0800

    Don't return stale data from fbsd_pid_to_exec_file for kernel processes.
    
    For processes without an associated executable (such as kernel processes),
    the kern.proc.pathname.<pid> system control node returns a length of zero
    without modifying the user's buffer.  Detect this case and return NULL
    rather than the previous contents of the static buffer 'buf'.
    
    gdb/ChangeLog:
    
            * fbsd-nat.c (fbsd_pid_to_exec_file) [KERN_PROC_PATHNAME]: Return
            NULL for an empty pathname.

diff --git gdb/fbsd-nat.c gdb/fbsd-nat.c
index ec4eed9abe..d0aaf89145 100644
--- gdb/fbsd-nat.c
+++ gdb/fbsd-nat.c
@@ -63,7 +63,10 @@ fbsd_pid_to_exec_file (struct target_ops *self, int pid)
   mib[3] = pid;
   buflen = sizeof buf;
   if (sysctl (mib, 4, buf, &buflen, NULL, 0) == 0)
-    return buf;
+    /* The kern.proc.pathname.<pid> sysctl returns a length of zero
+       for processes without an associated executable such as kernel
+       processes.  */
+    return buflen == 0 ? NULL : buf;
 #endif
 
   xsnprintf (name, PATH_MAX, "/proc/%d/exe", pid);
