When a controlled process performs a successful exec(2), the behavior of the debugger is controlled by the ::set -o follow_exec_mode option, as described in Summary of Command-line Options. If the debugger and victim process have the same data model, then the "stop" and "follow" modes determine whether MDB automatically continues the target or returns to the debugger prompt following the exec. If the debugger and victim process have a different data model, then the "follow" behavior causes MDB to automatically re-exec the MDB binary with the appropriate data model and re-attach to the process, still stopped on return from the exec. Not all debugger state is preserved across this re-exec.
If a 32-bit victim process execs a 64-bit program, then "stop" will return to the command prompt, but the debugger will no longer be able to examine the process because it is now using the 64-bit data model. To resume debugging, execute the ::release -a dcmd, quit MDB, and then execute mdb -p pid to re-attach the 64-bit debugger to the process.
If a 64-bit victim process execs a 32-bit program, then "stop" will return to the command prompt, but the debugger will only provide limited capabilities for examining the new process. All built-in dcmds will work as advertised, but loadable dcmds will not since they do not perform data model conversion of structures. The user should release and reattach the debugger to the process as described above in order to restore full debugging capabilities.