RTEMS 4.6.2 On-Line Library

A Rapid Tour of GDB Internals

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Chapter 4: A Rapid Tour of GDB Internals

To help the reader to understand what needs to be implemented, we will present briefly how GDB works regardless if the target is local or remote. A debugger is a tool which enables control of the execution of software on a target system. In most of cases, the debugger connects to a target system, attaches a process, inserts breakpoints and resumes execution. Then the normal execution is completely events driven (process execution stopped due to a breakpoint, process fault, single-step,...) coming from the debuggee. It can also directly access some parts of the target processor context (registers, data memory, code memory,...) and change their content. Native GDB debugger can just be seen as special cases where the host and the target are on the same machine and GDB can directly access the target system debug API.

In our case, the host and the target are not on the same machine and an Ethernet link is used to communicate between the different machines. Because GDB needs to be able to support various targets (including Unix core file, ...), each action that needs to be performed on the debuggee is materialized by a field of the following targets_ops structure :

struct target_ops
  char         *to_shortname;   /* Name this target type */
  char         *to_longname;    /* Name for printing */
  char         *to_doc;    /* Documentation.  Does not include trailing
                              newline, and starts with a one-line
                              description (probably similar to
                              to_longname). */
  void        (*to_open) PARAMS ((char *, int));
  void        (*to_close) PARAMS ((int));
  void        (*to_attach) PARAMS ((char *, int));
  void        (*to_detach) PARAMS ((char *, int));
  void        (*to_resume) PARAMS ((int, int, enum target_signal));
  int         (*to_wait) PARAMS ((int, struct target_waitstatus *));
  void        (*to_fetch_registers) PARAMS ((int));
  void        (*to_store_registers) PARAMS ((int));
  void        (*to_prepare_to_store) PARAMS ((void));

  /* Transfer LEN bytes of memory between GDB address MYADDR and
     target address MEMADDR.  If WRITE, transfer them to the target,
     else transfer them from the target.  TARGET is the target from
     which we get this function.

     Return value, N, is one of the following:

     0 means that we can't handle this.  If errno has been set,
     it is the error which prevented us from doing it (FIXME:
     What about bfd_error?).

     positive (call it N) means that we have transferred N bytes
     starting at MEMADDR.  We might be able to handle more bytes
     beyond this length, but no promises.

     negative (call its absolute value N) means that we cannot
     transfer right at MEMADDR, but we could transfer at least
     something at MEMADDR + N.  */

  int         (*to_xfer_memory)
                 PARAMS ((CORE_ADDR memaddr, char *myaddr,
                          int len, int write,
                          struct target_ops * target));

  void        (*to_files_info) PARAMS ((struct target_ops *));
  int         (*to_insert_breakpoint) PARAMS ((CORE_ADDR, char *));
  int         (*to_remove_breakpoint) PARAMS ((CORE_ADDR, char *));
  void        (*to_terminal_init) PARAMS ((void));
  void        (*to_terminal_inferior) PARAMS ((void));
  void        (*to_terminal_ours_for_output) PARAMS ((void));
  void        (*to_terminal_ours) PARAMS ((void));
  void        (*to_terminal_info) PARAMS ((char *, int));
  void        (*to_kill) PARAMS ((void));
  void        (*to_load) PARAMS ((char *, int));
  int         (*to_lookup_symbol) PARAMS ((char *, CORE_ADDR *));
  void        (*to_create_inferior) PARAMS ((char *, char *, char **));
  void        (*to_mourn_inferior) PARAMS ((void));
  int         (*to_can_run) PARAMS ((void));
  void        (*to_notice_signals) PARAMS ((int pid));
  int         (*to_thread_alive) PARAMS ((int pid));
  void        (*to_stop) PARAMS ((void));
  enum strata   to_stratum;
  struct target_ops
                *DONT_USE;      /* formerly to_next */
  int           to_has_all_memory;
  int           to_has_memory;
  int           to_has_stack;
  int           to_has_registers;
  int           to_has_execution;
  struct section_table
  struct section_table
  int           to_magic;
  /* Need sub-structure for target machine related rather than comm related? */

This structure contains pointers to functions (in C++, this would be called a virtual class). Each different target supported by GDB has its own structure with the relevant implementation of the functions (some functions may be not implemented). When a user connects GDB to a target via the ``target'' command, GDB points to the structure corresponding to this target. Then the user can attache GDB to a specific task via the ``attach'' command. We have therefore identified two steps to begin a remote debug session :

  1. the choice of the target type (in our case RTEMS),
  2. the choice of what to debug (entire system, specific task,...),
Note that in the case of natives debugger, the choice of the target is implicitly performed by commands like run, attach, detach. Several figures will now be described showing the main steps of a debug session.

Figure Debug session initialization explains how the debugger connects to the target :

  1. The debugger opens a connection to the target. The word ``connection'' doesn't only mean Ethernet or serial link connection but all the ways by which a process can communicate with another one (direct function call, messages mailbox, ...),
  2. The targets checks if it can accept or reject this connection,
  3. If the connection is accepted, the host ``attaches'' the process,
  4. the target stops the process, notifies a child's stop to the host and waits for command,
  5. the host can ask information about the debugged process (name, registers,...) or perform some action like setting breakpoints, ...

Figure Breakpoint and process execution explains how the debugger manages the breakpoints and controls the execution of a process :

  1. The host asks the debuggee what is the opcode at the concerned address in order for GDB to memorize this instruction,
  2. the host sends a CONTINUE command : it asks the target to write the ``DEBUG'' opcode (for example, the INTEL ``DEBUG'' opcode is INT3 which generate a breakpoint trap) instead of the debugged opcode.
  3. then the host waits for events,
  4. after the change of instruction, the target resumes the execution of the debuggee,
  5. when the ``DEBUG'' opcode is executed, the breakpoint exception handler is executed and it notifies the host that the process is stopped. Then it waits for commands (if no command is sent after a certain amount of time, the connection will be closed by the target).
  6. the host asks the target to re-write the right opcode instead of the ''DEBUG'' opcode and then can ask information

Figure Breakpoint and process execution also shows the case of other ``CONTINUE'' commands (remember that the ``DEBUG'' opcode has been replaced by the right instruction):

  1. Host sends first a ``single step'' command to execute the debugged instruction,
  2. It then waits for ``single step`` exception event,
  3. the target, once the single step executed, calls the debug exception handler. It notifies the host that execution is suspended and wait for commands.
  4. the host asks the target to re-write the ``DEBUG'' opcode (breakpoint trap) instead of the debugged one.
  5. then the host sends a ``CONTINUE'' command in order the target to resume the process execution to the next breakpoint.

Figure Detach a process and close a connection explains how the debugger disconnects from a target :

  1. the host sends a detach command to the target.
  2. the target detaches the concerned process, notifies the detachment and resumes the process execution.
  3. once notified, the host sends a close connection command.
  4. the target closes the connection.
These 3 examples show that the mains actions that are performed by the host debugger on the target are only simple actions which look like : Debug session initialization Breakpoint and process execution Detach a process and close a connection

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