Re: [PATCH 2/2] gdbserver: track current process as well as current thread

[Andrew Burgess <aburgess@redhat.com>]

Pedro Alves <pedro@palves.net> writes:

> The recent commit 421490af33bf ("gdbserver/linux: Access memory even
> if threads are running") caused a regression in
> gdb.threads/access-mem-running-thread-exit.exp with gdbserver, which I
> somehow missed.  Like so:
>
>  (gdb) print global_var
>  Cannot access memory at address 0x555555558010
>  (gdb) FAIL: gdb.threads/access-mem-running-thread-exit.exp: non-stop: access mem (print global_var after writing, inf=2, iter=1)
>
> The problem starts with GDB telling GDBserver to select a thread, via
> the Hg packet, which GDBserver complies with, then that thread exits,
> and GDB, without knowing the thread is gone, tries to write to memory,
> through the context of the previously selected Hg thread.
>
> GDBserver's GDB-facing memory access routines, gdb_read_memory and
> gdb_write_memory, call set_desired_thread to make GDBserver re-select
> the thread that GDB has selected with the Hg packet.  Since the thread
> is gone, set_desired_thread returns false, and the memory access
> fails.
>
> Now, to access memory, it doesn't really matter which thread is
> selected.  All we should need is the target process.  Even if the
> thread that GDB previously selected is gone, and GDB does not yet know
> about that exit, it shouldn't matter, GDBserver should still know
> which process that thread belonged to.
>
> Fix this by making GDBserver track the current process separately,
> like GDB also does.  Add a new set_desired_process routine that is
> similar to set_desired_thread, but just sets the current process,
> leaving the current thread as NULL.  Use it in the GDB-facing memory
> read and write routines, to avoid failing if the selected thread is
> gone, but the process is still around.
>
> Change-Id: I4ff97cb6f42558efbed224b30d5c71f6112d44cd
> ---
>  gdbserver/gdbthread.h  |  1 +
>  gdbserver/inferiors.cc | 26 +++++++++++++++++++------
>  gdbserver/server.cc    |  4 ++--
>  gdbserver/target.cc    | 44 ++++++++++++++++++++++++++++++++++++++++--
>  gdbserver/target.h     | 15 +++++++++++++-
>  5 files changed, 79 insertions(+), 11 deletions(-)
>
> diff --git a/gdbserver/gdbthread.h b/gdbserver/gdbthread.h
> index 10ae1cb7c87..8b897e73d33 100644
> --- a/gdbserver/gdbthread.h
> +++ b/gdbserver/gdbthread.h
> @@ -252,6 +252,7 @@ class scoped_restore_current_thread
>  
>  private:
>    bool m_dont_restore = false;
> +  process_info *m_process;
>    thread_info *m_thread;
>  };
>  
> diff --git a/gdbserver/inferiors.cc b/gdbserver/inferiors.cc
> index 678d93c77a1..3d0a8b0e716 100644
> --- a/gdbserver/inferiors.cc
> +++ b/gdbserver/inferiors.cc
> @@ -26,6 +26,11 @@
>  std::list<process_info *> all_processes;
>  std::list<thread_info *> all_threads;
>  
> +/* The current process.  */
> +static process_info *current_process_;
> +
> +/* The current thread.  This is either a thread of CURRENT_PROCESS, or
> +   NULL.  */
>  struct thread_info *current_thread;
>  
>  /* The current working directory used to start the inferior.
> @@ -130,6 +135,7 @@ clear_inferiors (void)
>    clear_dlls ();
>  
>    switch_to_thread (nullptr);
> +  current_process_ = nullptr;
>  }
>  
>  struct process_info *
> @@ -153,6 +159,8 @@ remove_process (struct process_info *process)
>    free_all_breakpoints (process);
>    gdb_assert (find_thread_process (process) == NULL);
>    all_processes.remove (process);
> +  if (current_process () == process)
> +    switch_to_process (nullptr);
>    delete process;
>  }
>  
> @@ -205,8 +213,7 @@ get_thread_process (const struct thread_info *thread)
>  struct process_info *
>  current_process (void)
>  {
> -  gdb_assert (current_thread != NULL);
> -  return get_thread_process (current_thread);
> +  return current_process_;

A bit late I know, but I wonder if the assert, or something similar,
should have been retained here?

The comment on this function currently (though Baris has a patch
proposed to change this), says this function should only be called in a
context where the result will not be nullptr.  Given that, not of the
(many) existing uses check the return value of this function against
nullptr.

Happy to roll a patch to add the assert back, just wondered if there was
a reason for its removal.

Thanks,
Andrew



>  }
>  
>  /* See gdbsupport/common-gdbthread.h.  */
> @@ -223,6 +230,10 @@ switch_to_thread (process_stratum_target *ops, ptid_t ptid)
>  void
>  switch_to_thread (thread_info *thread)
>  {
> +  if (thread != nullptr)
> +    current_process_ = get_thread_process (thread);
> +  else
> +    current_process_ = nullptr;
>    current_thread = thread;
>  }
>  
> @@ -231,9 +242,8 @@ switch_to_thread (thread_info *thread)
>  void
>  switch_to_process (process_info *proc)
>  {
> -  int pid = pid_of (proc);
> -
> -  switch_to_thread (find_any_thread_of_pid (pid));
> +  current_process_ = proc;
> +  current_thread = nullptr;
>  }
>  
>  /* See gdbsupport/common-inferior.h.  */
> @@ -254,6 +264,7 @@ set_inferior_cwd (std::string cwd)
>  
>  scoped_restore_current_thread::scoped_restore_current_thread ()
>  {
> +  m_process = current_process_;
>    m_thread = current_thread;
>  }
>  
> @@ -262,5 +273,8 @@ scoped_restore_current_thread::~scoped_restore_current_thread ()
>    if (m_dont_restore)
>      return;
>  
> -  switch_to_thread (m_thread);
> +  if (m_thread != nullptr)
> +    switch_to_thread (m_thread);
> +  else
> +    switch_to_process (m_process);
>  }
> diff --git a/gdbserver/server.cc b/gdbserver/server.cc
> index 33c42714e72..f9c02a9c6da 100644
> --- a/gdbserver/server.cc
> +++ b/gdbserver/server.cc
> @@ -1067,7 +1067,7 @@ gdb_read_memory (CORE_ADDR memaddr, unsigned char *myaddr, int len)
>        /* (assume no half-trace half-real blocks for now) */
>      }
>  
> -  if (set_desired_thread ())
> +  if (set_desired_process ())
>      res = read_inferior_memory (memaddr, myaddr, len);
>    else
>      res = 1;
> @@ -1088,7 +1088,7 @@ gdb_write_memory (CORE_ADDR memaddr, const unsigned char *myaddr, int len)
>      {
>        int ret;
>  
> -      if (set_desired_thread ())
> +      if (set_desired_process ())
>  	ret = target_write_memory (memaddr, myaddr, len);
>        else
>  	ret = EIO;
> diff --git a/gdbserver/target.cc b/gdbserver/target.cc
> index 883242377c0..adcfe6e7bcc 100644
> --- a/gdbserver/target.cc
> +++ b/gdbserver/target.cc
> @@ -29,16 +29,56 @@
>  
>  process_stratum_target *the_target;
>  
> -int
> +/* See target.h.  */
> +
> +bool
>  set_desired_thread ()
>  {
>    client_state &cs = get_client_state ();
>    thread_info *found = find_thread_ptid (cs.general_thread);
>  
> -  switch_to_thread (found);
> +  if (found == nullptr)
> +    {
> +      process_info *proc = find_process_pid (cs.general_thread.pid ());
> +      if (proc == nullptr)
> +	{
> +	  threads_debug_printf
> +	    ("did not find thread nor process for general_thread %s",
> +	     cs.general_thread.to_string ().c_str ());
> +	}
> +      else
> +	{
> +	  threads_debug_printf
> +	    ("did not find thread for general_thread %s, but found process",
> +	     cs.general_thread.to_string ().c_str ());
> +	}
> +      switch_to_process (proc);
> +    }
> +  else
> +    switch_to_thread (found);
> +
>    return (current_thread != NULL);
>  }
>  
> +/* See target.h.  */
> +
> +bool
> +set_desired_process ()
> +{
> +  client_state &cs = get_client_state ();
> +
> +  process_info *proc = find_process_pid (cs.general_thread.pid ());
> +  if (proc == nullptr)
> +    {
> +      threads_debug_printf
> +	("did not find process for general_thread %s",
> +	 cs.general_thread.to_string ().c_str ());
> +    }
> +  switch_to_process (proc);
> +
> +  return proc != nullptr;
> +}
> +
>  int
>  read_inferior_memory (CORE_ADDR memaddr, unsigned char *myaddr, int len)
>  {
> diff --git a/gdbserver/target.h b/gdbserver/target.h
> index f3172e2ed7e..6c536a30778 100644
> --- a/gdbserver/target.h
> +++ b/gdbserver/target.h
> @@ -699,7 +699,20 @@ target_thread_pending_child (thread_info *thread)
>  
>  int read_inferior_memory (CORE_ADDR memaddr, unsigned char *myaddr, int len);
>  
> -int set_desired_thread ();
> +/* Set GDBserver's current thread to the thread the client requested
> +   via Hg.  Also switches the current process to the requested
> +   process.  If the requested thread is not found in the thread list,
> +   then the current thread is set to NULL.  Likewise, if the requested
> +   process is not found in the process list, then the current process
> +   is set to NULL.  Returns true if the requested thread was found,
> +   false otherwise.  */
> +
> +bool set_desired_thread ();
> +
> +/* Set GDBserver's current process to the process the client requested
> +   via Hg.  The current thread is set to NULL.  */
> +
> +bool set_desired_process ();
>  
>  std::string target_pid_to_str (ptid_t);
>  
> -- 
> 2.26.2