Re: [PATCH] gdb: add 'maintenance print record-instruction' command

[Lancelot SIX <lsix@lancelotsix.com>]

Hi,

I do not not know this part well (to say the least), so my comments will
only focus on style

On Wed, Dec 07, 2022 at 02:50:00PM +0100, Bruno Larsen via Gdb-patches wrote:
> While chasing some reverse debugging bugs, I found myself wondering what
> was recorded by GDB to undo and redo a certain instruction. This commit
> implements a simple way of printing that information.
> ---
>  gdb/NEWS            |  6 ++++
>  gdb/doc/gdb.texinfo |  8 +++++
>  gdb/record-full.c   | 81 +++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 95 insertions(+)
> 
> diff --git a/gdb/NEWS b/gdb/NEWS
> index c4ccfcc9e32..d6ce6bf86a0 100644
> --- a/gdb/NEWS
> +++ b/gdb/NEWS
> @@ -103,6 +103,12 @@
>  
>  * New commands
>  
> +maintenance print record-instruction [ N ]
> +  Print the recorded information for a given instruction. If N is not given
> +  prints how GDB would undo the last instruction executed. If N is negative,
> +  prints how GDB would undo the N-th previous instruction, and if N is
> +  positive, it prints how GDB will redo the N-th following instruction.
> +
>  maintenance set ignore-prologue-end-flag on|off
>  maintenance show ignore-prologue-end-flag
>    This setting, which is off by default, controls whether GDB ignores the
> diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
> index 5b566669975..807af351e79 100644
> --- a/gdb/doc/gdb.texinfo
> +++ b/gdb/doc/gdb.texinfo
> @@ -40531,6 +40531,14 @@ that symbol is described.  The type chain produced by this command is
>  a recursive definition of the data type as stored in @value{GDBN}'s
>  data structures, including its flags and contained types.
>  
> +@kindex maint print record-instruction
> +@item maint print record-instruction
> +@itemx maint print record-instruction @var{N}
> +@cindex print how GDB recorded a given instruction.  If N is not positive
> +number, it prints the values stored by the inferior before the N-th previous
> +instruction was exectued.  If N is positive, print the values after the N-th
> +following instruction is executed.  If N is not given, 0 is assumed.
> +
>  @kindex maint selftest
>  @cindex self tests
>  @item maint selftest @r{[}-verbose@r{]} @r{[}@var{filter}@r{]}
> diff --git a/gdb/record-full.c b/gdb/record-full.c
> index 48b92281fe6..47cdf75eea4 100644
> --- a/gdb/record-full.c
> +++ b/gdb/record-full.c
> @@ -2764,6 +2764,79 @@ set_record_full_insn_max_num (const char *args, int from_tty,
>      }
>  }
>  
> +/* Implement the 'maintenance print record-instruction' command.  */
> +
> +static void
> +maintenance_print_record_instruction (const char *args, int from_tty)

I think, usually the command implementation have a _command suffix in
the name.  I do not know if this is a hard rule.

> +{
> +  struct record_full_entry* to_print = record_full_list;
> +
> +  if (args != nullptr)
> +    {
> +      int offset = value_as_long (parse_and_eval (args));
> +      if (offset > 0)
> +	{
> +	  /* Move forward OFFSET instructions.  We know we found the
> +	     end of an instruction when to_print->type is 0.  */
> +	  while (to_print->next && offset > 0)

to_print->next != nullptr

> +	    {
> +	      to_print = to_print->next;
> +	      if (!to_print->type)

I would find it easier to read if you spelled it as:

  if (to_print->type != record_full_end)

rather than relying on record_full_end's value being 0.  When dealing
with enum I find it clearer to use the symbolic name rather than the
underlying value.


> +		offset --;

Is the space before "--" intentional?

> +	    }
> +	  if (offset != 0)
> +	    error (_("Not enough recorded history"));
> +	}
> +      else
> +	{
> +	  while (to_print->prev && offset < 0)

to_print->prev != nullptr

> +	    {
> +	      to_print = to_print->prev;
> +	      if (!to_print->type)

to_print->type == record_full_end ?

> +		offset++;
> +	    }
> +	  if (offset != 0)
> +	    error (_("Not enough recorded history"));
> +	}
> +    }
> +  gdb_assert (to_print != nullptr);
> +
> +  /* Go back to the start of the instruction.  */
> +  while (to_print->prev && to_print->prev->type)

Similarly: to_print->prev != nullptr && to_print->prev->type != record_full_end?

Best,
Lancelot.

> +    to_print = to_print->prev;
> +
> +  while (to_print->type)
> +    {
> +      switch (to_print->type)
> +	{
> +	  case record_full_reg:
> +	    {
> +	      gdb_byte* b = record_full_get_loc (to_print);
> +	      gdb_printf ("Register %%%s changed:",
> +			  gdbarch_register_name (target_gdbarch (),
> +						 to_print->u.reg.num));
> +	      for (int i = 0; i < to_print->u.reg.len; i++)
> +		gdb_printf (" %02x",b[i]);
> +	      gdb_printf ("\n");
> +	      break;
> +	    }
> +	  case record_full_mem:
> +	    {
> +	      gdb_byte* b = record_full_get_loc (to_print);
> +	      gdb_printf ("%d bytes of memory at address %s changed from:",
> +			  to_print->u.mem.len,
> +			  print_core_address (target_gdbarch (),
> +					      to_print->u.mem.addr));
> +	      for (int i = 0; i < to_print->u.mem.len; i++)
> +		gdb_printf (" %02x",b[i]);
> +	      gdb_printf ("\n");
> +	      break;
> +	    }
> +	}
> +      to_print = to_print->next;
> +    }
> +}
> +
>  void _initialize_record_full ();
>  void
>  _initialize_record_full ()
> @@ -2868,4 +2941,12 @@ When ON, query if PREC cannot record memory change of next instruction."),
>    c = add_alias_cmd ("memory-query", record_full_memory_query_cmds.show,
>  		     no_class, 1,&show_record_cmdlist);
>    deprecate_cmd (c, "show record full memory-query");
> +
> +  add_cmd ("record-instruction", class_maintenance,
> +	   maintenance_print_record_instruction,
> +	   _("\
> +Print a recorded instruction.\nIf no argument is provided, print the last \
> +instruction recorded.\nIf a negative argument is given, prints how the nth \
> +previous instruction will be undone.\nIf a positive argument is given, prints \
> +how the nth following instruction will be redone."), &maintenanceprintlist);
>  }
> -- 
> 2.38.1
>