Re: [PATCH 7/7] Reset Windows hardware breakpoints on executable's entry point

[Pedro Alves <palves@redhat.com>]

On 5/25/20 7:56 PM, Hannes Domani via Gdb-patches wrote:

> This patch creates an internal breakpoint on the process entry point, which
> when it is reached, resets all active hardware breakpoints, and continues
> execution.

Missing ChangeLog entry.

> ---
>  gdb/windows-tdep.c | 130 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 130 insertions(+)
> 
> diff --git a/gdb/windows-tdep.c b/gdb/windows-tdep.c
> index aa0adeba99..90e4794fc5 100644
> --- a/gdb/windows-tdep.c
> +++ b/gdb/windows-tdep.c
> @@ -37,6 +37,7 @@
>  #include "coff/internal.h"
>  #include "libcoff.h"
>  #include "solist.h"
> +#include "observable.h"
>  
>  #define CYGWIN_DLL_NAME "cygwin1.dll"
>  
> @@ -870,6 +871,99 @@ windows_get_siginfo_type (struct gdbarch *gdbarch)
>    return siginfo_type;
>  }
>  
> +/* Windows-specific cached data.  This is used by GDB for caching
> +   purposes for each inferior.  This helps reduce the overhead of
> +   transfering data from a remote target to the local host.  */
> +struct windows_info
> +{
> +  CORE_ADDR entry_point = 0;
> +};
> +
> +/* Per-inferior data key.  */
> +static const struct inferior_key<windows_info> windows_inferior_data;

This should be program_space_key / per-program_space data, instead of
per-inferior data.

You may want to take a look at the jit.c code, which is doing similar
things.

> +
> +/* Frees whatever allocated space there is to be freed and sets INF's
> +   Windows cache data pointer to NULL.  */
> +
> +static void
> +invalidate_windows_cache_inf (struct inferior *inf)
> +{
> +  windows_inferior_data.clear (inf);
> +}
> +
> +/* Fetch the Windows cache info for INF.  This function always returns a
> +   valid INFO pointer.  */
> +
> +static struct windows_info *
> +get_windows_inferior_data (void)

Drop the (void), only old pre-C++ code has it.  You can also drop
redundant "struct" throughout if you like.

> +{
> +  struct windows_info *info;
> +  struct inferior *inf = current_inferior ();
> +
> +  info = windows_inferior_data.get (inf);
> +  if (info == NULL)
> +    info = windows_inferior_data.emplace (inf);
> +
> +  return info;
> +}
> +
> +/* Breakpoint on entry point where any active hardware breakpoints will
> +   be reset.  */

Please expand the comments, explaining why this is necessary
in the first place.

> +static struct breakpoint_ops entry_point_breakpoint_ops;
> +
> +/* Reset active hardware breakpoints.  */
> +
> +static bool
> +reset_hardware_breakpoints (struct breakpoint *b)
> +{
> +  if (b->type != bp_hardware_breakpoint
> +      && b->type != bp_hardware_watchpoint
> +      && b->type != bp_read_watchpoint
> +      && b->type != bp_access_watchpoint)
> +    return false;

This should instead look at locations and their bp_loc_type,
looking for bp_loc_hardware_breakpoint / bp_loc_hardware_watchpoint.
There are situations where the breakpoint is a software breakpoint,
but GDB still inserts a hardware breakpoint, like e.g., due
to "set breakpoint auto-hw".

> +
> +  struct bp_location *loc;
> +  for (loc = b->loc; loc; loc = loc->next)
> +    if (loc->enabled && loc->pspace == current_program_space
> +	&& b->ops->remove_location (loc, REMOVE_BREAKPOINT) == 0)
> +      b->ops->insert_location (loc);

This is incorrect for not considering whether a
breakpoint location was enabled but not inserted (e.g., the overall
breakpoint was disabled), or whether a breakpoint location was
a duplicate.

You should instead look at loc->inserted.

> +
> +  return false;
> +}
> +
> +/* This breakpoint type should never stop, but when reached, reset
> +   the active hardware breakpoints.  */

hardware breakpoints and watchpoints.

> +
> +static void
> +startup_breakpoint_check_status (bpstat bs)
> +{
> +  /* Never stop.  */
> +  bs->stop = 0;
> +
> +  iterate_over_breakpoints (reset_hardware_breakpoints);
> +}
> +
> +/* Update the breakpoint location to the current entry point.  */
> +
> +static void
> +startup_breakpoint_re_set (struct breakpoint *b)
> +{

This is called if/when the loaded executable changes, even
without re-starting an inferior.  E.g., if you use the
"file" command after starting the inferior.  So this
should re-fetch the new entry point from the executable.
Again, take a look at the jit.c code.

> +  struct windows_info *info = get_windows_inferior_data ();
> +  CORE_ADDR entry_point = info->entry_point;
> +
> +  /* Do nothing if the entry point didn't change.  */
> +  struct bp_location *loc;
> +  for (loc = b->loc; loc; loc = loc->next)
> +    if (loc->pspace == current_program_space && loc->address == entry_point)
> +      return;
> +
> +  event_location_up location
> +    = new_address_location (entry_point, nullptr, 0);
> +  std::vector<symtab_and_line> sals;
> +  sals = b->ops->decode_location (b, location.get (), current_program_space);

Merge the two statements, so that you end up copy initialization, instead of
initialization, and then assignment:

  std::vector<symtab_and_line> sals
    = b->ops->decode_location (b, location.get (), current_program_space);

> +  update_breakpoint_locations (b, current_program_space, sals, {});
> +}
> +
>  /* Implement the "solib_create_inferior_hook" target_so_ops method.  */
>  
>  static void
> @@ -914,6 +1008,30 @@ windows_solib_create_inferior_hook (int from_tty)
>        if (vmaddr != exec_base)
>  	objfile_rebase (symfile_objfile, exec_base - vmaddr);
>      }
> +
> +  /* Create the entry point breakpoint if it doesn't exist already.  */
> +  if (target_has_execution && exec_base != 0)
> +    {
> +      struct windows_info *info = get_windows_inferior_data ();
> +      CORE_ADDR entry_point = exec_base
> +	+ pe_data (exec_bfd)->pe_opthdr.AddressOfEntryPoint;
> +      info->entry_point = entry_point;
> +
> +      breakpoint *startup_breakpoint
> +	= iterate_over_breakpoints ([] (breakpoint *bp)
> +	  {
> +	    return bp->ops == &entry_point_breakpoint_ops;
> +	  });
> +      if (startup_breakpoint == nullptr)
> +	{
> +	  event_location_up location
> +	    = new_address_location (entry_point, nullptr, 0);
> +	  create_breakpoint (target_gdbarch(), location.get(), nullptr, -1,

Space before parens.

This looking up for the pre-existing breakpoint doesn't work
correctly when you consider multiple inferiors, where each will
need a location for its own entry pointer.  The Windows backend
doesn't support multi-process, but OTOH, if you do it like jit.c
does, which just basically always create a breakpoint and
stores the pointer in the per-pspace data, you're practically
good to go, and you'll make it easier for whomever comes next
and decides to all multi-process support.

> +			     nullptr, 0, 1, bp_breakpoint, 0,
> +			     AUTO_BOOLEAN_FALSE, &entry_point_breakpoint_ops,
> +			     0, 1, 1, 0);

Thanks,
Pedro Alves