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

[Hannes Domani <ssbssa@yahoo.de>]

 Am Sonntag, 31. Mai 2020, 18:38:06 MESZ hat Pedro Alves <palves@redhat.com> Folgendes geschrieben:

> 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.

OK. I will change it to program_space_key, but why are there no observers
to invalidate the a program-space, like these for inferiors?:

  /* Observers used to invalidate the cache when needed.  */
  gdb::observers::inferior_exit.attach (invalidate_windows_cache_inf);
  gdb::observers::inferior_appeared.attach (invalidate_windows_cache_inf);

Are they not needed for some reason?


> > +
> > +/* 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.

If I do "file" after "start", then windows_solib_create_inferior_hook is
called before startup_breakpoint_re_set, so the new entry point was
fetched already.


> > +  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.

I'm not sure what part here will not work.
I actually tested with multiple inferiors (not running at the same time),
and update_breakpoint_locations made a breakpoint location for each:

(gdb) maint info br
Num     Type           Disp Enb Address            What
-1      breakpoint     del  y   <MULTIPLE>
-1.1                        y   0x00000000004015b0 in mainCRTStartup at C:/gcc/src/mingw-w64-v7.0.0/mingw-w64-crt/crt/crtexe.c:218:1 inf 2
-1.2                        y   0x000000000117ee20 in mainCRTStartup at heob.c:7094:1 inf 1


Hannes