Re: [PATCH 1/3] Handle 64bit breakpoints of WOW64 processes as SIGINT

[Hannes Domani via Gdb-patches <gdb-patches@sourceware.org>]

 Am Donnerstag, 24. September 2020, 05:30:11 MESZ hat Simon Marchi <simark@simark.ca> Folgendes geschrieben:

> On 2020-09-23 1:21 p.m., Hannes Domani via Gdb-patches wrote:
> > When a WOW64 process triggers a breakpoint exception in 64bit code (which
> > happens when a 64bit gdb calls DebugBreakProcess for a 32bit target),
> > gdb ignores the breakpoint (because Wow64GetThreadContext can only report
> > the pc of 32bit code, and there is not int3 at this location).
> >
> > But if these 64bit breakpoint exceptions are handled as SIGINT, gdb
> > doesn't check for int3, and always stops the target.
> >
> > gdb/ChangeLog:
> >
> > 2020-09-23  Hannes Domani  <ssbssa@yahoo.de>
> >
> >     * nat/windows-nat.c (handle_exception): Handle 64bit breakpoints
> >     in WOW64 processes as SIGINT.
> >     * nat/windows-nat.h: Make wow64_process a shared variable.
> >     * windows-nat.c: Remove static wow64_process variable.
> >
> > gdbserver/ChangeLog:
> >
> > 2020-09-23  Hannes Domani  <ssbssa@yahoo.de>
> >
> >     * win32-low.cc: Remove local wow64_process variable.
> >     * win32-low.h: Remove local wow64_process variable.
>
> This is ok, but...
>
>
> > @@ -240,6 +241,13 @@ handle_exception (struct target_waitstatus *ourstatus, bool debug_exceptions)
> >        ourstatus->kind = TARGET_WAITKIND_SPURIOUS;
> >        ignore_first_breakpoint = false;
> >      }
> > +      else if (wow64_process)
> > +    {
> > +      DEBUG_EXCEPTION_SIMPLE ("EXCEPTION_BREAKPOINT");
> > +      rec->ExceptionCode = DBG_CONTROL_C;
> > +      ourstatus->value.sig = GDB_SIGNAL_INT;
> > +      break;
>
> > +    }
>
> ...please add a comment here saying why this particular handling exists,
> it's really not intuitive.

Is it ok with this addition?:

diff --git a/gdb/nat/windows-nat.c b/gdb/nat/windows-nat.c
index a277156138..2cbbc0f2cc 100644
--- a/gdb/nat/windows-nat.c
+++ b/gdb/nat/windows-nat.c
@@ -243,6 +243,13 @@ handle_exception (struct target_waitstatus *ourstatus, bool debug_exceptions)
        }
       else if (wow64_process)
        {
+         /* This breakpoint exception is triggered for WOW64 processes when
+            reaching an int3 instruction in 64bit code.
+            gdb checks for int3 in case of SIGTRAP, this fails because
+            Wow64GetThreadContext can only report the pc of 32bit code, and
+            gdb lets the target process continue.
+            So handle it as SIGINT instead, then the target is stopped
+            unconditionally.  */
          DEBUG_EXCEPTION_SIMPLE ("EXCEPTION_BREAKPOINT");
          rec->ExceptionCode = DBG_CONTROL_C;
          ourstatus->value.sig = GDB_SIGNAL_INT;


Hannes