Re: [PATCH] Fix ctrl-c when debugging WOW64 processes

[Simon Marchi <simark@simark.ca>]

On 2020-09-17 2:03 p.m., Hannes Domani via Gdb-patches wrote:
> DebugBreakProcess starts a new thread in the target process with the
> entry point DbgUiRemoteBreakin, where an int3 triggers a breakpoint
> exception for gdb.
>
> But this uses DbgUiRemoteBreakin of the 64bit ntdll.dll even for
> WOW64 processes.
> It stops in 64bit code, Wow64GetThreadContext reports a wrong pc without
> the int3, and gdb lets the target process continue.
>
> So this uses DbgUiRemoteBreakin of the 32bit ntdll.dll as the thread
> entry point for WOW64 processes instead.

Interesting.  Is there any reference somewhere that explains that things
must be done this way when a 64 bit process is debugging a 32 bit
process?

In any case, the patch is OK, with the formatting nits below fixed.

> @@ -1522,9 +1524,36 @@ ctrl_c_handler (DWORD event_type)
>    if (!new_console && !attach_flag)
>      return TRUE;
>
> -  if (!DebugBreakProcess (current_process_handle))
> -    warning (_("Could not interrupt program.  "
> -	       "Press Ctrl-c in the program console."));
> +#ifdef __x86_64__
> +  if (wow64_process)
> +    {
> +      /* Call DbgUiRemoteBreakin of the 32bit ntdll.dll in the target process.
> +	 DebugBreakProcess would call the one of the 64bit ntdll.dll, which
> +	 can't be correctly handled by gdb.  */
> +      if (!wow64_dbgbreak)

wow64_dbgbreak == nullptr

> +	{
> +	  CORE_ADDR addr;
> +	  if (!find_minimal_symbol_address ("ntdll!DbgUiRemoteBreakin",
> +					    &addr, 0))
> +	    wow64_dbgbreak = (void *) addr;
> +	}
> +
> +      if (wow64_dbgbreak)

wow64_dbgbreak != nullptr

Simon