Re: [PATCH v2] Windows gdb: Fix resetting of the debug-registers bit in ContextFlags

Sorry for the delay.  I'm traveling this week, and last week I thought I would manage to take
a look at this before leaving but something came up that prevented it.

FWIW, this version looks good to me.  The note about WOW64 in the commit log is exactly
the sort of thing that we were missing before.  Thank you.

Pedro Alves

On 2026-07-22 16:32, Hannes Domani wrote:
> The CONTEXT_DEBUG_REGISTERS also includes the arch-specific bit
> (CONTEXT_i386 or CONTEXT_AMD64) which is included in all CONTEXT_*
> defines.
> 
> So this basically just checks if any CONTEXT_* define is set:
>   if ((context->ContextFlags & CONTEXT_DEBUG_REGISTERS) != 0)
> 
> And similarly, unsetting CONTEXT_DEBUG_REGISTERS removes the
> arch-specific bit as well.
> 
> This isn't a problem per-se, since Windows seems to just ignore the
> arch-specific bit anyways in SetThreadContext.
> But it also creates inconsistent behavior for WOW64, because it uses
> the wrong arch-specific bit there.
> 
> So this creates a CONTEXT_DEBUG_REG_FLAG define with just the
> debug-registers bit, and uses it in these problematic locations.
> ---
> v2:
>   - updated commit message
>   - changed CONTEXT_DEBUG_REG_FLAG define to avoid hardcoded number
> ---
>  gdb/x86-windows-nat.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/gdb/x86-windows-nat.c b/gdb/x86-windows-nat.c
> index 27adeb1f154..2f556e47b39 100644
> --- a/gdb/x86-windows-nat.c
> +++ b/gdb/x86-windows-nat.c
> @@ -42,6 +42,10 @@ enum
>  
>  #define DR6_CLEAR_VALUE 0xffff0ff0
>  
> +/* The CONTEXT_DEBUG_REGISTERS define without the arch-specific bit
> +   (CONTEXT_i386 or CONTEXT_AMD64).  */
> +#define CONTEXT_DEBUG_REG_FLAG (CONTEXT_DEBUG_REGISTERS & ~CONTEXT_CONTROL)
> +
>  struct x86_windows_per_inferior : public windows_per_inferior
>  {
>    /* The function to use in order to determine whether a register is
> @@ -142,7 +146,7 @@ x86_windows_nat_target::thread_context_continue (windows_thread_info *th,
>  	{
>  	  windows_process->fill_thread_context (th);
>  
> -	  gdb_assert ((context->ContextFlags & CONTEXT_DEBUG_REGISTERS) != 0);
> +	  gdb_assert ((context->ContextFlags & CONTEXT_DEBUG_REG_FLAG) != 0);
>  
>  	  /* Check whether the thread has Dr6 set indicating a
>  	     watchpoint hit, and we haven't seen the watchpoint event
> @@ -173,13 +177,13 @@ x86_windows_nat_target::thread_context_continue (windows_thread_info *th,
>  		     update the debug registers later when the thread
>  		     is re-resumed by the core after the watchpoint
>  		     event.  */
> -		  context->ContextFlags &= ~CONTEXT_DEBUG_REGISTERS;
> +		  context->ContextFlags &= ~CONTEXT_DEBUG_REG_FLAG;
>  		}
>  	    }
>  	  else
>  	    DEBUG_EVENTS ("0x%x has no dr6 set", th->tid);
>  
> -	  if ((context->ContextFlags & CONTEXT_DEBUG_REGISTERS) != 0)
> +	  if ((context->ContextFlags & CONTEXT_DEBUG_REG_FLAG) != 0)
>  	    {
>  	      DEBUG_EVENTS ("0x%x changing dregs", th->tid);
>  	      context->Dr0 = state->dr_mirror[0];