Re: [patch/cygwin] Remove dependency on __COPY_CONTEXT_SIZE

[Pedro Alves <palves at redhat dot com>]

On 03/31/2015 03:36 PM, Corinna Vinschen wrote:
> On Mar 31 13:34, Pedro Alves wrote:
>> On 03/30/2015 11:04 AM, Corinna Vinschen wrote:
>>
>>> @@ -820,7 +819,7 @@ handle_output_debug_string (struct target_waitstatus *ourstatus)
>>>  #endif
>>>  	warning (("%s"), s);
>>>      }
>>> -#ifdef __COPY_CONTEXT_SIZE
>>> +#ifdef __CYGWIN__
>>>    else
>>>      {
>>>        /* Got a cygwin signal marker.  A cygwin signal is followed by
>>> @@ -847,8 +846,8 @@ handle_output_debug_string (struct target_waitstatus *ourstatus)
>>>  	  else if ((x = (LPCVOID) (uintptr_t) strtoull (p, NULL, 0))
>>>  		   && ReadProcessMemory (current_process_handle, x,
>>>  					 &saved_context,
>>> -					 __COPY_CONTEXT_SIZE, &n)
>>> -		   && n == __COPY_CONTEXT_SIZE)
>>> +					 sizeof (CONTEXT), &n)
>>
>> Is that really wise?  AFAIK, the size of the CONTEXT structure can
>> grow as MSFT adds more registers to support newer machines.
> 
> No, that's not possible.  The CONTEXT structure matches the platform.
> It doesn't even contain a version number.  Consider that the structure
> is available in user space.  If Microsoft changes the size on a given
> platform, applications built for this platform might crash due to
> overwritten memory.  They wouldn't do that.

That's not true.   GetThreadContext takes a size parameter,
and only writes to the bits that the caller requests with
context.ContextFlags.  A size parameter is common in Windows API land
to permit later versions.  If the structure grows, evidently the new
fields will need to be requested with a new context.ContextFlags flag.  Old
applications will never request that extra flag, and will be passing
a smaller SIZE to GetThreadContext, so it won't ever overwrite memory.

See the description of InitializeContext's parameters:

 https://msdn.microsoft.com/en-us/library/windows/desktop/hh134237%28v=vs.85%29.aspx

And the remarks section:

"InitializeContext can be used to initialize a CONTEXT structure within a buffer
with the required size and alignment characteristics. This routine is required if
the CONTEXT_XSTATE ContextFlag is specified since the required context
size and alignment may change depending on which processor features are
enabled on the system.
...
Applications may subsequently remove, but must never add, bits from
the ContextFlags member of CONTEXT.
"

Thanks,
Pedro Alves