[PATCH v2] Cygwin: clipboard: Fix a bug in read().

[Mark Geisert]

Brian Inglis wrote:
> On 2021-12-07 13:18, Thomas Wolff wrote:
>>
>> Am 07.12.2021 um 15:23 schrieb Corinna Vinschen:
>>> On Dec  7 23:00, Takashi Yano wrote:
>>>> - Fix a bug in fhandler_dev_clipboard::read() that the second read
>>>>    fails with 'Bad address'.
>>>>
>>>> Addresses:
>>>>    https://cygwin.com/pipermail/cygwin/2021-December/250141.html
>>>> ---
>>>>   winsup/cygwin/fhandler_clipboard.cc | 2 +-
>>>>   winsup/cygwin/release/3.3.4         | 6 ++++++
>>>>   2 files changed, 7 insertions(+), 1 deletion(-)
>>>>   create mode 100644 winsup/cygwin/release/3.3.4
>>>>
>>>> diff --git a/winsup/cygwin/fhandler_clipboard.cc 
>>>> b/winsup/cygwin/fhandler_clipboard.cc
>>>> index 0b87dd352..ae10228a7 100644
>>>> --- a/winsup/cygwin/fhandler_clipboard.cc
>>>> +++ b/winsup/cygwin/fhandler_clipboard.cc
>>>> @@ -229,7 +229,7 @@ fhandler_dev_clipboard::read (void *ptr, size_t& len)
>>>>         if (pos < (off_t) clipbuf->cb_size)
>>>>       {
>>>>         ret = (len > (clipbuf->cb_size - pos)) ? clipbuf->cb_size - pos : len;
>>>> -      memcpy (ptr, &clipbuf[1] + pos , ret);
>>>> +      memcpy (ptr, (char *) &clipbuf[1] + pos, ret);
> 
>>> I'm always cringing a bit when I see this kind of expression. Personally
>>> I think (ptr + offset) is easier to read than &ptr[offset], but of course
>>> that's just me.  If you agree, would it be ok to change the above to
>>>
>>>    (char *) (clipbuf + 1)
>>>
>>> while you're at it?  If you like the ampersand expression more, it's ok,
>>> too, of course.  Please push.
> 
>> In this specific case I think it's actually more confusing because of the type 
>> mangling that's intended in the clipbuf.
>> At quick glance, it looks a bit as if the following were meant:
>>
>>    (char *) clipbuf + 1
>>
>> I'd even make it clearer like
>>
>> +      memcpy (ptr, ((char *) &clipbuf[1]) + pos, ret);
>> or even
>> +      memcpy (ptr, ((char *) (&clipbuf[1])) + pos, ret);
> 
> If the intent is to address:
> 
>      clipbuf + pos + 1
> 
> use either that or:
> 
>      &clipbuf[pos + 1]
> 
> to avoid obscuring the intent,
> and add comments to make it clearer!

Boy am I kicking myself for screwing up the original here and opening this can of 
worms.  Brian, you'd be correct if clipbuf was (char *) like anything-buf often 
is.  But here it's a struct defining the initial part of a dynamic char buffer.

So my original
     &clipbuf[1]
to mean "just after the defining struct" was OK.  But the code needed a ptr to 
some char offset after that and
     &clipbuf[1] + pos
was wrong.  Casting the left term to (char *) would fix it.  But I like Corinna's 
choice of
     (char *) (clipbuf + 1)
to be most elegant and clear of all.  Now enclose that in parens and append the 
char offset so the new expression is
     ((char *) (clipbuf + 1)) + pos
and all should be golden.  I don't think extra commentary is needed in code.

(I think.)

..mark