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

[Brian Inglis]

On 2021-12-07 23:30, Mark Geisert wrote:
> 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

In this context, (char *)clipbuf + sizeof clipbuf would actually be 
clearer. Or hide the expression in a cb_text(clipbuf[,pos?]) macro.

You could make the format and intent clear and obvious by appending 
cb_text[0|1|...] (some C++ compatible form) into cygcb_t in 
sys/clipboard.h.

> and all should be golden.  I don't think extra commentary is needed
> in code. (I think.)
Where else put commentary?
In this case it should be mandatory, as this is an address hack to 
access the clipboard text, and there were questions about that expression.

-- 
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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