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

[Brian Inglis <Brian.Inglis@SystematicSw.ab.ca>]

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!

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

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