Re: [Qemu-devel] [PATCH] jazz_led: fix bad snprintf

[Markus Armbruster]

Paolo Bonzini <address@hidden> writes:

> On 03/05/2017 13:38, Markus Armbruster wrote:
>> Paolo Bonzini <address@hidden> writes:
>> 
>>> Detected by GCC 7's -Wformat-truncation.  snprintf writes at most
>>> 2 bytes here including the terminating NUL, so the result is
>>> truncated.  In addition, the newline at the end is pointless.
>>> Fix the buffer size and the format string.
>>> ---
>>>  hw/display/jazz_led.c | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/hw/display/jazz_led.c b/hw/display/jazz_led.c
>>> index b72fdb1717..3c97d56434 100644
>>> --- a/hw/display/jazz_led.c
>>> +++ b/hw/display/jazz_led.c
>>> @@ -227,13 +227,13 @@ static void jazz_led_invalidate_display(void *opaque)
>>>  static void jazz_led_text_update(void *opaque, console_ch_t *chardata)
>>>  {
>>>      LedState *s = opaque;
>>> -    char buf[2];
>>> +    char buf[3];
>>>  
>>>      dpy_text_cursor(s->con, -1, -1);
>>>      qemu_console_resize(s->con, 2, 1);
>>>  
>>>      /* TODO: draw the segments */
>>> -    snprintf(buf, 2, "%02hhx\n", s->segments);
>>> +    snprintf(buf, 3, "%02hhx", s->segments);
>>>      console_write_ch(chardata++, ATTR2CHTYPE(buf[0], QEMU_COLOR_BLUE,
>>>                                               QEMU_COLOR_BLACK, 1));
>>>      console_write_ch(chardata++, ATTR2CHTYPE(buf[1], QEMU_COLOR_BLUE,
>> 
>> Since we're only every interested in the first two characters, the
>> truncation is totally harmless.  Thus, your patch cleans doesn't really
>> "fix bad snprintf", it cleans up an unclean one.  Consider rewording the
>> commit message accordingly.
>
> My reading is that buf[1] is always '\0' in the current code: "snprintf
> writes at most 2 bytes here including the terminating NUL, so the result
> is truncated".

You're right; I forgot that snprintf() always adds a NUL.  So this *is*
broken: we write NUL instead of the second digit.  Mentioning this in
the commit message wouldn't hurt.

>
> Paolo
>
>> Regardless,
>> Reviewed-by: Markus Armbruster <address@hidden>
>>