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Re: [Qemu-trivial] [Qemu-devel] [PATCH] jazz_led: fix bad snprintf
From: |
Laurent Vivier |
Subject: |
Re: [Qemu-trivial] [Qemu-devel] [PATCH] jazz_led: fix bad snprintf |
Date: |
Wed, 3 May 2017 14:12:42 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.0 |
On 03/05/2017 13:54, Paolo Bonzini wrote:
>
>
> 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".
I agree, from printf(3):
"int snprintf(char *str, size_t size, const char *format, ...);
...
The functions snprintf() and vsnprintf() write at most size bytes
(including the terminating null byte ('\0')) to str."
Laurent
Re: [Qemu-trivial] [PATCH] jazz_led: fix bad snprintf, Michael Tokarev, 2017/05/05
Re: [Qemu-trivial] [PATCH] jazz_led: fix bad snprintf, Michael Tokarev, 2017/05/05