Re: [PATCH] drivers/video: Fix -Wstringop-truncation in hdmi.c

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi

On 22.10.20 01:06, Laurent Pinchart wrote:
> Hi Thomas,
> 
> Thank you for the patch.
> 
> On Wed, Oct 21, 2020 at 02:12:41PM +0200, Thomas Zimmermann wrote:
>> Trying to copy into the string fields with strncpy() gives a warning from
>> gcc. Both fields are part of a packed HDMI header and do not require a
>> terminating \0 character.
>>
>> ../drivers/video/hdmi.c: In function 'hdmi_spd_infoframe_init':
>> ../drivers/video/hdmi.c:230:2: warning: 'strncpy' specified bound 8 equals destination size [-Wstringop-truncation]
>>   230 |  strncpy(frame->vendor, vendor, sizeof(frame->vendor));
>>       |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> ../drivers/video/hdmi.c:231:2: warning: 'strncpy' specified bound 16 equals destination size [-Wstringop-truncation]
>>   231 |  strncpy(frame->product, product, sizeof(frame->product));
>>       |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>> Just use memcpy() instead.
>>
>> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
>> ---
>>  drivers/video/hdmi.c | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/video/hdmi.c b/drivers/video/hdmi.c
>> index b7a1d6fae90d..1e4cb63d0d11 100644
>> --- a/drivers/video/hdmi.c
>> +++ b/drivers/video/hdmi.c
>> @@ -221,14 +221,18 @@ EXPORT_SYMBOL(hdmi_avi_infoframe_pack);
>>  int hdmi_spd_infoframe_init(struct hdmi_spd_infoframe *frame,
>>  			    const char *vendor, const char *product)
>>  {
>> +	size_t len;
>> +
>>  	memset(frame, 0, sizeof(*frame));
>>  
>>  	frame->type = HDMI_INFOFRAME_TYPE_SPD;
>>  	frame->version = 1;
>>  	frame->length = HDMI_SPD_INFOFRAME_SIZE;
>>  
>> -	strncpy(frame->vendor, vendor, sizeof(frame->vendor));
>> -	strncpy(frame->product, product, sizeof(frame->product));
>> +	len = strlen(vendor);
>> +	memcpy(frame->vendor, vendor, min(len, sizeof(frame->vendor)));
>> +	len = strlen(product);
>> +	memcpy(frame->product, product, min(len, sizeof(frame->product)));
> 
> As this seems to be a legitimate use of strncpy(), isn't there a way to
> silence the warning without requiring this additional runtime complexity
> ?

Yes, the original code this correct. I looked through include/string.h
if there's better string function, but none fits. Most of them
0-terminate the output string.

The only simple fix seems to be to set gcc's -Wno-stringop-truncation
here. I'd expect that would be an even less preferable change.

Best regards
Thomas

> 
>>  
>>  	return 0;
>>  }
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer
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