linux-kernel - Re: [PATCH] tty/metag_da: initialize number

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Date:	Thu, 28 Jan 2016 18:48:17 +0000
From:	Colin Ian King <colin.king@...onical.com>
To:	James Hogan <james.hogan@...tec.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.com>, linux-metag@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty/metag_da: initialize number_written to zero

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On 27/01/16 11:42, James Hogan wrote:
> Hi Colin,
> 
> On Tue, Jan 26, 2016 at 11:37:25PM +0000, Colin King wrote:
>> From: Colin Ian King <colin.king@...onical.com>
>>
>> number_written is not initialized, so it can be any value. In the
>> case where dport->xmit_cnt is zero, number_written is not set
>> and subsequent accesses to it will be reading a garbage value.
> 
> the only subsequent accesses when dport->xmit_cnt == 0 are:
> 
> 	/* if we've made more data available, wake up tty */
> 	if (count && number_written) {
> 
> and:
> 
> 	/* did the write fail? */
> 	return count && !number_written;
> 
> but dport->xmit_cnt == 0 implies count == 0, so number_written shouldn
't
> be used, and both will evaluate to false regardless of the uninitialis
ed
> value, so it looks fine as it is to me.
> 
> Is this tripping up some static analysis tool or something?

It was found using cppcheck, namely:

[drivers/tty/metag_da.c:269]: (error) Uninitialized variable: number_wri
tten


> 
> Thanks
> James
> 
>> Fix this by initializing it to zero for the case when
>> dport->xmit_count is zero.
>>
>> Signed-off-by: Colin Ian King <colin.king@...onical.com>
>> ---
>>  drivers/tty/metag_da.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/tty/metag_da.c b/drivers/tty/metag_da.c
>> index 9325262..3da89c1 100644
>> --- a/drivers/tty/metag_da.c
>> +++ b/drivers/tty/metag_da.c
>> @@ -230,7 +230,7 @@ static int put_channel_data(unsigned int chan)
>>  {
>>  	struct dashtty_port *dport;
>>  	struct tty_struct *tty;
>> -	int number_written;
>> +	int number_written = 0;
>>  	unsigned int count = 0;
>>  
>>  	dport = &dashtty_ports[chan];
>> -- 
>> 2.7.0.rc3
>>

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