linux-kernel - Re: [PATCH] iio: gyro: adis16136: use scnprintf instead of snprintf

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Date:   Sun, 22 Mar 2020 11:41:02 +0530
From:   Rohit Sarkar <rohitsarkar5398@...il.com>
To:     Andy Shevchenko <andriy.shevchenko@...el.com>
Cc:     linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
        jic23@...nel.org, dragos.bogdan@...log.com,
        Lars-Peter Clausen <lars@...afoo.de>,
        Michael Hennerich <Michael.Hennerich@...log.com>,
        Stefan Popa <stefan.popa@...log.com>, knaack.h@....de,
        pmeerw@...erw.net
Subject: Re: [PATCH] iio: gyro: adis16136: use scnprintf instead of snprintf

On Sun, Mar 22, 2020 at 02:25:42AM +0200, Andy Shevchenko wrote:
> On Wed, Mar 18, 2020 at 08:25:22PM +0530, Rohit Sarkar wrote:
> > scnprintf returns the actual number of bytes written into the buffer as
> > opposed to snprintf which returns the number of bytes that would have
> > been written if the buffer was big enough. Using the output of snprintf
> > may lead to difficult to detect bugs.
> 
> Nice. Have you investigate the code?
> 
> > @@ -96,7 +96,7 @@ static ssize_t adis16136_show_serial(struct file *file,
> >  	if (ret)
> >  		return ret;
> >  
> > -	len = snprintf(buf, sizeof(buf), "%.4x%.4x%.4x-%.4x\n", lot1, lot2,
> > +	len = scnprintf(buf, sizeof(buf), "%.4x%.4x%.4x-%.4x\n", lot1, lot2,
> >  		lot3, serial);
> >  
> >  	return simple_read_from_buffer(userbuf, count, ppos, buf, len);
> 
> The buffer size is 20, the pattern size I count to 19. Do you think snprintf()
> can fail?
That might be the case, but IMO using scnprintf can be considered as a
best practice. There is no overhead with this change and further if the
pattern is changed by someone in the future they might overlook the
buffersize
Thanks,
Rohit
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>