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

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Date:   Mon, 23 Mar 2020 18:05:32 +0200
From:   'Andy Shevchenko' <andy.shevchenko@...il.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Rohit Sarkar <rohitsarkar5398@...il.com>,
        linux-iio <linux-iio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jonathan Cameron <jic23@...nel.org>,
        "dragos.bogdan@...log.com" <dragos.bogdan@...log.com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Michael Hennerich <Michael.Hennerich@...log.com>,
        Stefan Popa <stefan.popa@...log.com>,
        Hartmut Knaack <knaack.h@....de>,
        Peter Meerwald <pmeerw@...erw.net>
Subject: Re: [PATCH] iio: gyro: adis16136: use scnprintf instead of snprintf

On Mon, Mar 23, 2020 at 03:04:23PM +0000, David Laight wrote:
> From: Andy Shevchenko
> > Sent: 22 March 2020 10:27
> > On Sun, Mar 22, 2020 at 8:11 AM Rohit Sarkar <rohitsarkar5398@...il.com> wrote:
> > >
> > > 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
> > 
> > If we cut the string above we will give wrong information to the user space.
> > I think scnprintf() change is a noise and does not improve the situation anyhow.
> 
> If, for any reason, any of the values are large the user will get
> corrupt data.

> But you don't want to leak random kernel memory to the user.

How? Kernel already got crashed at this point.

> 
> So while you may be able to prove that this particular snprintf()
> can't overflow, in general checking it requires knowledge of the code.

Here it's still a noise.

> With scnprintf() you know nothing odd will happen.

...and quite likely hide a lot of issues.

Really any "micro" / "small" correction / optimization to be very carefully
thought through.

> FWIW I suspect the 'standard' return value from snprintf() comes
> from the return value of the original user-space implementations
> which faked-up a FILE structure on stack and just silently discarded
> the output bytes that wouldn't fit in the buffer (they'd usually
> by flushed to a real file).
> The original sprintf() just specified a very big length so the
> flush would never be requested.

-- 
With Best Regards,
Andy Shevchenko