Re: [PATCH 2/3] PCI: don't use snprintf() in driver_override_show()

[Bjorn Helgaas]

On Mon, Sep 11, 2017 at 04:55:11AM -0700, Greg Kroah-Hartman wrote:
> On Mon, Sep 11, 2017 at 09:45:41AM +0200, Nicolai Stange wrote:
> > Quote from Documentation/filesystems/sysfs.txt:
> > 
> >   show() must not use snprintf() when formatting the value to be
> >   returned to user space. If you can guarantee that an overflow
> >   will never happen you can use sprintf() otherwise you must use
> >   scnprintf().
> > 
> > Commit 4efe874aace5 ("PCI: Don't read past the end of sysfs
> > "driver_override" buffer") introduced such a snprintf() usage from
> > driver_override_show() while at the same time tweaking
> > driver_override_store() such that the write buffer can't ever get
> > overflowed.
> > 
> > Reasoning:
> > Since aforementioned commit, driver_override_store() only accepts to be
> > written buffers less than PAGE_SIZE - 1 in size.
> > 
> > The then kstrndup()'ed driver_override string will be at most PAGE_SIZE - 1
> > in length, including the trailing '\0'.
> > 
> > After the addition of a '\n' in driver_override_show(), the result won't
> > exceed PAGE_SIZE characters in length, again including the trailing '\0'.
> > 
> > Hence, snprintf(buf, PAGE_SIZE, ...) and sprintf(buf, ...) are equivalent
> > at this point.
> > 
> > Replace the former by the latter in order to adhere to the rules in
> > Documentation/filesystems/sysfs.txt.
> > 
> > This is a style fix only and there's no change in functionality.
> > 
> > Signed-off-by: Nicolai Stange <nstange@xxxxxxx>
> > ---
> >  drivers/pci/pci-sysfs.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> > index 8e075ea2743e..43f7fbede448 100644
> > --- a/drivers/pci/pci-sysfs.c
> > +++ b/drivers/pci/pci-sysfs.c
> > @@ -722,7 +722,7 @@ static ssize_t driver_override_show(struct device *dev,
> >  	ssize_t len;
> >  
> >  	device_lock(dev);
> > -	len = snprintf(buf, PAGE_SIZE, "%s\n", pdev->driver_override);
> > +	len = sprintf(buf, "%s\n", pdev->driver_override);
> 
> While I'm all for changes like this, it's an uphill battle to change
> them, usually it's best to just catch them before they go into the tree.
> 
> Anyway, nice summary, very good job with that.
> 
> Acked-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

Why use snprintf() instead of scnprintf()?  It looks like snprintf()
is probably safe, but requires a bunch of analysis to prove that,
while scnprintf() would be obviously safe.

Bjorn