[PATCH] drm: sysfs: Use scnprintf() for avoiding potential buffer overflow

[Takashi Iwai]

On Wed, 11 Mar 2020 09:10:56 +0100,
Thomas Zimmermann wrote:
> 
> Hi Takashi
> 
> Am 11.03.20 um 08:35 schrieb Takashi Iwai:
> > Since snprintf() returns the would-be-output size instead of the
> > actual output size, the succeeding calls may go beyond the given
> > buffer limit.  Fix it by replacing with scnprintf().
> > 
> > Signed-off-by: Takashi Iwai <tiwai at suse.de>
> > ---
> >  drivers/gpu/drm/drm_sysfs.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_sysfs.c b/drivers/gpu/drm/drm_sysfs.c
> > index dd2bc85f43cc..9b3180e8c12f 100644
> > --- a/drivers/gpu/drm/drm_sysfs.c
> > +++ b/drivers/gpu/drm/drm_sysfs.c
> > @@ -230,7 +230,7 @@ static ssize_t modes_show(struct device *device,
> >  
> >  	mutex_lock(&connector->dev->mode_config.mutex);
> >  	list_for_each_entry(mode, &connector->modes, head) {
> > -		written += snprintf(buf + written, PAGE_SIZE - written, "%s\n",
> > +		written += scnprintf(buf + written, PAGE_SIZE - written, "%s\n",
> >  				    mode->name);
> >  	}
> >  	mutex_unlock(&connector->dev->mode_config.mutex);
> > 
> 
> In drm_sysfs.c, there are more _show functions with calls to snprintf()
> that could be replaced by scnprintf(). ATM they don't return the correct
> length for output that exceeds PAGE_SIZE. since you're at it, you may
> replace them as well.

Well, the rest snprintf() calls are single calls and can't be over
PAGE_SIZE obviously.  IOW, they could be rather replaced with
sprintf() instead, for a sake of simplicity. 

> But in any case
> 
> Reviewed-by: Thomas Zimmermann <tzimmermann at suse.de>
> 
> for this patch.
> 
> Do you want me to merge the patch into drm-misc-next?

Yes, please.


thanks,

Takashi