Re: [Qemu-devel] [PATCH] Do not use slow [*] expansion for GPIO creation

[Daniel P. Berrange]

On Wed, Jul 29, 2015 at 05:08:18PM +0300, Pavel Fedin wrote:
>  Hello!
> 
> > > +    l = strlen(name);
> > > +    propname = g_malloc(l + 13); /* 10 characters for UINT_MAX plus "[]" 
> > > */
> > > +    memcpy(propname, name, l);
> > 
> > Please don't do manual string length calculations in combination with
> > unbounded sprintf calls. It is a recipe for future security bugs.
> 
> [skip]
> 
> > >      for (i = gpio_list->num_in; i < gpio_list->num_in + n; i++) {
> > > +        g_sprintf(&propname[l], "[%u]", i);
> > 
> > Replace this with
> > 
> >     gchar *propname = g_strdup_printf("%s[%u]", name, i)
> > 
> > >          object_property_add_child(OBJECT(dev), propname,
> > >                                    OBJECT(gpio_list->in[i]), 
> > > &error_abort);
> > 
> >     g_free(propname);
> 
>  IMHO it's not really good because of repeating allocation-free. This
>  is not VERY slow, but still slower than it could be (imagine that this
>  repeats ~1000 times).

Unless this repeated allocation overhead is illustrated to be a real world
problem, I think using g_strdup_printf is preferrable.

>  I have a better idea instead. What if instead:
> 
> propname = g_malloc(l + 13); /* 10 characters for UINT_MAX plus "[]" */
> 
>  i do:
> 
> propname = g_strdup_printf("%s[%u]", name, -1)
> 
>  ? This will automatically give me a buffer to fit in the largest possible 
> integer.

I think it is premature/unneccesary optimization unless there are bench
marks to show this is a real world problem.

Regards,
Daniel
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