Re: [RFC] slab: introduce auto_kfree macro

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Wed, Apr 02, 2025 at 01:32:52PM +0300, Andy Shevchenko wrote:
> On Tue, Apr 01, 2025 at 03:44:08PM +0200, Przemek Kitszel wrote:
> > Add auto_kfree macro that acts as a higher level wrapper for manual
> > __free(kfree) invocation, and sets the pointer to NULL - to have both
> > well defined behavior also for the case code would lack other assignement.
> > 
> > Consider the following code:
> > int my_foo(int arg)
> > {
> > 	struct my_dev_foo *foo __free(kfree); /* no assignement */
> > 
> > 	foo = kzalloc(sizeof(*foo), GFP_KERNEL);
> > 	/* ... */
> > }
> > 
> > So far it is fine and even optimal in terms of not assigning when
> > not needed. But it is typical to don't touch (and sadly to don't
> > think about) code that is not related to the change, so let's consider
> > an extension to the above, namely an "early return" style to check
> > arg prior to allocation:
> > int my_foo(int arg)
> > {
> >         struct my_dev_foo *foo __free(kfree); /* no assignement */
> > +
> > +	if (!arg)
> > +		return -EINVAL;
> >         foo = kzalloc(sizeof(*foo), GFP_KERNEL);
> >         /* ... */
> > }
> > Now we have uninitialized foo passed to kfree, what likely will crash.
> > One could argue that `= NULL` should be added to this patch, but it is
> > easy to forgot, especially when the foo declaration is outside of the
> > default git context.
> > 
> > With new auto_kfree, we simply will start with
> > 	struct my_dev_foo *foo auto_kfree;
> > and be safe against future extensions.
> > 
> > I believe this will open up way for broader adoption of Scope Based
> > Resource Management, say in networking.
> > I also believe that my proposed name is special enough that it will
> > be easy to know/spot that the assignement is hidden.
> 
> 
> I understand the issue and the problem it solves, but...
> 
> > +#define auto_kfree __free(kfree) = NULL
> 
> ...I do not like this syntax at all (note, you forgot to show the result
> in the code how it will look like).
> 
> What would be better in my opinion is to have it something like DEFINE_*()
> type, which will look more naturally in the current kernel codebase
> (as we have tons of DEFINE_FOO().
> 
> 	DEFINE_AUTO_KFREE_VAR(name, struct foo);

Maybe slightly better name is

	DEFINE_AUTO_KFREE_PTR()

as we expect this to be a pointer.

> with equivalent to
> 
> 	struct foo *name __free(kfree) = NULL

-- 
With Best Regards,
Andy Shevchenko