linux-kernel - Re: [PATCH v4] gpio: sim: fix an invalid _

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Date:   Mon, 25 Sep 2023 09:58:49 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     brgl@...ev.pl, Linus Walleij <linus.walleij@...aro.org>,
        Kent Gibson <warthog618@...il.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        akpm@...ux-foundation.org, linux-gpio@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v4] gpio: sim: fix an invalid __free() usage

On Sat, Sep 23, 2023 at 11:19:08AM -0700, Linus Torvalds wrote:
> On Tue, 19 Sept 2023 at 03:49, Andy Shevchenko
> <andriy.shevchenko@...ux.intel.com> wrote:
> >
> > Nope, k*alloc*() returns ZERO or NULL on failure. That's what most developers
> > are missing :-)
> 
> Absolutely not.
> 
> k*alloc() returns NULL on failure. Absolutely nothing else.
> 
> On *success*, it can return the special ZERO_SIZE_PTR. But that is
> *not* a failure at all. It's very much a successful pointer.
> 
> Now, it's a pointer that you can't actually dereference, but that's
> very much intentional. You can't dereference it, because you asked for
> a zero-sized allocation. You got a zero-sized allocation.

"A-ha" moment to me, thank you for elaboration!

> But please never *ever* think it's a failure. It's very much not a
> failure case, and it is very much intentional.
> 
> It's different from NULL exactly *because* it's successful, and
> exactly so that you can write
> 
>      ptr = kmalloc(size);
>      if (!ptr)
>           return -ENOMEM;
> 
> without having to worry about the "size is zero" case.
> 
> The standard user-space "malloc()" library is misdesigned. Surprise
> surprise. The kernel isn't.

-- 
With Best Regards,
Andy Shevchenko