linux-kernel - Re: ERR_PTR(0) in a couple of places

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Message-ID: <ZRAoiGafRSv7SM6C@gallifrey>
Date:   Sun, 24 Sep 2023 12:16:08 +0000
From:   "Dr. David Alan Gilbert" <dave@...blig.org>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     matthew.brost@...el.com, mgreer@...malcreek.com,
        linux-kernel@...r.kernel.org, airlied@...il.com,
        intel-gfx@...ts.freedesktop.org
Subject: Re: ERR_PTR(0) in a couple of places

* Krzysztof Kozlowski (krzysztof.kozlowski@...aro.org) wrote:
> On 24/09/2023 02:41, Dr. David Alan Gilbert wrote:
> > Hi,
> >   I randomly noticed there are a couple of places in the kernel that
> > do
> >    ERR_PTR(0);
> > 
> > and thought that was odd - shouldn't those just be NULL's ?
> > 
> > 1) i915
> >   drivers/gpu/drm/i915/gt/uc/selftest_guc_multi_lrc.c : 47
> > 
> >     if (i <= 1)
> >       return ERR_PTR(0);
> > 
> >   from f9d72092cb490 
> > 
> > 2) trf7970a
> >   drivers/nfc/trf7970a.c : 896
> > 
> >       trf->ignore_timeout =
> >          !cancel_delayed_work(&trf->timeout_work);
> >       trf->rx_skb = ERR_PTR(0);
> 
> I would guess that code is relying on rx_skb being valid pointer or ERR
> (if (!IS_ERR(...))).

If seems mixed, that function calls trf7970a_send_upstream which has
both:

  if (trf->rx_skb && !IS_ERR(trf->rx_skb) && !trf->aborting)
    print_hex_dump_debug("trf7970a rx data: ", DUMP_PREFIX_NONE,
             16, 1, trf->rx_skb->data, trf->rx_skb->len,
             false);
and
    if (!IS_ERR(trf->rx_skb)) {
      kfree_skb(trf->rx_skb);
      trf->rx_skb = ERR_PTR(-ECANCELED);
    }

It's not clear to me whether it's expecteing that 2nd if to happen or
not.

I notice err.h gained a IS_ERR_OR_NULL to help that case as well.

Dave

> Best regards,
> Krzysztof
> 
-- 
 -----Open up your eyes, open up your mind, open up your code -------   
/ Dr. David Alan Gilbert    |       Running GNU/Linux       | Happy  \ 
\        dave @ treblig.org |                               | In Hex /
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