linux-kernel - Re: [PATCH] Intel: Skylake: Fix inconsistent IS_ERR and PTR

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Date:   Sun, 23 Feb 2020 16:59:30 +0100
From:   Cezary Rojewski <cezary.rojewski@...el.com>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        Joe Perches <joe@...ches.com>, Xu Wang <vulab@...as.ac.cn>,
        "Slawinski, AmadeuszX" <amadeuszx.slawinski@...el.com>
Cc:     perex@...ex.cz, tiwai@...e.com, alsa-devel@...a-project.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Intel: Skylake: Fix inconsistent IS_ERR and PTR_ERR

On 2020-02-21 16:40, Pierre-Louis Bossart wrote:
> On 2/21/20 8:41 AM, Joe Perches wrote:
>> On Fri, 2020-02-21 at 18:11 +0800, Xu Wang wrote:
>>> PTR_ERR should access the value just tested by IS_ERR.
>>> In skl_clk_dev_probe(),it is inconsistent.

Please include all maintainers of given driver when submitting the 
patch, thank you.

>> []
>>> diff --git a/sound/soc/intel/skylake/skl-ssp-clk.c 
>>> b/sound/soc/intel/skylake/skl-ssp-clk.c
>> []
>>> @@ -384,7 +384,7 @@ static int skl_clk_dev_probe(struct 
>>> platform_device *pdev)
>>>                   &clks[i], clk_pdata, i);
>>>           if (IS_ERR(data->clk[data->avail_clk_cnt])) {
>>> -            ret = PTR_ERR(data->clk[data->avail_clk_cnt++]);
>>> +            ret = PTR_ERR(data->clk[data->avail_clk_cnt]);
>>
>> NAK.
>>
>> This is not inconsistent and you are removing the ++
>> which is a post increment.  Likely that is necessary.
>>
>> You could write the access and the increment as two
>> separate statements if it confuses you.
> 
> Well to be fair the code is far from clear.

Thanks for notifying, Pierre.

Although NAK is upheld here. Proposed change is likely to introduce 
regression.

> 
> the post-increment is likely needed because of the error handling in 
> unregister_src_clk 1
>          data->clk[data->avail_clk_cnt] = register_skl_clk(dev,
>                  &clks[i], clk_pdata, i);
> 
>          if (IS_ERR(data->clk[data->avail_clk_cnt])) {
>              ret = PTR_ERR(data->clk[data->avail_clk_cnt++]);
>              goto err_unreg_skl_clk;
>          }
>      }
> 
>      platform_set_drvdata(pdev, data);
> 
>      return 0;
> 
> err_unreg_skl_clk:
>      unregister_src_clk(data);
> 
> static void unregister_src_clk(struct skl_clk_data *dclk)
> {
>      while (dclk->avail_clk_cnt--)
>          clkdev_drop(dclk->clk[dclk->avail_clk_cnt]->lookup);
> }
> 
> So the post-increment is cancelled in the while().
> 
> That said, the avail_clk_cnt field is never initialized or incremented 
> in normal usages so the code looks quite suspicious indeed.

As basically entire old Skylake code, so no surprises here : )
struct skl_clk_data::avail_clk_cnt field is initialized with 0 via 
devm_kzalloc in skl_clk_dev_probe().

> 
> gitk tells me this patch is likely the culprit:
> 
> 6ee927f2f01466 ('ASoC: Intel: Skylake: Fix NULL ptr dereference when 
> unloading clk dev')
> 
> -        data->clk[i] = register_skl_clk(dev, &clks[i], clk_pdata, i);
> -        if (IS_ERR(data->clk[i])) {
> -            ret = PTR_ERR(data->clk[i]);
> +        data->clk[data->avail_clk_cnt] = register_skl_clk(dev,
> +                &clks[i], clk_pdata, i);
> +
> +        if (IS_ERR(data->clk[data->avail_clk_cnt])) {
> +            ret = PTR_ERR(data->clk[data->avail_clk_cnt++]);
>               goto err_unreg_skl_clk;
>           }
> -
> -        data->avail_clk_cnt++;
> 
> That last removal is probably wrong. Cezary and Amadeusz, you may want 
> to look at this?

Indeed, code looks wrong. Idk what are we even dropping in 
unregister_src_clk() if register_skl_clk() fails and avail_clk_cnt gets 
incremented anyway.

In general usage of while(ptr->counter--) (example of which is present 
in unregister_src_clk()) is prone to errors. Decrementation happens 
regardless of while's check outcome and caller may receive back handle 
in invalid state.

Amadeo, your thoughts?

Czarek