linux-kernel - Re: [PATCH v2 1/2] scsi: ufs: Fix a possible NULL pointer issue

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Date:   Sat, 16 Jan 2021 21:27:34 +0800
From:   Can Guo <cang@...eaurora.org>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Bart Van Assche <bvanassche@....org>, asutoshd@...eaurora.org,
        nguyenb@...eaurora.org, hongwus@...eaurora.org,
        ziqichen@...eaurora.org, rnayak@...eaurora.org,
        linux-scsi@...r.kernel.org, kernel-team@...roid.com,
        saravanak@...gle.com, salyzyn@...gle.com,
        Alim Akhtar <alim.akhtar@...sung.com>,
        Avri Altman <avri.altman@....com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Stanley Chu <stanley.chu@...iatek.com>,
        Bean Huo <beanhuo@...ron.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] scsi: ufs: Fix a possible NULL pointer issue

On 2021-01-15 21:07, Adrian Hunter wrote:
> On 2/01/21 3:10 pm, Can Guo wrote:
>> On 2021-01-02 20:29, Can Guo wrote:
>>> On 2021-01-02 00:05, Bart Van Assche wrote:
>>>> On 12/31/20 9:44 PM, Can Guo wrote:
>>>>> During system resume/suspend, hba could be NULL. In this case, do 
>>>>> not touch
>>>>> eh_sem.
>>>>> 
>>>>> Fixes: 88a92d6ae4fe ("scsi: ufs: Serialize eh_work with system PM 
>>>>> events
>>>>> and async scan")
>>>>> 
>>>>> Signed-off-by: Can Guo <cang@...eaurora.org>
>>>>> ---
>>>>>  drivers/scsi/ufs/ufshcd.c | 9 +++++----
>>>>>  1 file changed, 5 insertions(+), 4 deletions(-)
>>>>> 
>>>>> diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
>>>>> index e221add..34e2541 100644
>>>>> --- a/drivers/scsi/ufs/ufshcd.c
>>>>> +++ b/drivers/scsi/ufs/ufshcd.c
>>>>> @@ -8896,8 +8896,11 @@ int ufshcd_system_suspend(struct ufs_hba 
>>>>> *hba)
>>>>>      int ret = 0;
>>>>>      ktime_t start = ktime_get();
>>>>> 
>>>>> +    if (!hba)
>>>>> +        return 0;
>>>>> +
>>>>>      down(&hba->eh_sem);
>>>>> -    if (!hba || !hba->is_powered)
>>>>> +    if (!hba->is_powered)
>>>>>          return 0;
>>>>> 
>>>>>      if ((ufs_get_pm_lvl_to_dev_pwr_mode(hba->spm_lvl) ==
>>>>> @@ -8945,10 +8948,8 @@ int ufshcd_system_resume(struct ufs_hba 
>>>>> *hba)
>>>>>      int ret = 0;
>>>>>      ktime_t start = ktime_get();
>>>>> 
>>>>> -    if (!hba) {
>>>>> -        up(&hba->eh_sem);
>>>>> +    if (!hba)
>>>>>          return -EINVAL;
>>>>> -    }
>>>>> 
>>>>>      if (!hba->is_powered || pm_runtime_suspended(hba->dev))
>>>>>          /*
>>>> 
>>>> Hi Can,
>>>> 
>>>> How can ufshcd_system_suspend() or ufshcd_system_resume() be called 
>>>> with a
>>>> NULL argument? In ufshcd_pci_probe() I see that pci_set_drvdata() is 
>>>> called
>>>> before pm_runtime_allow(). ufshcd_pci_remove() calls 
>>>> pm_runtime_forbid().
>>>> 
>>>> Thanks,
>>>> 
>>>> Bart.
>>> 
>>> Hi Bart,
>>> 
>>> You are right about ufshcd_RUNTIME_suspend/resume() - 
>>> platform_set_drvdata()
>>> is called before pm_runtime_enable(), so runtime suspend/resume 
>>> cannot happen
>>> before pm_runtime_enable() is called. We can remove the sanity checks 
>>> of
>>> !hba there, they are outdated.
>> 
>> Add more history here - before Stanley's change (see below),
>> platform_set_drvdata()
>> is called AFTER pm_runtime_enable(), which was why we needed sanity 
>> checks
>> of !hba.
>> But now the sanity checks are unnecessary in
>> ufshcd_RUNTIME_suspend/resume(), so
>> feel free to remove them.
>> 
>> But still, things are a bit different for 
>> ufshcd_SYSTEM_suspend/resume(), we
>> need
>> the sanity checks of !hba there if my understanding is correct.
>> 
>> commit 24e2e7a19f7e4b83d0d5189040d997bce3596473
>> Author: Stanley Chu <stanley.chu@...iatek.com>
>> Date:   Wed Jun 12 23:19:05 2019 +0800
>> 
>>     scsi: ufs: Avoid runtime suspend possibly being blocked forever
>> 
>> Thanks,
>> Can Guo.
>> 
>>> 
>>> But for ufshcd_SYSTEM_suspend/resume() callbacks (not runtime ones), 
>>> my
>>> understanding is that system suspend/resume may happen after probe 
>>> (vendor
>>> driver probe calls ufshcd_pltfrm_init()) starts but before
>>> platform_set_drvdata()
>>> is called, in this case hba is NULL.
>>> 
>>> int ufshcd_pltfrm_init(struct platform_device *pdev,
>>>                const struct ufs_hba_variant_ops *vops)
>>> {
>>> ...
>>>      platform_set_drvdata(pdev, hba);
>>> 
>>>     pm_runtime_set_active(&pdev->dev);
>>>     pm_runtime_enable(&pdev->dev);
>>> }
> 
> Hi Can
> 
> I expect probe and system suspend are synchronized e.g. by 
> device_lock(), so
> hba would not be NULL.  Is there any example of it being NULL in system 
> suspend?
> 
> Regards
> Adrian

Hi Adrian,

Thanks for the remind - I didn't notice they are protected by 
device_lock().
You are right, hba cannot be NULL in current code... Maybe if (!hba) was
there just for a sanity check. I will make a change to remove these 
checks.

Thanks,
Can Guo.