linux-kernel - Re: [PATCH 2/2] iommu/vt-d: Use BUG_ON to check NULL value of 'table'

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Message-ID: <e735528a-7bbd-b9a8-7945-10f779fb750e@linux.intel.com>
Date:   Thu, 1 Jun 2023 09:23:44 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Yanfei Xu <yanfei.xu@...el.com>, dwmw2@...radead.org,
        joro@...tes.org, will@...nel.org, robin.murphy@....com
Cc:     baolu.lu@...ux.intel.com, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] iommu/vt-d: Use BUG_ON to check NULL value of 'table'

On 5/31/23 3:09 PM, Yanfei Xu wrote:
> On 5/31/2023 11:26 AM, Baolu Lu wrote:
>> On 5/30/23 5:25 PM, Yanfei Xu wrote:
>>> Checking NULL value of 'table' variable deserves a BUG_ON as the
>>> following code will trigger a crash by dereferencing the NULL
>>> 'table' pointer. Crash in advance with BUG_ON to avoid WARN_ON
>>> plus NULL pointer dereferencing can simplify the crash log.
>>>
>>> Signed-off-by: Yanfei Xu<yanfei.xu@...el.com>
>>> ---
>>>   drivers/iommu/intel/iommu.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
>>> index e98f1b122b49..8aa3bfdb7f95 100644
>>> --- a/drivers/iommu/intel/iommu.c
>>> +++ b/drivers/iommu/intel/iommu.c
>>> @@ -1944,7 +1944,7 @@ static int domain_context_mapping_one(struct 
>>> dmar_domain *domain,
>>>       if (sm_supported(iommu)) {
>>>           unsigned long pds;
>>>   -        WARN_ON(!table);
>>> +        BUG_ON(!table);
>>
>> BUG_ON() is not recommended. Perhaps,
>>
>>         if (!table)
>>             -ENODEV;
>>
> Agree:) It is always better to handle the error than crash kernel.
> 
> How about:
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 8096273b034c..7f077e3a4128 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -1944,7 +1944,10 @@ static int domain_context_mapping_one(struct 
> dmar_domain *domain,
>          if (sm_supported(iommu)) {
>                  unsigned long pds;
> 
> -               WARN_ON(!table);
> +               if (WARN_ON(!table)) {
> +                       ret = -ENODEV;
> +                       goto out_unlock;
> +               }

I'd recommend to remove this line directly. This pointer will be
accessed in the following code, if empty "table" really happens, the
kernel will report a NULL pointer reference warning at the first place.

In the same function, I also saw "WARN_ON(did == 0);". It's unnecessary
as domain_id_iommu() will never return 0. Perhaps we can clean it up as
well.

Best regards,
baolu