linux-kernel - Re: [PATCH v3 07/10] iommu: Use right way to retrieve iommu

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Message-ID: <0bf678af-1905-de87-322b-43703992030b@linux.intel.com>
Date:   Tue, 15 Feb 2022 10:01:24 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     baolu.lu@...ux.intel.com, Jason Gunthorpe <jgg@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        Ben Skeggs <bskeggs@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Eric Auger <eric.auger@...hat.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v3 07/10] iommu: Use right way to retrieve iommu_ops

On 2/15/22 9:46 AM, Lu Baolu wrote:
> On 2/14/22 8:49 PM, Joerg Roedel wrote:
>> On Mon, Feb 14, 2022 at 09:55:35AM +0800, Lu Baolu wrote:
>>> +static inline const struct iommu_ops *dev_iommu_ops(struct device *dev)
>>> +{
>>> +    /*
>>> +     * Assume that valid ops must be installed if iommu_probe_device()
>>> +     * has succeeded. The device ops are essentially for internal use
>>> +     * within the IOMMU subsystem itself, so we should be able to trust
>>> +     * ourselves not to misuse the helper.
>>> +     */
>>> +    WARN_ON(!dev || !dev->iommu || !dev->iommu->iommu_dev ||
>>> +        !dev->iommu->iommu_dev->ops);
>>
>> There is no need for this WARN_ON, the code will oops anyway when one of
>> the pointers checked here is NULL.
>>
> 
> We really don't need to WARN_ON intermediate null pointers. But I would
> argue that we could add a WARN() on null dev->iommu->iommu_dev->ops, so
> that callers have no need to check the returned ops.

Oh, sorry! We don't need to check null ops either. That will also result
in a null pointer reference oops in the caller.

So, yes. No need for this WARN_ON().

Best regards,
baolu