linux-kernel - Re: [PATCH v2 4/6] iommufd: Deliver fault messages to user space

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Date:   Fri, 8 Dec 2023 13:47:35 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...pe.ca>,
        Joel Granados <j.granados@...sung.com>
Cc:     baolu.lu@...ux.intel.com, Kevin Tian <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Nicolin Chen <nicolinc@...dia.com>,
        Yi Liu <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        iommu@...ts.linux.dev, linux-kselftest@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/6] iommufd: Deliver fault messages to user space

On 12/8/23 1:17 AM, Jason Gunthorpe wrote:
> On Thu, Dec 07, 2023 at 05:34:10PM +0100, Joel Granados wrote:
>>> @@ -58,6 +255,8 @@ static void hw_pagetable_fault_free(struct hw_pgtable_fault *fault)
>>>   	WARN_ON(!list_empty(&fault->deliver));
>>>   	WARN_ON(!list_empty(&fault->response));
>>>   
>>> +	fput(fault->fault_file);
>>> +	put_unused_fd(fault->fault_fd);
>> I have resolved this in a naive way by just not calling the
>> put_unused_fd function.
> That is correct.
> 
> put_unused_fd() should only be called on error paths prior to the
> syscall return.
> 
> The design of a FD must follow this pattern
> 
>   syscall():
>     fdno = get_unused_fd_flags(O_CLOEXEC);
>     filep = [..]
>   
>     // syscall MUST succeed after this statement:
>     fd_install(fdno, filep);
>     return 0;
> 
>    err:
>      put_unused_fd(fdno)
>      return -ERRNO

Yes. Agreed.

> 
> Also the refcounting looks a little strange, the filep reference is
> consumed by fd_install, so what is that fput pairing with in fault_free?

fput() pairs with get_unused_fd_flags()? fd_install() does not seem to
increase any reference.

Best regards,
baolu