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Date:   Thu, 25 Aug 2022 12:22:37 +0200
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Subject: Re: [PATCH v2] xen/privcmd: fix error exit of privcmd_ioctl_dm_op()
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To:     Juergen Gross <jgross@suse.com>
Cc:     Stefano Stabellini <sstabellini@kernel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>,
        stable@vger.kernel.org,
        Rustam Subkhankulov <subkhankulov@ispras.ru>,
        xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org
References: <20220825092600.7188-1-jgross@suse.com>
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 <a69d7917-daae-c8d9-5f4b-2300c99168b8@suse.com>
From:   Jan Beulich <jbeulich@suse.com>
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On 25.08.2022 12:13, Juergen Gross wrote:
> On 25.08.22 11:50, Jan Beulich wrote:
>> On 25.08.2022 11:26, Juergen Gross wrote:
>>> --- a/drivers/xen/privcmd.c
>>> +++ b/drivers/xen/privcmd.c
>>> @@ -602,6 +602,10 @@ static int lock_pages(
>>>   		*pinned += page_count;
>>>   		nr_pages -= page_count;
>>>   		pages += page_count;
>>> +
>>> +		/* Exact reason isn't known, EFAULT is one possibility. */
>>> +		if (page_count < requested)
>>> +			return -EFAULT;
>>>   	}
>>
>> I don't really know the inner workings of pin_user_pages_fast()
>> nor what future plans there are with it. To be as independent of
>> its behavior as possible, how about bailing here only when
>> page_count actually is zero (i.e. no forward progress)?
> 
> This would require to rework the loop in lock_pages() to be able to
> handle only a partial buffer.

Oh, I see - I've misread the code as if the loop was capping each
iteration's count to the capacity of some internal buffer (as iirc
is being done elsewhere). So ...

> This would add some complexity, but OTOH I'd get an exact error code
> back in case of failure.

... perhaps not worth it then, ...

> I'll have a try and see how the result would look like.

... unless you think this might be relevant in certain cases.

Jan