linux-kernel - Re: [v8] misc: pci_endpoint_test: Fix overflow of bar

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Message-ID: <bcb4e21c-f7e9-4b5e-8712-9ab462bdf46c@163.com>
Date: Thu, 9 Jan 2025 17:19:23 +0800
From: Hans Zhang <18255117159@....com>
To: Arnd Bergmann <arnd@...db.de>, Niklas Cassel <cassel@...nel.org>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
 Krzysztof Wilczyński <kw@...ux.com>,
 Kishon Vijay Abraham I <kishon@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>, linux-pci@...r.kernel.org,
 linux-kernel@...r.kernel.org, rockswang7@...il.com
Subject: Re: [v8] misc: pci_endpoint_test: Fix overflow of bar_size



On 2025/1/9 14:29, Arnd Bergmann wrote:
>> On 2025/1/8 22:13, Niklas Cassel wrote:
>>>>> Ok. Looking at do_div(), it seems to be the correct API to use
>>>>> for this problem. Just change bar_size type to u64 (instead of casting)
>>>>> and use do_div() ? That is how it is seems to be used in other drivers.
>>>>
>>>> I think using div_u64_rem() instead of do_div() would make this
>>>> more readable as this is always an inline function, so the type can
>>>> remain resource_size_t, and the division gets optimized well when
>>>> that is a 32-bit type.
>>>
>>> After patch 1/2, we no longer care about the remainder, so I guess
>>> div64_u64() is the correct function to use then?
> 
> div_u64() is the correct interface here, div64_u64() is the
> even slower version where both arguments are 64-bit wide.
> 
>>   >> drivers/misc/pci_endpoint_test.c:311:11: warning: comparison of
>> distinct pointer types ('typeof ((bar_size)) *' (aka 'unsigned int *')
>> and 'uint64_t *' (aka 'unsigned long long *'))
>> [-Wcompare-distinct-pointer-types]
>>        311 |         remain = do_div(bar_size, buf_size);
>>            |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> You don't use div_u64() or div64_u64() here, do_div() is the macro
> version that must be called with a 64-bit argument.
> 
>      Arnd

Thank you so much Niklas and Arnd.

Best regards
Hans