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Re: [RFC PATCH 2/4] xen/arm64: bitops: justify uninitialized variable inside a macro
- To: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 17 Jul 2023 16:06:48 +0200
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- Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, "michal.orzel@xxxxxxx" <michal.orzel@xxxxxxx>, "xenia.ragiadakou@xxxxxxx" <xenia.ragiadakou@xxxxxxx>, "ayan.kumar.halder@xxxxxxx" <ayan.kumar.halder@xxxxxxx>, "consulting@xxxxxxxxxxx" <consulting@xxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Bertrand Marquis <Bertrand.Marquis@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Luca Fancellu <Luca.Fancellu@xxxxxxx>, Julien Grall <julien@xxxxxxx>
- Delivery-date: Mon, 17 Jul 2023 14:07:10 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 17.07.2023 14:16, Nicola Vetrini wrote:
> On 16/07/23 18:50, Julien Grall wrote:
>> On 16/07/2023 10:20, Luca Fancellu wrote:
>>>> On 14 Jul 2023, at 14:05, Julien Grall <julien@xxxxxxx> wrote:
>>>> On 14/07/2023 12:49, Nicola Vetrini wrote:
>>>>> The macro 'testop' expands to a function that declares the local
>>>>> variable 'oldbit', which is written before being set, but is such a
>>>>> way that is not amenable to automatic checking.
>>>>
>>>> The code is pretty straightforward. So I am not entirely sure why
>>>> Eclair is not happy. Is it because the value is set by assembly code?
>
> Exactly. The reason why I didn't just state that oldbit is always
> written or never read before being written in that function is that I
> was unsure about the meaning of the assembly.
So I'm pretty sure the tool wouldn't take apart the string literal passed
first to the asm(). Instead I expect it goes from the operands specified,
which for oldbit is "=&r". There's nothing conditional here, so if the
tool didn't trust that outputs are written, it would need to flag such an
issue on about any asm() having outputs.
I hope the issue isn't that the tool doesn't properly deal with the
do/while.
In any event: I may misremember earlier discussions, but isn't this a
pretty obvious false positive? In which case didn't we mean to flag
those as such (because really an improved checking tool could avoid them)?
Jan
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