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Re: [XEN PATCH] xen/vpci: Fix UB in mask_write
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Mykyta Poturai <Mykyta_Poturai@xxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 6 Nov 2024 12:31:58 +0100
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Wed, 06 Nov 2024 11:32:04 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 06.11.2024 10:07, Roger Pau Monné wrote:
> On Wed, Nov 06, 2024 at 10:00:07AM +0100, Roger Pau Monné wrote:
>> On Wed, Nov 06, 2024 at 08:05:19AM +0000, Mykyta Poturai wrote:
>>> During the construction of dmask value, it gets shifted by
>>> (32 - msi->vectors) bits. If msi->vectors is 0, the result of the shift
>>> becomes undefined due to shifting by a size of the type. While this
>>> works fine on x86, on ARM the resulting mask becomes 0xFFFFFFFF, which
>>> is incorrect.
>>>
>>> Fix this by adding an explicit check for msi->vectors == 0.
>
> Wait - how can msi->vectors ever be 0? AFAICT there's no way in the
> MSI logic to configure 0 vectors, there will always be at least 1 vector
> enabled.
>
> Maybe what you want, if this fix is for compliance reasons, is an
> assert unreachable that msi->vectors > 0?
Which raises a question as to (lack of) context: Was this spotted by
mere code inspection? Or by a static analyzer? If so, which one? That
may help figure whether some workaround like the one suggested is
necessary, or whether it can simply be left alone.
Jan
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