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Re: [XEN for-4.19 PATCH] x86/apic: Fix signing in left bitshift


  • To: Matthew Barnes <matthew.barnes@xxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Thu, 20 Jun 2024 16:16:37 +0100
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  • Cc: Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
  • Delivery-date: Thu, 20 Jun 2024 15:16:44 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 20/06/2024 3:31 pm, Matthew Barnes wrote:
> There exists a bitshift in the IOAPIC code where a signed integer is
> shifted to the left by at most 31 bits. This is undefined behaviour,
> and can cause faults in xtf tests such as pv64-pv-iopl~hypercall.
>
> This patch fixes this by changing the integer from signed to unsigned.
>
> Signed-off-by: Matthew Barnes <matthew.barnes@xxxxxxxxx>

The code change itself is fine, but I'm going to recommend some
adjustments to the commit message.

Its "x86/ioapic";  apic implies the Local APIC which is apic.c and
distinct from the IO-APIC.  The subject would be clearer as "Fix signed
shift in end_level_ioapic_irq_new()".

The XTF test has nothing to do with this, so shouldn't be mentioned like
this.  The UBSAN failure was in an interrupt handler, and it was
complete chance that it triggered while pv64-pv-iopl~hypercall was the
test being ran.

I'm happy to fix all of that up on commit.

CC Oleksii for 4.19.  This is low risk, and found during testing with
UBSAN active.

~Andrew



 


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