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

["Oleksii K." <oleksii.kurochko@xxxxxxxxx>]

On Thu, 2024-06-20 at 16:16 +0100, Andrew Cooper wrote:
> 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.
Release-Acked-By: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

~ Oleksii