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Re: [PATCH for-4.22] xen/pdx: fix off-by-one index in offset mask calculation

To: Roger Pau Monne <roger.pau@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx

From: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

Date: Tue, 9 Jun 2026 11:39:42 +0200

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Cc: Anthony PERARD <anthony.perard@xxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>

Delivery-date: Tue, 09 Jun 2026 09:39:50 +0000

List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 6/9/26 10:53 AM, Roger Pau Monne wrote:

Adjust the mask calculation in case the last range is merged with the
previous one, as then the mask must be calculated from the previous range,
which the current one has been merged into.

Instead of fixing the off-by-one in place, move the calculation of the bit
change mask to the next loop, after the ranges have been merged.  This
simplifies the logic by consolidating mask calculation in a single place,
possibly making it less error prone in the future.

Also add a test case that triggers the bug being fixed by this commit.

Fixes: c5c45bcbd6a1 ("pdx: introduce a new compression algorithm based on region 
offsets")
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx> Thanks. ~ Oleksii

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