Re: [Xen-devel] [PATCH] memory: don't depend on guest_handle_subrange_okay() implementation details

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Wed, Jun 5, 2019 at 3:30 PM Jan Beulich <JBeulich@xxxxxxxx> wrote:
>
> guest_handle_subrange_okay() takes inclusive first and last parameters,
> i.e. checks that [first, last] is valid. Many callers, however, actually
> need to see whether [first, limit) is valid (i.e., limit is non-
> inclusive), and to do this they subtract 1 from the size. This is
> normally correct, except in cases where first == limit, in which case
> guest_handle_subrange_okay() will be passed a second parameter less than
> its first.
>
> As it happens, due to the way the math is implemented in x86's
> guest_handle_subrange_okay(), the return value turns out to be correct;
> but we shouldn’t rely on this behavior.
>
> Make sure all callers handle first == limit explicitly before calling
> guest_handle_subrange_okay().
>
> Note that the other uses (increase-reservation, populate-physmap, and
> decrease-reservation) are already fine due to a suitable check in
> do_memory_op().
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>

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