Re: [Qemu-devel] [PATCH] spapr_pci: Fix unsafe signed/unsigned comparisons

[Markus Armbruster <armbru@redhat.com>]

David Gibson <david@gibson.dropbear.id.au> writes:

> spapr_pci.c contains a number of expressions of the form (uval == -1) or
> (uval != -1), where 'uval' is an unsigned value.
>
> This mostly works in practice, because as long as the width of uval is
> greater or equal than that of (int), the -1 will be promoted to the
> unsigned type, which is the expected outcome.
>
> However, at least for the cases where uval is uint32_t, this would break
> on platforms where sizeof(int) > 4 (and a few such do exist), because then
> the uint32_t value would be promoted to the larger int type, and never be
> equal to -1.

We may not care for portability to such systems.  However, a comparison
between signed and unsigned values still makes careful readers pause to
consider width.

gcc can warn (-Wsign-compare), but we don't enable this warning,
probably because we'd get too many of them.

> This patch fixes these errors.  The fixes for the (uint32_t) cases are
> necessary as described above.  I've made similar fixes to (uint64_t) and
> (hwaddr) cases.  Those are strictly theoretical, since I don't know of any
> platforms where sizeof(int) > 8, but hey, it's not that hard so we might
> as well be strictly C standard compliant.

It fixes all -Wsign-compare warnings from this file (but not from
included headers, but that's outside the scope of this patch).

> Reported-by: Markus Armbruster <armbru@redhat.com>
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Reviewed-by: Markus Armbruster <armbru@redhat.com>