Re: [PULL v2 25/31] osdep: Make MIN/MAX evaluate arguments only once

[Eric Blake]

On 6/27/20 4:35 PM, Peter Maydell wrote:
> On Fri, 26 Jun 2020 at 14:57, Paolo Bonzini <pbonzini@redhat.com> wrote:
> > From: Eric Blake <eblake@redhat.com>
> >
> > I'm not aware of any immediate bugs in qemu where a second runtime
> > evaluation of the arguments to MIN() or MAX() causes a problem, but
> > proactively preventing such abuse is easier than falling prey to an
> > unintended case down the road.  At any rate, here's the conversation
> > that sparked the current patch:
> > https://lists.gnu.org/archive/html/qemu-devel/2018-12/msg05718.html
>
> Hi; the changes in this patch seem to confuse Coverity.

Oh dear.

> In particular, where MIN_CONST or MAX_CONST are used to
> define values that must be const, eg in qemu-file.c:
>   50    DECLARE_BITMAP(may_free, MAX_IOV_SIZE);
> or in hcd-xhci.h:
> 217    USBPort  uports[MAX_CONST(MAXPORTS_2, MAXPORTS_3)];
>
> Coverity reports:
>
> CID 1429992 (#1 of 1): Unrecoverable parse warning (PARSE_ERROR)1.
> expr_not_constant: expression must have a constant value
>
> Can we do something (eg providing fallback less-intelligent
> versions of the macro ifdef __COVERITY__) to help it?

Absolutely; I see we've done similar in include/qemu/thread.h.  I'll 
post something later today.

> (This is the cause of CID 1429992, 1429995, 1429997,
> 1429999. Parse errors are unfortunate because Coverity
> abandons analysis of the affected function entirely,
> and analysis of its callers is also limited.)
--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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